Strength, plasticity, fracture and fatigue behaviour controlled by interfaces and grain boundaries

Resume : Refractory high-entropy alloys (HEAs) have attracted significant attention due to their superior mechanical properties at elevated temperature. However, most of them are brittle and suffer from low ductility and toughness at room temperature, and their usage is limitted due to the inadequate fracture-resistance property. Grain boundaries play an important role in the extraordinarily high temperature strength and stability [1] of bcc HEAs and can also be potential sites for fracture. Here, strongly textured, columnar and nanometer-size grains NbMoTaW HEA thin flims with and without ion beam assisted deposition are produced. Mechanical properties, especially fracture toughness are determined by in situ micro-pillar and micro-cantilever tests. Further characterization is conducted by using high-resolution SEM images and TEM analysis. References [1] Y. Zou, et al., Nano Lett, 17 (2017) 1569-1574.

Resume : The erratic behaviour of short cracks propagation under low cyclic loading in ductile metals is commonly attributed to a complex interplay between stabilisation mechanisms that occur at the mescopic scale. Among these mechanisms, the interaction with the existing dislocation microstructure play a major role. The dislocation microstructure is source of plastic deformation and heat transfer that reduce the specimen stored elastic energy, screen the crack field due to its self-generated stress field or change the crack geometry through blunting mechanisms. In this study, these mechanisms are investigated with 3D Dislocation Dynamics simulations using the Discrete-Continuous Model, modelling three different crack orientations under monotonic traction loading, promoting mode I crack opening. Surprisingly, screening and blunting effects do not seem to have a key role on mode I crack stabilisation. Rather, the capability of the specimen to deform plastically without strong forest hardening is found to be the leading mechanism. Additional investigations of two different size effects (plastic zone confined introducing grain boundaries around the crack tip and reduction of the physical size of the system) confirm those results and show the minor contribution of a polarised dislocations density and the associated kinematic hardening on crack stabilisation.

Resume : Despite the fact that Ni-based superalloys have been used for decades, its micromechanical behavior during the first stages of fatigue is still a field under study. Grain boundaries are thought to play an important role on this research for two reasons: on the one hand, twin boundaries are suspicious of being responsible for a significant proportion of strain localization prior to the nucleation of the crack. On the other hand, the microcrack propagation path through the microstructure is governed by crystal misorientation between adjacent grains. Thus, the development of novel experimental tests to develop a better understanding of the mechanisms involved in the nucleation and propagation of short cracks is crucial. In this work, we present some examples of such tests, all applied to a forged Inconel 718 alloy with an average grain size of approximately 100 µm. In order to assess the effect of twins on strain localization, High Resolution Digital Image Correlation (HR-DIC) is employed to track strain throughout the sample surface during High Cycle Fatigue (HCF) at the subgrain level. Regarding the analysis of the crack path under the surface, HCF tests were carried out at the European Synchrotron Research Facility (ESRF). Diffraction Contrast Tomography (DCT) was initially performed to reveal the 3D grain structure of each specimen and Phase Contrast Tomographs (PCT) were acquired at regular intervals using white beam radiation, to study the evolution of crack nucleation and propagation with the number of fatigue cycles. The combination of both, DCT and PCT techniques, allows tracking of the crack path through the interior of the sample, thus leading to an exhaustive 3D study of that path as a function of grain misorientation.

Resume : The deformation of materials at the sub-micron length scale can be different from that of bulk / coarse grained materials because of a limited dislocation source size and source availability. Thus, detailed studies on the fundamental mechanisms are required (e.g. [1, 2]). In this work, stochastic aspects during deformation in single crystals and near crystal-interfaces are investigated by nanoindentation using a spherical nanoindenter. Our focus is on the nucleation and source activation of the dislocation sources. Therefore, we are using pop-in statistics such as performed by Morris et al [3], which requires a high number of individual experiments. The dislocation activation stress-distribution is correlated to the density and nearest neighbour distribution of dislocations obtained by the etch pitting technique. To avoid the influence of deformation zones of one indent with respect to the subsequent indents, the minimum distance for measuring a reliable, unaffected pop-in behaviour was investigated. It is significantly higher than the minimum distance for Young’s modulus and hardness evaluation. Also, the minimum distance for reliable pop-in statistics is strongly influenced by the density of pre-existing dislocations and the lattice friction stress. It was found that the pop-in behaviour was heavily influenced by the density and the distribution of pre-existing dislocations, and it was much more sensitive to pre-existing dislocations than other mechanical properties like the hardness. After these sanity experiments the work focuses on the source activation strength of different grain boundaries. Therefore, the pop-in behaviour of copper close to coherent Σ3 twin boundaries was studied. Interestingly, the mean pop-in stress close to the boundary was significantly smaller than in the bulk. The talk will provide a comprehensive overview on experimental boundary conditions for measuring reliable pop-in statistics and, in the second part, review the important role of twin boundaries during dislocation multiplication. References [1] Uchic, M. D., Dimiduk, D. M., Florando, J. N., and Nix, W. D., Sample dimensions influence strength and crystal plasticity. Science, 305(5686), 986-989 (2004). [2] Kraft, O., Gruber, P. A., Mönig, R., & Weygand, D.. Plasticity in confined dimensions. Annual review of materials research, 40, 293-317 (2010). [3] Morris J.R., Bei H., Pharr G.M., George E.P., Phys. Rev. Lett. 106 (2011) 165502.

Resume : Nowadays it is well known that grain boundaries (GBs) in metallic materials act not only as obstacles for dislocation motion, but also as sources and sinks for dislocations. Moreover, transmission through GBs can occur under specific conditions. Which interaction mode dominates is defined by several factors such as the crystal structure, geometric constraints from surrounding grains and the slip geometry of the adjacent glide planes. To resolve the interplay of dislocations with a single GB, a small sample testing volume is beneficial, as dislocation-dislocation interactions in this volume are minimized, thus, enhancing dislocation-GB interactions. Our previous study focuses on different dislocation transmission mechanisms through copper twin boundary and a random high angle GB at the micron scale. Details of the dislocation transmission mechanism suggest that dislocation transmission could be strain rate sensitive. Furthermore, in bulk polycrystalline samples a spectrum of different strain rates may locally act on dislocations interacting with grain boundaries. Thus, the present study aims to reveal the influence of strain rate on the transmission properties of two GBs: a penetrable high angle GB and a twin boundary. The discussion will focus on mechanisms of slip transfer depending on the strain rate, on evolution of the strain rate sensitivity with regard to higher strains and, finally, on the statistical significance of strain sensitivity factors at the micron scale.

Resume : This study deals with an analytical approach based on the Stroh formalism which provides the elastic fields of single straight edge dislocations and different dislocation pile-up configurations in anisotropic homogeneous crystal, half-space crystal, bi-crystals and tri-materials. The calculation of dislocation positions in an equilibrated pile-up, stress and displacement fields are carried out by an iterative relaxation scheme that minimizes the Peach-Koehler force on each dislocation. The tri-material configuration allows considering a non-zero thickness in the nanometer range and a specific stiffness for the grain boundary region. The effects of anisotropic elasticity, crystallographic orientation, grain boundary stiffness and applied stresses are studied. In parallel, in-situ micromechanical tests of micron-sized bi-crystals and observations coupling SEM, AFM and EBSD are conducted. On using FIB (focused ion beam), different FCC bi-crystals are obtained. Then the specimens are notched and mechanically loaded to produce slip bands interacting with the GB. Dislocation distribution function is measured by AFM. By comparing the theoretical calculations with the experimental results for metals with different elastic anisotropy, the driving force for slip activation in the neighboring grain and the resistance of different grain boundaries against incoming slip will be estimated to quantify the contribution of anisotropic elasticity on slip transmission at grain boundary.

Resume : The investigations into collective dynamics of dislocations attract an increasing attention as a key to understanding the micro-macro scale transition in the plastic flow. The self-organization of dislocations usually manifests itself in a range of mesoscopic scales accessible, e.g., to the acoustic emission (AE) technique. Its application to various materials has led to a general conclusion that the dislocation motion has an avalanche nature reflected in scale-free, power-law statistics of the AE. Such avalanche dynamics is usually levelled off on the macroscopic scale of deformation curves. However, collective behavior of dislocations can also manifest itself on the macroscopic scale, e.g., in the case of jerky flow in alloys, or the Portevin-Le Chatelier (PLC) effect. The fact that the complexity of plastic flow goes up to the macroscale makes this phenomenon especially interesting. Particularly, it involves specific correlations between the “dislocation avalanches”. The power-law exponents occur to depend on the material microstructure. For example, their analysis can bring information on the effect of fine elements, such as nanosize precipitates and/or submicron grains, featuring real materials. Indeed, whereas model binary alloys with coarse grains display well-known patterns of stress serrations and strain localizations, recent studies of more complex alloys revealed many peculiar features. In the present work, jerky flow in an AlMgScZr alloy with coarse and ultrafine grains is studied using statistical analysis of both the acoustic emission and stress serrations. The results are put into a general framework including behaviors of model alloys and smoothly deforming solids.

Resume : Nanoscale face-centered-cubic metals have become increasingly important engineering materials for energy-related applications, while fundamentally pushing the scientific frontiers of mechanics and materials science at the nanoscale. Examples vary from Ag nanowire networks for next-generation transparent conductive electrodes in flexible touchscreens, organic solar cells, and stretchable electronics, to structural metallic alloys strengthened by nanoscale interfaces resisting extreme environments. This talk will present research to understand the small-scale mechanics of strength and superplasticity in nanoscale silver metals, by using combined large-scale molecular dynamics simulations and in-situ nanomechanical experiments. First, following the “smaller is stronger” trend, we will show unusual room-temperature super-elongation without softening in single-crystalline Ag nanowires over a sample diameter range between 15 nm and 50 nm, which extends far beyond the maximum size for pure surface diffusion-mediated deformation (e.g. Coble-type creep). Over this diameter range, it is observed experimentally and theoretically that crystal slip can serve as a stimulus to diffusional creep of atomic surface ledges. Second, the ability of twin boundaries in strengthening and maintaining ductility in bulk nanostructures has been well documented; yet most understanding of the origin of these properties relies on perfect-interface assumptions. We will show that growth twins in bulk nanotwinned metals are inherently defective with kink-like steps, and that these atomic-scale imperfections also play a key role in plastic deformation mechanisms and the Hall-Petch strength limit. Specifically, this talk will highlight the role of solute atom segregation as a fundamentally new mechanism of twin stability and strengthening in nanotwinned Ag containing trace concentrations of solute Cu atoms.

Resume : Although twin boundaries are known to cause simultaneously high strength and ductility, the dislocation-grain boundary interactions are not thoroughly understood. At twin boundaries – and more generally at grain boundaries - dislocations can be emitted, absorbed or transmitted. All different mechanisms depend on the types of grain boundaries, orientation of grain boundaries and grains, density of grain boundaries and types of dislocations. Studies on the deformation of bicrystals with coherent twin boundaries have shown the perfect slip transfer across the boundary [1, 2]. This can have implications on deformation of nanotwinned materials that show an unusual combination of high strength and ductility [3]. Slip transfer has also been reported in nanotwinned materials tested under specific conditions [4]. However, there are relatively fewer studies on nanotwinned materials without grain boundaries, for example on a nanotwinned wire [5]. To address these aspects, in this study highly epitaxial Ag thin films with a large number of coherent twins are deposited on a Si substrate. Further, these are annealed to obtain films with varying twin spacings. Micropillar compression experiments are carried out to understand dislocation – twin boundary interactions. Wafer curvature measurements are carried out to study the elastic – plastic behavior under biaxial thermal stress. The experiments effectively decouple the effect of nanotwins on deformation from microstructural constraints compared to the polycrystalline nanotwinned materials where deformation is often constrained by surrounding grains. In the talk, the results obtained from micropillar compression on pillars containing a single twin boundary and on pillars with multiple boundaries will be discussed with respect to the deformation of nanotwinned bulk materials. References: 1. PJ Imrich, C Kirchlechner, C Motz, G Dehm, "Differences in deformation behavior of bicrystalline Cu micropillars containing a twin boundary or a large-angle grain boundary", Acta Materialia 73 (2014) 240-250 2. NV Malyar, JS Micha, G Dehm and C Kirchlechner, “Dislocation – twin boundary interactions in small scale Cu bicrystals loaded in different crystallographic directions”, Acta Mater., 129 (2017) 91-97 3. L Lu, Y Shen, X Chen, L Qian and K Lu, “Ultrahigh strength and high electrical conductivity in copper”, Science, 304 (2004) 422-426 4. Q Lu, Z You, X Huang, N. Hansen and L Lu, “Dependence of dislocation structure on orientation and slip systems in highly oriented nanotwinned Cu”, Acta Mater., 127 (2017) 85 - 97 5. D Jang, X Li, H Gao and JR Greer, “Deformation mechanisms in nanotwinned metal nanopillars”, Nature Nanotech., 7 (2012) 594 - 601

Resume : Nano-crystals exhibit particularly promising mechanical properties as increased yield strength and ductility that are associated to a key change in terms of dislocation process i.e. surface dislocation nucleation supplying classical bulk dislocation multiplication. Experimentally, mechanical tests can be performed on individual nano-object in situ in the TEM. However, very few studies investigate the role of surface oxide or amorphous layers that might strongly influence the mechanical properties of metallic nano-crystals. In this context, we use molecular dynamics (MD) simulations to investigate the influence of amorphous shells on the mechanical properties of spherical nanoparticles under compression. We create a monoatomic model system made of pure nickel for both the crystalline and amorphous phases. Firstly, we focus on the monoatomic bulk metallic glass and core/crystalline-shell/amorphous Ni NP fabrication methodology. Based on several EAM potentials and fabrication methods, a unique methodology that stabilizes the monoatomic amorphous-crystalline interface is proposed. Secondly, MD compression tests are presented. The mechanical response disparities between crystalline, core-shell and purely glassy NPs will be discussed, and specific attention will be paid to the main differences in dislocation-based plastic deformation processes.

Resume : Surface Mechanical Attrition Treatment (SMAT) is one of the most promising mechanical surface treatment techniques. It is based on the repetitive multi-directional impacts between the surface of a material and spherical shot boosted by an ultrasonic generator. For a mechanical part treated by SMAT, a gradient microstructure can be generated from the treated surface to the interior region, with the presence of a top surface nano-sized grains layer. This nanostructured layer, even if it is generally thin, can have significant effect on the performance of materials, since engineering components are mostly loaded on their surface. The nanostructured layer obtained by SMAT is so thin that it is difficult to prepare samples to perform uniaxial mechanical tests. In this work, micro-pillar compression technique is used to investigate the mechanical properties of a gradient microstructure generated by SMAT for a biomedical grade 316L stainless steel under both monotonic and cyclic loadings. A particular attention was paid to the mechanical property of the 5 microns thick nanostructured layer. The results show that the strength for the micro-pillars located in the nanostructured layer is much higher than that in the non-affected SMAT region. In addition, micro-pillars located at different distances from the treated surface show different cyclic behaviors and different deformation mechanisms. The effectiveness of this micro-pillar compression technique coupled with EBSD observation is demonstrated.

Resume : Nanoporous gold (np-Au) have been studied for catalyst, sensor, actuator and other applications due to their high surface-to-volume ratio and bio-compatibility. Np-Au shows brittle fracture by open cell structure that failure of the weakest ligament could cause fast crack propagation due to local stress concentration. Enhancing tensile strength is crucial to increasing the applicability of np-Au and other nanoporous materials. In recent studies, nanotwin(nt) structure in metals and other ceramic materials can significantly enhance both strength and ductility compared to ultra-fine grain or coarse grain structure due to a large density of twin boundaries. In this study, we fabricated nanotwined nanoporous gold (nt np-Au) thin film and measured its mechanical properties using in-situ tensile test. We fabricated nt Ag-Au thin film by co-sputtering of Ag and Au targets in magnetron sputter. After deposition, we controlled grain size and twin boundary density in nt precursor thin film by annealing process. Microstructures of precursor and np-Au are observed before and after dealloying by TEM and SEM and mechanical properties are investigated by in-situ tensile test using push-to-pull device in SEM chamber. We find that nt np-Au show 3 times greater tensile strength than np-Au with rare twins due to high density of twin boundaries in ligaments.

Resume : A recent study on commercially produced severely cold drawn Ni microwires has shown significant size effects, where the tensile strength of the wires was shown to approach the theoretical strength with a reduction of diameter from 120 µm down to 20 µm, by electropolishing [1]. Thus, in-situ deformation (monotonous and cyclic tensile tests) study of Ni microwires under synchrotron radiation has been performed to achieve a fundamental understanding of the observed size effect. To get more insights into the mechanical response of different grain families within the microwires, X-Ray diffraction (XRD) peaks are followed during deformation providing the possibility to follow the evolution of microstructure (including dislocation storage) and to detect the elastic-plastic transition. The measurements were carried out on several microwires with diameter ranging from 100 µm down to 40 µm: a first batch of wires has been thinned down by electropolishing, while a second batch has been cold drawn. Comparison between these two series allows for discriminating the contribution of internal microstructure and external size on the observed mechanical properties. To understand the influence of different microstructural parameters on the mechanical properties of microwires, 3D microstructures were generated using statistics from several large EBSD maps combined with texture information coming from high-energy X-ray diffraction pole figures. The obtained microstructures were used to perform crystal plasticity finite element (CPFE) simulations within the framework of small perturbations assumption. In-situ XRD analysis indicates successive yielding of grain families under uniform deformation. The gradient in the microstructure of microwire activates deformation mechanisms which are not seen for microwires with homogeneous microstructure. CPFE simulation will help in understanding the role of the spatial gradient in texture and grain size to the observed strength and ductility. References [1] N. Warthi et al., Scripta Mat., 68 (2013) 225-228.

Resume : Although extensive research has been performed in the last recent years on the mechanics of nanocrystalline (nc) metallic thin films, the nanoscale fundamental mechanisms controlling the plastic deformation of this class of materials are still not well understood. One of the main challenges is to isolate extrinsic (i.e. sample dimensions) from intrinsic (i.e. microstructure) size effect. This is mainly due to the difficulty to control grain size during the deposition of the films. In this research, an original method combining simultaneous mechanical and electrical loadings on in-house developed micro-electromechanical-systems (MEMS) was used to produce freestanding nc palladium (Pd) thin films with same thickness but with gradients of grain size. The films were first investigated using automated crystal orientation mapping (ACOM) TEM in order to obtain statistical info regarding grain size distribution, character of grain boundaries and crystallographic texture. Small-sized samples with different grain size (coarse-grained, ultrafine-grained and nanocrystalline) were thus cut by focused ion beam (FIB) and mounted on a Push-To-Pull device in order to achieve quantitative in-situ TEM tensile tests using the PI 95 Picoindenter instrument (Brucker.Inc). Both conventional diffraction contrast imaging TEM and ACOM-TEM have been used to unravel the elementary mechanisms controlling the plasticity/failure of the Pd films. The method indicates guidelines for the study of the mechanical behaviour and nanoscale deformation mechanisms of freestanding thin films with well-controlled micro/nanostructures.

Resume : In order to get better resolution and less sample preparation during experiments, coherent X-ray diffraction allows to characterize (or even to image) in three dimensions the geometry of an object and its defects (dislocations, twins, stacking faults) at nanometers scale [1]. In this experiment, the sample used are thin monocrystalline gold films with twins [2]. In a first step we simulate these experiments to anticipate the different kinds of diffraction patterns available. We are interested about the resolution that we can reach in real space related to the different parameters of the experiment, as the energy of the beam, the detector’s characteristics, etc. Then thanks to these simulations based on strain field of twins inspired from literature [3], we are trying to reproduce experimental coherent diffraction patterns acquired on the beam line ID 13 at the European Radiation Synchrotron Facility (ESRF) at Grenoble. These experiments have been realized with a focused beam of 300nm x 300nm on gold thin films of 50nm and 200 nm thickness. We are hoping to apply this new technic to in situ characterization of plastic events and microstructure evolution during a deformation process. The objective of the present study is for example to understand twins growth through a finite thickness. [1] M. Dupraz, et al, J. Appl. Cryst. 48 (2015) 621-644 [2] S.H. Oh, et al, Acta Mater. 55 (2007) 5558–5571 [3] F.D. Fischer, et al, European J. Mech. A 22 (2003) 709–726

Resume : The selection and design of modern high-performance engineering materials are driven by control and optimization of various mechanical and thermal properties such as strength, ductility, plasticity, toughness and so on. Nanoscale metallic multilayers (NMMs) are the relatively new class of materials with a high potential of replacing their conventional counterparts for the high-end applications in nuclear, space and aerospace industries. The superior properties of the NMMs are mostly correlated with the advanced design and fabrication of the interfaces at a very confined space (2- 20 nm). In this presentation, we will present a novel nanoscale interface architecture of the Zr-Nb NMMs that are fabricated by the epitaxial and physical vapor deposition based film-growth techniques. Then, we will discuss the effect of polycrystalline, epitaxial and amorphous interfaces on the deformation kinetics. In addition to the experimental investigations, we will also present the dynamic response maps of the molecular dynamics simulations and correlate them to dislocation-interface reactions.

Resume : Metallic thin films are often used as conductive or barrier layers on polymers. When deposited on elastomers, for stretchable microelectronic devices for instance, the deformability of the metal coating becomes an outstanding challenge. Elastomeric substrates might indeed undergo extreme deformations compared to metals, which may fracture, leading to a loss of the required properties. Here, we study the growth and deformability of copper and gold thin films (< 60 nm) deposited by thermal evaporation on silicone elastomers of different viscoelastic properties. This growth and deformability is discussed as a function of the nature of the metal and the mechanical properties of the substrate, on the basis of transmission electron microscope observations and fragmentation tests. As expected, our results demonstrate a dependence of the size and surface density of the growing metal islands with the nature of the metal, i.e. with the atom/substrate interaction. But, they also show a remarkable dependence of this island growth pattern with the mechanical properties of the substrate. Additionally, we have observed a similar dependence with the nature of the metal and the mechanical properties of the substrate, this time of the crack pattern growing in 60 nm thick metallic films during the deformation of the substrate. Our results emphasize the fundamental role of metal-elastomer interface on the mechanical response of metallic thin films deposited on strechable substrates.

Resume : The present work is a successful application of a powder metallurgy process to fabricate surface composites by using graphene nanoplatelets (GNP) as reinforcement. The powder metallurgy assisted friction surfacing (PMAFS) involves a two-stage homogeneous distribution of GNP in the aluminium matrix. In the first stage, a custom-made expendable tool of Al-GNP composite is fabricated by using a powder metallurgy technique. In the second stage, this expendable tool is utilized for the fabrication of Al6061-GNP surface composite by friction surfacing (FS). Experimental results reveal that the combination of above two strategies leads to homogeneous distribution and binding of GNPs in the Al substrate. Both friction surfacing tool and surface composites are characterized by using FE-SEM and TEM. Morphology and damage caused to the GNPs during the friction surfacing are studied by using Raman spectroscopy. The nanomechanical behavior of the surface composite and its correlation with the hardness of FS tool is studied by using nanoindentation. Further, the influence of process parameters such as GNP content in tool and tool rotational speed during FS on nanomechanical properties and microstructural changes were studied. The homogeneous distribution of GNPs and optimum mechanical properties are obtained for the surface composites fabricated with relatively lower GNP content in the tool and processed at lower tool rotational speed. With an optimum set of processing parameters, more than 100 % increase in the nano-hardness of the surface composite is observed.

Resume : The fatigue resistance of metal films is improved by decreasing the film thickness into the sub-micron regime, as a result of the inhibition of dislocation motion and the accompanying increase in strength. It remains unclear, however, what happens to the fatigue resistance of very thin films (less than 100 nm), where diffusive processes may become important and pose a threat to the film reliability. In this study, we investigate the ultra-high cycle fatigue behavior of supported Cu films with thicknesses between 40 and 360 nm in order to search for a transition from dislocation plasticity to diffusive deformation control of fatigue resistance. We use a novel AFM-based resonance method to investigate the damage created under strain controlled fatigue loading as a function of applied strain, film thickness and cycle numbers up to 5E10. For films thicker than 100 nm, extrusions and boundary cracks limit the fatigue performance but only appear above a threshold in the applied strain amplitude which scales inversely with the square root of the film thickness. This damage formation is attributed to dislocation activation which is controlled by the film thickness and grain size. In films of 100 nm and thinner, the grain boundary cracks are replaced by grain boundary grooves which are believed to form by diffusion mediated creep processes, similar to observations at higher temperatures but here driven by cyclic stresses and capillarity. In contrast to the thicker films, no threshold strain is observed for the formation of damage formed by diffusion mediated processes. These results indicate that films thinner than approximately 100 nm are less resistant to fatigue damage formation than thicker films due to diffusive processes and suffer from an increased reliability threat, particularly at high cycle numbers.

Resume : A novel method [1] was develop in order to investigate the local fatigue properties of ultrafine-grained copper on the micrometer-scale up to the high cycle fatigue (HCF) range, relying only on widely available nanoindentation hardware. This breakthrough was achieved by combining the widely used micropillar compression method with the fast actuation (40 Hz) provided by the continuous stiffness measurement (CSM) module, originally intended for pyramidal nanoindentation. Cyclic testing was performed at constant nominal stress amplitude for up to several million (3.10^6) cycles. The resulting strain amplitude was directly recorded and the plastic strain was evaluated from the phase angle measured by the lock-in amplifier during testing. Defining a threshold for strain amplitude decrease as failure criterion further enabled the determination of S-N curves. The fatigue behavior of the tested ECAP copper micropillars was found to be dominated by cyclic softening, which is in line with previous macroscopic observations on similar samples. The calculated plastic strain amplitude also matches the literature data closely. Generally, the new method has a great potential for studying the local cyclic effects taking place at interfaces in complex micro-architectured materials and coatings. References [1] Merle, B., Höppel, H.W. Microscale High-Cycle Fatigue Testing by Dynamic Micropillar Compression Using Continuous Stiffness Measurement, Experimental Mechanics, DOI:10.1007/s11340-017-0362-3 (2017)

Resume : For wearable flexible electronics, such as bendable displays, backplane electrodes have to cope with bending fatigue. In this study the bending fatigue behavior of Aluminum/Molybdenum thin films will be presented. Bilayer systems deposited by DC magnetron sputtering with Al as the top conductive layer and Mo as the adhesion layer on polyimide substrates were investigated. With a 30 nm Mo layer at the bottom, different thickness ratios (1:1, 2:1, 5:1 and 10:1) of Al/Mo were produced in order to better understand the role of brittle adhesion layers on the failure of the ductile Al layer. The bending experiments were performed with the custom-built FLEX-E-TEST which allows bending in tensile, compressive and a combined tensile and compression modes under different bending strains. Interruption of the bending test after specific cycle numbers was performed in order to assess the electrical resistance via 4-point-probe method and the surface topography with confocal laser scanning microscopy. These measurements allow for a correlation of the surface damage with the electrical conductivity. Additionally, post-testing focused ion beam cross-sections and scanning electron microscopy aid in the characterization of the fracture paths, deformation and role of interfaces. As the ductile film thickness decreases, more brittle fracture was observed. The results obtained within this study aid the use of brittle interlayers in future flexible electronics.

Resume : Tungsten (W) alloys and P91 grade ferritic-martensitic steels are promising materials for structural and high heat flux components in future fusion and fission nuclear reactors, respectively. W is a candidate material for plasma facing divertor components in ITER and DEMO, while P91 is a material of choice for steam generators or other reactor components where high temperature capabilities are critical. Instrumented high temperature nanoindentation is a powerful technique for obtaining various mechanical properties from small regions of interest, e.g. welds, coatings or ion-irradiated layers. Indentations with pyramidal and spherical indenters allow the determination of hardness and Young modulus as a function of depth, from which a number of microstructurally controlled plasticity phenomena can be studied. In addition, spherical indentations induce plastic strains which increase with depth thus allowing the derivation of nanoindentation stress−strain curves from a single measurement. This is of high relevance for engineering design, provided one succeeds in finding a robust translation from the microstructurally controlled local deformation to the macroscopic constitutive behaviour, for which comprehensive data and analyses are still needed. In this work, W and P91 have been characterised by nanoindentation using Berkovich and spherical indenters, respectively, at room temperature and elevated temperatures up to 600 °C. In doing so, progressive multicycle and continuous stiffness measurement methods have been combined and compared. Two temperature dependent transitions have been captured in the measurements; namely, the reduction of the indentation size effect at high temperatures in W and the dynamic strain ageing of P91 at intermediate temperatures.

Resume : Surface modification techniques, such as deep cold rolling (DCR) and shot peening (SP) are being used to induce the beneficial residual compressive stresses (RCS) in the material to improve the fatigue resistance. Although, both DCR and SP induce residual stresses, RCS of DCR has a greater depth of influence while that of SP has a greater magnitude. In this research work, the combined effect of DCR and SP processes on RCS, microstructure and mechanical property on nickel-based superalloy (Udimet 720Li) was studied. The residual stresses and strain hardening were quantified using X-Ray diffraction (XRD) and electron backscattered diffraction (EBSD) method respectively. The statistical approach was used to quantify the strain hardening depth using local mis-orientation and grain orientation spread (GOS) data. It is observed that the RCS magnitude is higher and deeper into the sub-surface which was not possible using a single process alone. Significant grain refinement and an addition of low angle grain boundaries (LAGBs) are noticed near the deformation edge which is gradually reducing towards the depth. The similar trend is observed for the local mis-orientation and micro-hardness which represents a positive correlation between them. Special attention is paid to the Intragranular deformation mechanism in the sub-surface and illustrated using inverse pole figure (IPF) map in EBSD. Overall, the comprehensive understanding of the microstructural features could generate the pathway to develop material with excellent mechanical performance. Keywords: Nickel-based superalloys, microstructure, residual stresses, strain hardening, intragranular deformation, deep cold rolling, shot peening

Resume : The purpose is to design and characterise novel aeroengine shaft alloys with better combinations of low and high temperature properties. The shafts have specific requirements of strength, ductility, fatigue- and creep-resistance, high purity and thermal stability. One approach involves the re-examination of secondary hardening alloy systems, where carbides are used to achieve strength and creep resistance. The characterisation of a commercially available alloy has revealed poor creep ductility and unacceptable thermal stability. The intention is to redesign the alloy and validate it experimentally. Based on previous work, reduced austenite grain size contributes to the creep properties, therefore the second approach is to microalloying F1E by adding small concentrations of niobium and carbon, to control the austenite grain size. It is found that the niobium and carbon are effective in refining the austenite grain size, but their role on a range of other properties needs to be thoroughly assessed.

Resume : The high temperature deformation behaviour of low-density Fe-10Mn-9Al-4Ni-0.8C duplex steel has been studied in a wider range of temperature (1223-1423 K) and strain rates (0.001-10 s-1). The value of apparent activation energy for the entire hot deformation regime of the studied low-density duplex steel was estimated to be 400 kJ mol-1. The processing map was constructed based on dynamic material model theories and experimental stress-strain data obtained from the hot compression tests. The microstructural response corresponding to the various processing conditions were examined employing optical microscopy, electron back scatter diffraction, and transmission electron microscopy. The optimum processing domain is identified as 1223-1350 K and 0.001-0.01 s-1 which is characterized by steady and low apparent activation energy and dominant continuous dynamic recrystallization (CDRX) mechanism where typical conversion of low-angle grain boundary to high-angle grain boundary was observed. At higher strain rate (1-10 s-1) and lower temperature (1223-1323 K) regime, the unstable domain was manifested by the presence of micro-cracks along the austenite/δ-ferrite interphase and the micro-band formation. Furthermore, at lower strain rate (0.001s-1) and higher temperature (1373 K) unstable domain, wedge crack at the triple junction of δ-ferrite/δ-ferrite grain boundaries was manifested as signature of instabilities.

Resume : In this study, the incipient plasticity and mechanical properties of Ni-Ti-Cu alloys were investigated using in-situ transmission electron microscope (TEM) compression and nanoindentation experiments. Results shows that crack growth and plastic zone of the Ni-Ti-Cu shape memory alloy after indentation loading. The strain-induced microstructure variation was observed. The hardness and Young's modulus of Ni-Ti-Cu alloys decreased with the annealing temperature increased. Both induced cracking and reduced flow stress were observed. The present provides necessary information for better understanding of the mechanical properties, plasticity and deformation of Ni-Ti-Cu shape memory alloys.

Resume : In the view of achieving optimum balance between strength and ductility, medium Mn, high Al multicomponent steel is designed and developed to meet the requirement of automotive applications. Low stacking fault energy (SFE) in the medium Mn range results in the poor mechanical properties of an alloy. So Al is purposefully added to increase the overall SFE of the developed material. The alloy after being processed through melting-casting route is subjected to thermo-mechanical treatment like hot forging, hot rolling and cold rolling, followed by annealing at 973K-1273K for 15-150 mins and thereafter air cooled. The microstructure with the duplex ferrite-austenite as the starting microstructure undergoes the massive phase transformation with the variation in annealing temperature and time. However during annealing within 973K-1273K for 15-150 mins, not only the phase transformation accelerates, but also recrystallization is initiated in both ferrite and austenite grains. Nevertheless in contrast to the hot forged-annealed specimen, the hot rolled and cold rolled annealed specimens takes sufficiently shorter time period and lesser temperature to attain the complete recrystallization owing to the presence of larger amount of stored energy during higher degree of deformation. Thus crystallization rates estimated from GOS study and Johnson-Mehl-Avrami equation states about migration of the dislocation along the grain boundary during annealing. Even the material with better recrystallized fraction (~> 90%) and Avrami exponent (~2) exhibits better strength-ductility relationship (UTS~ 1150 MPa and elongation ~ 25%) as compared to the material with the poor one.

Resume : In order to improve the application of SiCp-Al2O3 nanocomposites as one of the potential materials in hybrid bearings of rocket turbopumps at the low temperatures, the strengthen effects of microstresses on grain boundaries caused by SiC nanoparticles are discussed. The microstresses on grain boundaries were detected by in-situ Raman spectra and fluorescence spectra from 293 K to 4 K, and the results were in agreement with the theoretical calculation. Furthermore, the fracture mode of the nanocomposites transformed to transgranular, due to that the grain boundaries was strengthened by higher compressive microstresses during cooling. Therefore, the fracture strength of SiCp-Al2O3 nanocomposites increased obviously with the temperature decreasing. The optimal strength at 77 K was obtained at 5 vol.% addition and was trending to shift to the low concentration of SiC particles at low temperatures, which may in turn pave a way for optimizing strength at different temperatures.

Resume : We present a systematic ab initio study of segregation of 12 non-magnetic sp-impurities (Al, Si, P, S, Ga, Ge, As, Se, In, Sn, Sb and Te) at Σ5(210) grain boundary (GB) and (210) free surface (FS) in fcc ferromagnetic cobalt and nickel and analyze their effect on structure, magnetic and mechanical properties. We determine preferred segregation sites at the Σ5(210) GB for the sp-impurities studied, their segregation enthalpies and strengthening/embrittling energies with their decomposition into the chemical and mechanical components. In nickel, most of the above impurities nearly kill or substantially reduce the magnetic moments at the FS and, when segregating interstitially (i.e. Si, P, S, Ge, As, Se), also at the GB so that they provide atomically thin magnetically dead layers which may be very desirable in spintronics. Reduction of magnetic moments at the Σ5(210) GB in fcc ferromagnetic cobalt is, in absolute values, very similar to that in nickel. However, as the magnetic moment in bulk cobalt is higher, we do not observe magnetically dead layers here. It turns out that by focused impurity segregation we can generate atomically thin magnetic layers with tailored magnetization, which can contribute to a new development of technologically important materials. Further, we use ab initio methods to assess tensorial elastic properties of interface configurations associated with GBs in intermetallics. Focusing on the Σ5(210) GBs in Ni3Al compound as a case study, we evaluate the mechanical stability of the interface configurations by checking Born-Huang’s stability criteria. The elastic constant C55 is found three-/five-fold lower than in the bulk and, as a result, the mechanical stability of interface states is reduced. The tensorial elasto-chemical complexity of the interface configurations is demonstrated by a high sensitivity of elastic constants to the GB composition. In particular, we study elasticity changes induced by Si atoms segregating into the atomic layers close to the GBs and substituting Al atoms. If wisely exploited, our study paves the way towards solute-controlled design of GB-related interface configurations with tailored stability and/or tensorial properties. Most of our results are theoretical predictions and we hope that they may motivate experimentalists to conduct new investigations in this field.

Resume : Interfaces determine the overall reliability of multi-material components since they have to bear the distinct physical and chemical properties of the different adhering materials. In microelectronic applications, where several materials are implemented at small length scales, the main interest is on identifying the weakest interface, since it dictates the overall reliability of the implemented packages. The focus of the present study is set on a multi-layer stack composed of a rigid Si substrate with dielectric borophosphosilicate glass (BPSG) and a thin TiW film acting as adhesion promoter and diffusion barrier to the electro-deposited copper film, which are finally covered with 6 µm of polyimide (PI). Of main interest is the characterization of the delamination of the various interfaces, which allow for a better understanding of the adhesion and the stresses present in the complex material stack. As a first, a peel test was carried out to reveal the weakest interfaces resulting in three different delamination zones. In one zone, straight buckles formed in the Cu-TiW layer parallel to the peeling direction at the TiW-BPSG interface. Using these buckles, the evolution of the shape as a function of film thickness and layer stress was investigated using atomic force microscopy and X-ray diffraction. Of interest is that the Cu layer the buckles have a straight geometry indicating an isotropic stress. However, the buckle morphology changes to a telephone cord geometry, maintaining the outer boundaries from the previous straight buckles shape after etching away the Cu layer. The change in geometry is due to the change in film stress from isotropic to biaxial as well as the fact that the out of plane plasticity is constrained while the copper film is present.

Resume : Growing scientific interest to mechanical properties of nanoscale materials requires appropriate experimental techniques with commensurate tools for detail investigations. Such technique as Scanning X-ray nanodiffraction may shed additinal light on number of aspects as local strain fields, crystaline quality, dominating crystal orientation etc. P03 nanofocus endstation at PETRA III in DESY operated by Helmholtz Zentrum Geesthacht provides highly stabile experimental setup with high spatial resolution using a nanosized beam. It is one of only few places in the world where the experimental conditions for scanning X-ray nanodiffraction are provided and it offers a hard Xray beam with a size of only 250 x 350 nm^2. The strong focus on materials science at P03 is demonstrated by the wide range of experiments already performed with in-situ sample environment including mechanical testing with strain resolution of 10^-5. In this presentation the setup for high spatial resolution experiments at the Nanofocus Endstation of P03 beamline (PETRA III, DESY) will be presented as well as technical characteristic and examples of successful experiments.

Resume : The drop tube technique and single roller quenching technique were used to study the rapid solidification mechanism of ternary Al52.18Fe24.59Si23.23 peritectic alloy. DSC and EDS analysis show that the liquidus temperature of the alloy is 1252 K, and a ternary peritectic reaction takes place during the solidification process. The final microstructure consists of the primary 1 phase, the cell like peritectic 2 phase and the Al68Fe12Si20 phase. With the decrease of droplet diameter, the thickness of the peritectic layer decreases from 21.59 m to 7.07 m and the microhardness increases from 927.9 kg/ mm2 to 1637.5 kg/mm2 in the free falling condition. In the single-roller quenching experiment, the solidified products are flakes, particles, and metal ribbons with free-surface protrusions. With the increase of the roller speed, the τ2 phase changes from long strip to equiaxed grain, and the solidified structure of the alloy ribbon is significantly refined compared with the solidification structure of the droplets in the single roller.

Resume : The present study focuses on the surface modification of aluminium (Al) by the formation of surface composite through the mechanical insertion of graphene nanoplatelets (GNPs), which improves the surface properties of aluminium. The GNPs were successfully embedded into the Al matrix through the application of pressure onto a GNP coated aluminium substrate which are locally softened by electrically resistance heating. The degree of softening can be controlled by altering the process parameters, i.e. current and time. The microstructural characterization of GNP/Al surface composite was carried out by XRD, FESEM, Raman spectroscopy and the micromechanical property study was carried out through Vickers hardness test. The nanomechanical property study was carried out through nanoindentation test. It was observed that the depth of impregnation of the GNP particles is less than 250 µm. The microhardness is improved by more than 400%. The mechanical properties were improved by different hardening mechanisms. Tribological studies were carried out on the surface modified Al and the post analysis of wear track was carried out with SEM and Raman spectroscopy. It was observed that the wear volume was reduced drastically due to the smearing of GNP particles on the wear track and the coefficient of friction was also reduced.

Resume : Due to the various applications in microelectronics such as insulator and diffusion barrier, many researchers have reported about mechanical behavior of inorganic materials. However, they are difficult to apply to deformable electronics due to their low deformation characteristics compared to organic materials. In case of inorganic materials of large scale, obtaining required strain for stretchable device is difficult since flaws such as pinhole and grain boundary in the material act as weak spots that initiate failure and reduce strength. On the other hand, stretchability of nano-scale thick inorganic materials was improved by reduced thickness below the critical thickness. As the thickness of inorganic material decreases, the number of defect decreases stochastically, and the material reaches a high strength with the same elastic modulus as large samples. There is lack of research on mechanical properties of nano-scale inorganic materials due to challenges in sample preparation and testing process in nano-scale. In this work, nano-scale thick silicon dioxide was fabricated by sol-gel process, and stretchability of silicon dioxide was evaluated by uni-axial tensile tests. Microstructure of silicon was confirmed as amorphous phase by TEM analysis. We conducted uniaxial tensile test and cyclic loading test in SEM with Push-to-Pull device to accurately measure elastic limit of silicon dioxide.

Resume : Micro- and nano-sized structures have attracted substantial interest due to their special mechanical behaviour: they generally show an increased yield strength compared to the bulk material as well as an improved ductility. However, while metallic and silicon nanoparticles have been widely investigated since almost two decades, less is known about the strength of ceramic NPs maybe due to the brittleness of their bulk counterparts. Here we investigate the mechanical behaviour of <100>-oriented MgO nanocubes. First, incipient plasticity mechanisms are studied using MD and nanocompression tests at constant strain rate. Results show that the plastic deformation of MgO nanocubes initiates with the nucleation from the surface of perfect 1⁄2<110>{110} dislocations under ultra-high stresses, one order of magnitude larger than what is generally observed in fcc metals. Thus, using the nudged-elastic-band method, we calculate the activation energy for dislocation nucleation as a function of the stress, particle size and dislocation nucleation site. With the help of the transition state theory, the nucleation stress can then be estimated as a function of temperature and experimental strain rate. We find that due to the large stiffness of the activation energy vs stress curve, the strain rate has a much smaller influence on the nucleation stress than what is found for metallic nanoparticles.

Resume : Ceramic hard tool materials with a matrix of silicon nitride are characterized by good chemical resistance, high strength and fracture toughness and a very high hardness. The silicon based refractory possess excellent oxidation resistance up to 1700°. Si3N4-Al2O3-Y2O3 samples were prepared in two stages. In the first stage, the materials were compacted using different methods: isostatic pressing, HP (High Pressure) and Shock Compaction. In the second stage, obtained green body samples were sintered by the free sintering method. Parameters of compaction green bodies and sintering processes were optimized for density, hardness and Young’s modulus. Density od green body samples were in the range 54%-58% for isostatic compaction, 82%-86% for HP-HT compaction and for the Shock Compaction 82% of theoretical density. Vickers harnesses was only measurable for Shock Compacted green body samples and were depending on the applied load from 10.8GPa - 14 GPa. The largest change in lattice constant parameters were measured for the Y2O3 phase for the dynamic compaction method (a = 0.01% - 2.26%). The highest relative density were measured for sinters obtained by HP-HT (95%). Young's modulus for sinters obtained by HP-HT and isostatic method was 284 GPa and 236-241 GPa, respectively. The hardness of the free sintering - isostatic samples was 11.7 GPa, for the HP-HT 14.8 GPa and 17.6 GPa for the Shock Compaction samples.

Resume : Advanced high strength strip steel with bainitic-martensitic microstructure is widely used at present for the structural applications due to their high strength along with satisfactory ductility and toughness. The present study is undertaken after failure was observed in industrial high strength strip steels while performing bend tests as per standard specification. Two different industrial steel grades having different compositions were hot rolled and coiled at different coiling temperatures ranging from 450°C to 382°C. Microstructural and textural characterization was carried out by scanning electron microscopy attached with EBSD facility and transmission electron microscopy. Bend tests were conducted following ASTM E290 standard. Results showed that microstructures of the investigated steels were comprised of different constituents such as granular bainite, upper bainite, lower bainite and martensite. Factors such as superior microstructural homogeneity, high intensity and uniform distribution of gamma fiber texture (<111>//ND) along with {332}<113> texture component and high post uniform elongation played beneficial effect in terms of bending performance. On the other hand dominance of cube and Goss {110} <001> texture components, higher martensite fraction and severe microstructural heterogeneity are the factors that deteriorated the bendability. An effort has been made to establish processing-microstructure-texture-bendability correlation for the investigated steels.

Resume : Bonding of glass-ceramics is one of the challenging tasks in various technologies including especially aerospace and space technologies. ZERODUR which has a low coefficient of thermal expansion (CTE) is widely used as glass-ceramic material in opto-mechanical industry [1]. We compared three different state-of-the art bonding methods to contact Zerodur test plates. Cylindrical Zerodur samples with a diameter of 20 mm and with a length of 40 mm were used as test specimens. For comparison, dry bonding (direct bonding), chemical bonding and hydroxide catalysis bonding were applied to these specimens. In hydroxide catalysis bonding, NaOH and KOH solutions were used as bonding agent. At same concentration of both solutions, the use of NaOH led to a higher mechanical strength [2]. In chemical bonding, Si-based additives such as sodium silicate (Na2SiO3) were used and the bonding time was correlated with varying the pH value of solution [2]. Specimens prepared for all bonding types were exposed to heating-cooling cycles (-40˚C to 80 ˚C.). Subsequently, tension and shear tests were applied. Contact zones were examined by non-destructive test methods such as AFM, white light interferometry and XPS [3]. A correlation between nondestructive test methods and other destructive test methods was presented to reveal the most suitable method for high-end applications. References: [1]T. Döhring and et al, “Properties of Zerodur Mirror Blanks for Extremely Large Telescopes”, 2nd Intl. Symp. On Advanced Optical Manufacturing and Testing Technologies: Large Mirrors and Telescopes, Proc. Of SPIE Vol. 6148 61480G-1 [2]E. J. Elliffe and et al ”Hydroxide-catalysis bonding for stable opticalsystems for space,” Class. Quantum Grav. 22, S257-S267, 2005. [3]J-J. Fermé, ”Optical Contacting,” Optical System Design 2003.

Resume : Silicon nitride (Si3N4) ceramics with high density and fine, uniform grains were consolidated by a novel oscillatory pressure sintering (OPS) approach. For the first time, the oscillatory pressure was applied at three stages (initial, intermediate, and final) during sintering process of silicon nitride ceramics.Compared with the Silicon nitride by hot pressing (HP) at the same sintering temperature, the microstructure of the specimen develops in a more homogeneous manner, leading to a higher final density, a smaller average grain size, and a higher aspect ratio. The flexural strength of the OPS-fabricated specimen increased markedly; a maximum flexural strength of 1291.7 MPa can be achieved at the sintering temperature of 1850 °C. The enhancement of flexural strength of the OPS-fabricated specimen can be attributed to the oscillatory pressure can facilitates grain boundary sliding and diffusional creep and accelerates the removal of isolated pores in the intermediate and final stage of sintering. It was suggested that OPS was an effective approach to fabricate silicon nitride ceramics with ultrahigh density and high strength.

Resume : One of the most important problems of science and technology is to ensure the durability and reliability of the machine parts and tools that works in difficult conditions. The direction of solving this problem is hardening of the surface layer of products. A promising method of creating of coatings, which operated in extreme conditions, is electric-spark alloying. In this method by means of concentrated flows of electric energy, surface layer chemical composition, structure-phase state and properties is being modified. The aim of this work is to investigate the influence of sequence of electric-spark alloying by titanium and chromium on the structure, phase composition, microhardness, and wear resistance of surface layers of the steel Art.3. Discovered, that with changing the sequence of alloying with titanium and chromium leads to surface micro hardness changes. For instance, if the chromium is coated before titanium the micro hardness increases to 6,6 GPa; if it is coated on the contrary the micro hardness increases to 10 GPа due to the formation of the following compounds: Cr7C3 carbide, Cr2Ti, Fe2Ti intermetallics, electrode-based solid solution. It was found that after ESA in the Cr-Ti sequence, the resistance to the wearing of the steel Art.3 surface is increased by 25 times, and in the Ti-Cr sequence – by19 times compared to the sample that was not processed.

Resume : Nowadays there is a growing request on composite materials because they are able to combine, in one compound, different properties. In many applications, such as for example vacuum technologies or aerospace engineering, it is often necessary to join ceramics and metals together and keeping high stability. Ceramics and metals are not easily joined because of their expansion coefficient. So, to avoid this problem metal-ceramic brazing is applied. This consists in depositing thin film metals on ceramics, diffuse them by annealing in order to be able to solder the metallic film to the metals. Usually to metallize ceramics, moly-manganese plating methods are used. In this work a new technique to metallize ceramics using different thin-film methods is presented. From a Al2O3 ceramic substrate different thin film layers of Ti, Mo and Ni are deposited with different thicknesses. Usually metals deposited in thin-films show weak mechanical properties as tensile strength and hardness. To enhance these properties an additional layer of Ti is added to the standard configuration between ceramic and molybdenum. In this way titanium layer enhances the molybdenum diffusion in alumina and better mechanicals properties on the stack are obtained. Samples will be characterized by atomic force microscopy, scanning electron microscopy and x-ray diffraction and finally their physical and mechanical properties will be shown.

Resume : In photovoltaic, the cost reduction is currently the main factor to improve the competitiveness of solar cell systems. Indeed, in silicon-based solar cells, their cost strongly depends on the silicon consumption which represents up to 60% of the total solar cell cost. Stress-induced spalling process (SIS) is one of the easiest kerf-free techniques developed for the detachment of thin silicon layers using a thermo-mechanical stress between the silicon and stressor layer. However, the detached wafer thickness is inhomogeneous and difficult to control. In this work, using the (SIS) process, we could show that the thickness of detached silicon foils can be controlled by choosing the most appropriated: stressor and glue thicknesses as well as stressor nature. Silicon foils with different thicknesses (48-165µm) were successfully obtained. To investigate the detachment mechanism of the silicon foils, we studied the process by analytical and numerical modelling using principles of the fracture mechanics to predict the conditions required for silicon foils detachment with desired thicknesses. The detached depths of silicon foils are in agreement with the calculated depths by modelling. Moreover, the residual stress of the foils after exfoliation was characterized using Raman spectroscopy.

Resume : While bulk silicon is brittle at temperatures below 600-700K, the compression of nanopillars has shown that a decrease of the diameter below few hundreds of nanometers could change the silicon behavior from brittle to ductile [1]. As silicon is free of initial defect, this size effect should be controlled by the cracks and/or the dislocations nucleation from the surface. The identification of the parameters governing the brittle to ductile transition in size and the understanding of the mechanisms are the key points to prevent the failure of microelectronic components based on the silicon strained technology. Nowadays the respective improvements in simulations and experiments allow to investigate the mechanical properties of objects of similar sizes, close to hundreds of nanometers. We considered atomistic simulations based on two different semi-empirical potentials fitted to better reproduce the ductile and brittle properties of bulk silicon. The simulations under tensile load show the nucleation of perfect dislocations from the surface that can lead to cavity opening when they interact [2]. Second, we show that the brittle to ductile transition is not abrupt and mainly controlled by the diameter of the nanopillars, as observed experimentally in compression. The underlying mechanisms will be detailed, and a criterion based on the quantification of the plastic deformation before cracks opening will be presented to measure the degree of ductility of the breaking. [1] F. Oestlund et al. AFM 19 (2009) 1 [2] F. Abed El Nabi et al MSMSE 23 (2015) 025010

Resume : Nine stacked sheets of Fe-48%Ni alloy were severely deformed by equal channel angular pressing (ECAP) with two inner angles of φ = 90º and 110° at room temperature following route A up to two passes. The impact of die channel angle on the evolution of microstructure, texture and mechanical properties along the normal direction were investigated using electron backscattered diffraction, X-ray diffraction and microhardness. That’s why, the initial state, peripherals (in the top and the bottom) and central sheets of the stack were characterized after each ECAP pass. In the present study large deformations were achieved in the matrix with smaller angle after 2 ECAP passes and exceptionally in the upper sheet of the stack. During the first ECAP pass, the microstructure developed a large fraction of low angle boundaries associated with subgrain formation. A heterogeneous microstructure and texture were obtained in different samples in each pass. After 1 pass, for both φ =90 and 110°, we principally got rolling texture. Shear texture was obtained after 2 passes and only with channel angle of 90°. Significant increase in hardness was observed from the first pass but results are lower using the die with φ = 110º after 2 passes.

Resume : In the last years the mechanical properties of Nb-Zr alloys have proven both scientifically unique and of potential practical interest. Nb–Zr alloys has been studied in many environments, from nuclear reactors (due to their small capture cross-section for thermal neutrons, their relatively good high-temperature strength, resistance to corrosion and radiation tolerance) [1, 2], to orthopedic applications [3]. Today, chemical vapor deposition and sputtering techniques are widely used to produce layered Nb-Zr materials and coatings. Generally speaking, the stability of layered alloys, sometimes amorphous, is depended on the microstructure, however, the underlying deformation physics of these materials remains less firmly established [4]. In that regard Molecular Dynamics (MD) simulations are playing today an important role in order to understand the various mechanisms and processes that determine and govern the crystal structure and different physical properties of all kind of materials. In this work a three-dimensional MD model has been developed and used to simulate the growth of Nb-Zr multi-layers of different concentrations. Our results describes the structural properties and phase stability changes of Nb–Zr under different scenarios. Further, our simulations show the time evolution of different physical properties such as roughness, asperity, volume, as well as the the crystal structure (a mix of bcc or fcc and amorphous phases), vacancies, and their dependence with several parameters like temperature, rate of deposition or the velocity of the deposited atoms. For instance, an optimum deposition rate and velocity combination can be found to maximize the bcc or fcc phase for a given concentration, or to minimize the roughness of the surface. At the same time we have studied the deformation mechanism under nano-indentation as a function of layer thickness and amorphous/crystalline thickness ratio. References: [1] E. Tenckhoff, in: Deformation Mechanisms, Texture, and Anisotropy in Zirconium and Zircaloy, ASTM Special Technical Publication (STP 966), 1916 Race Street, Philadelphia, 1988. [2] M. Callisti, S. Lozano-Perez, T. Polcar, Structural and mechanical properties of gamma-irradiated Zr/Nb multilayer nanocomposites, Mater. Lett. 163 (2016) 138e141. [3] Rajarshi Banerjee Soumya Nag John Stechschulte Hamish L.Fraser. Strengthening mechanisms in Ti–Nb–Zr–Ta and Ti–Mo–Zr–Fe orthopaedic alloys. Biomaterials. Vol 25, 17, (2004), 3413-3419 [4] E. Frutos, M. Callisti, M. Karlik, T. Polcar. Length-scale-dependent mechanical behaviour of Zr/Nb multilayers as a function of individual layer thickness. Materials Science & Engineering A 632 (2015) 137–146

Resume : Among the physical phenomena under which polycrystalline microstructures evolve, grain boundary migration (GBm) remains poorly understood, due to the variety of possible configurations (i.e. GB type and crystalline orientation) and influencing parameters (temperature and plastic strain). We detail our recent efforts to improve the understanding of GBm from multiscale experiments and simulations. Taking pure Al and Cu as FCC model metals, we propose a methodology to correlate GBm to local geometry, orientation and plastic activity. Samples are heated in-situ in a SEM, and crystalline orientations are determined through EBSD cartography before and after key steps. We employed active contours post-processing to define local GB curvature which is one of the motive force to GBm. A second one is the stored energy associated to dislocation microstructures formed during prestrain. With also topological measurements performed by AFM we quantified plastic activity within grains. The migration of hundreds of GBs has been followed via this protocol. Connections are drawn with simulations: developed tools to generate and analyze the atomic configurations of arbitrary CSL GBs, with specific attention to mixed twist + tilt GBs, al-lowed investigating the atomic details of many GBm cases using Molecular Dynamics. The concept of stored energy associated to dislocation microstructures was revisited using Dislocation Dynamics. At macroscale, data are implemented in a crystal plasticity model.

Resume : Dry rolling/sliding wear behaviour of two newly designed low carbon, continuously cooled, carbide-free bainitic rail steels has been studied and compared with pearlitic rail steel. Indentation damage from deformation of asperities, smearing of generated hard wear debris with the interfaces and delamination due to coalescence of rolling contact fatigue (RCF) cracks has been found to be main material removal process during wear. Carbide-free bainitic rail steels showed considerably better wear resistance than pearlitic rail steel because of their higher initial hardness, lesser number of crack initiation sites and better RCF damage resistance. Detailed microstructural characterization of the steels undergoing wear were done and analyzed. TRIP effect coming from stress induced transformation of retained austenite into martensite has been studied carefully. Effect of prior austenite grain sizes and bainitic ferrite lath thickness on the wear behaviour of carbide-free bainitic steels has been investigated.

Resume : We have probed melting of nano confined organic compounds using a differential scanning calorimeter (DSC). In agreement with the Gibbs-Thomson predictions, the melting temperature of the confined materials decreased as the size of the confinement decreased, and a linear relationship was observed between the change in melting temperature and the inverse of the physical size of the confinement. In contradiction to the assumptions in the Gibbs-Thomson, however, the heat of fusion changed with the physical size of the materials. We demonstrate that the change in the apparent heat of fusion can be explained by the presence of an amorphous phase at the interfaces and in between grain boundaries.

Resume : Smart power MOSFETs are now found in many fields of automotive, domotics or energy conversion. These devices, that can operate in extreme conditions (overload or short circuit mode), need to dissipate important energy levels and experience fast temperature gradients. Those gradients can quickly lead to failure [1-3] and previous studies have shown that the metallic parts of the power components, such as the source top metallization and the bond wire, are the location where failure is mainly prone to happen [4-7]. This is expected because they possess the lower yield stress. However, their aging mode is significantly different from bulk metals. This is in part due to their thin film configuration, but also to the mode of stress that include high temperature compression and lower temperature tension. We focused here on the top aluminum metallization of NXP® low-voltage power devices. Its microstructure is characterized before and after aging through ion microscopy, electron microscopy and grain structure mapping. We have investigated areas away and under the wire bondings that are cold welded prior to aging. In the former area the metallization undergoes an apparent grain shrinkage and cracks run perpendicularly to the surface down to the substrate, following the grain boundaries (GBs), due to enhanced self-diffusion of Al atoms along the boundaries. In the latter area a strong grain reduction has been observed, due to the initial plastic deformation; cracks propagate along the wire-metallization interface under the bonding because of the presence of Al oxide [8]. The change in the Al grain microstructure at the wire–metallization interface is systematically investigated: from statistical analysis, in-situ characterization and 3D visualization techniques we reached the conclusion that crack opening along the GBs occurred because of accelerated diffusion followed by local oxidation. The physical phenomena behind the Al metallization degradation is therefore specific and would now require modeling to predict its evolution. [1] Glavanovics M., Kock H., Kosel V., Smorodin T. Microelectron Reliab, 2007, 47, 1790–4 [2] Khong B. et al., Microelectron Reliab 2005, 45(9–11), 1717 [3] Bernoux B., Escoffier R., Jalbaud P., Dorkel J.M., Microelectron Reliab, 2009, 49, 1341–5 [4] Caccuri V., Milhet X., Gadaud P., Bertheau D., Gerland M.J., Electron Mater, 2014, 43, 4510–4 [5] LeHenaff F., Azzopardi S., Deletage J.Y., Woirgard E., Bontemps S., Joguet J., Microelectron Reliab, 2012, 52, 2321–5 [6] Irace A., Breglio G., Spirito P., Letor R., Russo S., Microelectron Reliab, 2005, 45, 1706. [7] Glavanovics M., Detzel T., Weber K., Proceedings of the 34th European Solid-State Device Research Conference, 2004, 273–6 [8] Ruffilli R., Berkani M., Dupuy P., Lefebvre S., Weber Y., Legros M., Microelectron Reliab, 2015, 55 (9-10), 1966-1970

Resume : Nanocomposites have attracted attention over the past decades because of their promising technological applications and exceptional properties [1]. Among these composites, the Cu-Ni nanocomposites which have been used as a magnetic microactuator [2] and also as a coating [3]. In the present work, we investigate the effect of spherical nickel inclusion on mechanical properties of Cu-Ni nanocomposite during nanoindentation process by means of Molecular Dynamics Simulations, based on the embedded atomic method (EAM). We investigate how Ni inclusion influences the copper nanoindentation. The results of the simulation show that the presence of nickel inclusion influenced the strength of the material. We found that the hardness of the Cu matrix containing Ni inclusion is greater than that of pure Cu. In addition, the hardness of the Cu-Ni nanocomposite increases when the depth of Ni inclusion, under the indenter, decreases. This result is in good agreement with previous work [4,5]. Keywords : Nanoindentation, MD simulation, Inclusion, Nanocomposite, Hardness. References: [1] J. Parameswaranpillai,'Nanocomposite Materials: Synthesis, Properties and Applications', Taylor & Francis, 324 p, .2016. [2] Y. W. Huang et al., Appl. Phys. Letter., 90, 244105, 2007͒. [3] E. Branningand & A. F. Jaukowski, Journal of Mater. Sci. Res., vol. 2, n°2, 2013. [4] T. Fu et al., Scientific Reports; 6:35665; doi: 10.1038/srep35665, 2016. [5] E. Kulej et al., Cent. Eur. J. Phys., 9(6) , P 1421-1425, 2011.

Resume : In the present study, stress relief annealing process for automotive springs is experimentally investigated in the experiment electric current periodically applied to a full size commercial automotive spring. As confirmed by the experiment result the EA stress relief annealing is clearly more effective than the conventional process using a furnace, even with a significantly shorter process time and less energy. The result of XRD analysis confirms the occurrence of electric current induced annealing during EA stress relief annealing.

Resume : Many applications in consumer electronics, microfluidic and sensing devices or healthcare for instance, are evolving towards miniaturization and flexibility in design. To fulfil the material requirements, polymer grades need to comply with specifications for injection moulding of small parts with extremely thin walls, while retaining other properties, in particular mechanical properties, aesthetics and chemical resistance. ValoxTM resin is a SABIC grade consisting of Poly-Butylene-Terephtalate (PBT), a semi-crystalline thermoplastic material. Upon standard injection moulding conditions, it presents a certain degree of crystallinity, providing chemical resistance and mechanical stability. When manufacturing small parts, the melt is subjected to faster cooling, influencing the crystallization behaviour and resulting in dimension stability and shrinkage challenges. In order to improve the performance of the material, control over the crystallization in different moulding conditions is desirable. The crystallization mechanisms and kinetics are investigated by Differential Scanning Calorimetry (DSC) and in-situ birefringence microscopy. The influence of different nucleating agents has been assessed, demonstrating that the crystallization mechanisms and the resulting morphology on the micro-scale have a strong effect on the appearance (colour) and mechanical properties as characterized by the Heat Deflection Temperature (HDT) and the Vicat Softening Temperature of the samples.

Resume : Increasing the density of interfaces is an option that was largely employed in order to improve the mechanical properties of fcc metals. For example, nanotwinned fcc metals exhibit enhanced yield stresses while keeping a good ductility with larger strains-to-fracture. Interesting mechanical properties are also encountered in nanolayered materials in which heterophase interfaces were found to be responsible for a strengthening effect. In this context, we investigated the role played by interfaces on the formation and extension of deformation twins. Molecular dynamics simulations were performed for Al, Ag and Cu in presence of a preexisting twin boundary (TB) and in Cu/Ag multilayered systems. Thin films were considered with atomic surface steps acting as dislocation sources and with a crystal orientation such that mechanical twinning is always promoted. Our results suggest that the size distribution of twins significantly depends on the structure of the considered interface. For TB or Cu/Ag heterotwin-type interface, deformation twins are smaller and more numerous. There, the nucleation of Lomer dislocations from the interface plays a crucial role in the formation of new twins. On the contrary, the Cu/Ag cube-on-cube interface is a weaker obstacle for the transmission of partial dislocations. But it appears that twin thickening is correlated to the glide of misfit dislocations and that the final twins thickness might depend on the distance between misfit dislocations.

Resume : The fracture toughness of ceramics is often dominated by the structure of their grain boundaries. Our capacity to improve the performance of ceramic components depends on our ability to investigate the properties of individual grain boundaries. This requires development of new fracture testing methods providing high accuracy and high spatial resolution. Recently, several techniques have been developed using small scaled mechanical testing, based within a nanoindenter, using a variety of tip and sample geometries, including: micropillar compression, microcantilever bending and double-cantilever compression. However, the majority of the published work relies on load-displacement curves for the identification of crack initiation and the geometries can result in a complex analysis of force distribution and stress intensity factor. Our approach uses a double cantilever geometry to obtain stable crack growth and we calculate the fracture energy under a constant wedging displacement. The tests are carried out within an SEM, this has two benefits: the sample is well aligned for a controlled test and images are recorded during the test for later analysis. Crucially this allows us to use beam deflection and crack length rather than critical load to measure fracture toughness. Our tests have proved it is possible to initiate and stably grow a crack in a controlled manner in ceramic materials for several microns. This approach has been validated on SiC and Diamond where it gives a good approximation of the surface energy and then extended to SiC bi-crystals along with Ni-Al2O3 interfaces where crack blunting and bridging mechanism can be observed and measured.

Resume : During manufacturing, handling and testing of printed circuit board assemblies, mechanical stress can result in small cracks in multilayer ceramic capacitors (MLCCs). Examining the influence of ceramic grain structure and interfaces on damage and fractures in these MLCCs is important in order to understand and mitigate their failure mechanisms. Micro-cracking due to excessive bending has been examined by means of mechanical bending experiments up to 6000uStr in different bending orientations (90°, 45°, and 0°), and characterization methods using X-rays to confirm the presence of cracks have been implemented. Structural examination methods of Scanning Transmission Electron Microscopy (STEM) and Energy Dispersive X-rays (EDX) was used to link the influence of interfaces and grain boundaries on the damage and fracture of the different MLCC ceramic structures. It was found that the shell diffusion sector and their dopants surrounding the BaTiO3 containing core in the ceramic grains had an influence not only on the breakdown voltage of the capacitors, but also played a role in the flex cracking behavior. Over 300 1812-sized 25V 22uF standard and flex end termination MLCCs from different commercial suppliers were examined electrically, mechanically, and with X-rays. The results were correlated with the structural analysis of the different ceramic core and shell diffusion layers of the grains, as well as dopants at the grain boundaries and distributions of grain sizes.

Resume : Magnesium oxide is an important ceramic material, used as a substrate, coating material, or for its refractory properties. It may also form under natural conditions, and iron-enriched MgO is known to be an important mineral phase belonging to the Earth's lower mantle, where it deforms at high pressure and high temperature. The mechanisms for its deformation are expected to depend strongly on the conditions of pressure, temperature, and strain rate. Among the defects that contribute to deformation, grain boundaries are known to be very sensitive to pressure. We use systematic atomic-scale simulations to investigate the properties of over 200 symmetric tilt grain boundaries. Their atomic structure is analyzed in details, revealing several structural transitions with increasing pressure. The evolution of formation enthalpies and excess volumes with pressure is quantified, providing insight into the mechanisms for grain growth, recrystallization, and segregation.

Resume : Functional graded composites are the materials with structures and properties changed gradually, which is beneficial to improve strength and radiation resistance at high temperatures. In this work, 0.33 wt. % and 1 wt. % SiC or Si3N4 ceramic nanoparticles were mixed in 316L austenitic steel matrix. The high efficient attritor milling provides size reduction of the 316L steel grains and homogeneous distribution of the ceramic particles before sintering process. Spark plasma sintering (SPS) was used for compaction of milled composites. The effect of the ceramic addition on the milling efficiency and the structure of the composites have been studied. It was found that the amount of ceramic addition influenced the efficiency of milling process resulting in powder mixtures with different 316L stainless steel grain size and shapes. The intensive milling assured an optimal coverage of 316L stainless steel grains with ceramic submicron sized particles in both cases. The milling efficiency decreased with the increase of the ceramic amount, which resulted in the presence of different morphologies in the mixture. The sintered composites showed high densities with the presence of small amount of closed porosities. A comparison of the structure, mechanical and tribological properties of 316L/SiC and 316L/ Si3N4 composites is presented in this work.

Resume : The structural integrity of functional thin-film multilayers is often determined by the strength of the constituent metal/oxide interfaces. Controlling de-adhesion at these interfaces is crucial to maintaining the performance of the system. An important example is the Ag (111) / ZnO (0001) interface frequently found in solar coatings and in this study we show, using first principles modelling, how suitable doping of the oxide can control its strength. Donor doping of ZnO is known to improve the conductivity of this oxide but a recent computational study has also shown that it can inhibit Ag inter-diffusion, a process which may contribute to the interfacial de-adhesion. In this work we systematically study the impact of 12 different Zn substitutional dopants, both aliovalent and isovalent, on the work of separation of the Ag/ZnO interface. Interfacial bond strengths are also determined and correlated with the works of separation. It is found that the interfacial strength is highly dependent of the valence of the dopant. Monovalent dopants adjacent to the interface strengthen it while dopants with valence greater than two weaken it. Isovalent dopants have little effect. The results are understood in terms of charge suppression and donation at the interface as well as atomic size effects. It is concluded that enhancing interfacial strength while at the same time inhibiting Ag diffusion may not be possible or at least require a careful choice dopant types and concentrations.

Resume : Atomic layer deposition (ALD) method is commonly used to deposit oxide, nitride and sulfur layers for selected industrial applications. Currently, a big potential of the ALD method in photovoltaic (PV) applications is demonstrated for aluminum oxide (Al2O3), zinc oxide (ZnO) and aluminum doped zinc oxide (AZO) thin films. ALD-grown ZnO and AZO layers are cheap alternatives to expensive ITO (Indium Tin Oxide). Moreover, passivated layers based on Al2O3 (ALD-grown) are already applied in the production of crystalline silicon PV structures. The present results indicate a good adhesion of ZnO films to substrates. Even with a load of 10 N these films maintain a resistance to wear of up to 500 cycles during sliding against alumina ball with 6 mm of diameter [1]. Furthermore, other ALD films are used as so-called high-k materials in electronics. Therefore, in addition to ZnO, AZO and Al2O3, we studied tribological properties of hafnium dioxide, which is dielectric materials for gate oxides of MOS devices. At a load of 10 N, the thin film of HfO2 revealed resistance to wear above >300 cycles and very low friction coefficient at the level of 0.12. These are very advantageous properties. This work was partially supported by the National Science Center (Decision Nos. DEC-2012/06/A/ST7/00398), and AGH 11.11.130.174, AGH 11.11.230.017. [1] R. Pietruszka, B.S. Witkowski, S. Zimowski, T. Stapinski, M. Godlewski, Surface & Coatings Technology 319 (2017) 164-169.

Resume : Metallic glasses have attracted significant engineering material due to superior strength and high elastic limit, but they lack tensile ductility that result in sudden and show catastrophic failure. Substantial work has been investigated to improve the tensile ductility of metallic glasses. Two basic techniques are employed to attain high tensile ductility: inducing generation of local shear banding around the secondary phase by introducing a softer secondary phase in the metallic glass matrix and suppressing a propagation of shear bands by reducing its external dimensions, leading to enhanced strength and ductility. To investigate both approaches, we have developed metallic glass-graphene nanolaminate structures that utilize advantages of graphene, including 2-dimensionality, high modulus, and high strength. In this research, nanolaminate samples are fabricated with alternating layers of metallic glass and graphene. Metallic glass is deposited on a Si substrate using RF magnetron co-sputtering and CVD-graphene synthesized on copper foil, is transferred on metallic glass layer. Tensile samples are fabricated by undercutting the Si substrate and patterning to dog-bone-shaped using FIB. In-situ tensile testing reveal that addition of graphene in the nanolaminate improve the modulus and yield strength by 9.6% and 14%, respectively and metallic glass film in the nanolaminate show explicit homogeneous flow, leading to improved tensile ductility of the nanolaminate.

Resume : Metallic glasses are known for their high yield strength and resilience. Their most severe shortcoming, however, remains the brittle failure mechanism at room temperature due to strain localization in a dominant shear band. Different strategies have been pursued to enforce a more homogeneous deformation behavior, either through the introduction of crystalline secondary phases [1], by rejuvenation through thermal cycling or by realizing fully amorphous nanoglass architectures [2,3]. Nanoglasses consist of glassy grains connected by glass-glass interfaces, that exhibit a different density and composition than the core of the glassy grains [3,4]. While internal interfaces in nanoglasses help to prevent brittle failure, they are usually not beneficial to the overall strength of the nanoglass. In this contribution, we present molecular dynamics simulations of conventional and reinforced nanoglasses. We manipulate the glass-glass interfaces of a Cu-Zr nanoglass, such that they are replaced by stronger crystalline interphases. Analogous to grain boundary strengthening in crystalline materials, we show that it is possible to reinforce the nanoglass without compromising its ductility [5]. [1] C. Kalcher et al., Acta Mater. 141 (2017) 251-260. [2] J.X. Fang et al., Nano Lett. 12 (2012) 458-463. [3] O. Adjaoud et al., Acta Mater. 145 (2018) 322-330. [4] O. Adjaoud et al., Acta Mater. 113 (2016) 284-292 [5] C. Kalcher et al., Scr. Mater. 141 (2017) 115-119.

Resume : Multiphase polymer blends with nano-scale impact modifiers and TiO2 pigment offer unique properties, including improved chemical resistance, flame-retardancy and impact toughness. To accelerate industrial material development and further expand applications, it is crucial to build an in-depth understanding of the toughening mechanisms for complex multiphase polymer systems. Here we present a method that combines AFM nano-mechanical mapping and fracture mechanics to investigate the mechanisms of toughening in a multiphase polycarbonate-based polymer blend. Single edge notch beams were tested (ASTMD6068) to measure the crack propagation resistance (J-R curve). In selected samples, a crack was arrested after 2-3 mm extension. Modulus mapping images were produced in the vicinity of the arrested crack tip using AFM nano-mechanical mapping. By quantitatively comparing the morphology between cracked region and reference region, the deformation and interface changes from nano-size impact modifier and TiO2 pigment were investigated and correlated to toughness. Interface changes between polymer phases during crack growth were recognized by modulus mapping, suggesting a key role of the polymer interface in toughening. This work highlights using AFM nano-mechanical mapping to obtain quantitative morphology and modulus changes at interfaces during crack growth, which provides new insights into building an in-depth understanding on toughening mechanisms in complex multiphase polymers.

Resume : Automation, intellectual, and complexity are in the mainstream of modern machinery development theme along with the light-weight and high-quality designs. This constant upgrading trend makes the application of aluminum alloy and plastic material in high demand. A wide variety of applications including many types of transportation vehicles and wind turbines need such aluminum alloy and plastic joining techniques. However, cyclic vibration during the operation may bring a big challenge to such dissimilar joining functions. The aluminum alloy 5052 and the plastic acrylonitrile butadiene styrene (ABS) were adopted as the experimental materials for this study. Both were first processed into 120 mm × 40 mm × 3 mm sheets before welding, with a joint area of 30 mm × 30 mm, after which they were joined in an overlapping mode. The micro drilling array was used for the 5052 aluminum alloy surface-roughening treatment, followed by adding the ABS powder to the specimen. The plastic powder could be used as an interface coupling to the welding process. It could greatly improve the joining structure and enhance the joining strength. The cantilever beam vibration system was used to study the vibration behavior during the actual operation. On the other hand, the finite element software could effectively evaluate the mode of vibration from the ABS and aluminum alloy welding compartments. This simulation can support the understanding and prediction of breaking behavior on the specimen under different vibration conditions. Furthermore, the laser Doppler vibrometer (LDV) has been used to measure the mode of vibration on the specimens. The acceleration-vibration curve will be calculated from vibration force of the specimen on the electromagnetic control vibration platform at resonant state. In this study, the vibration fatigue life of a sample can be defined as the number of cycles at the beginning of the descending deflection amplitude. The experimental results show that the vibration fatigue life of the specimen can be improved from 507 cycles to 46270 cycles when the ABS powders were added to the interface of joint, which have a 1.5 mm depth on the micro hole array and welding time of 3.5 s. It can be understood that the increase in depth of hole and welding time can improve the vibration fatigue life of the welding specimen. This improved approach can produce the high quality plastic-aluminum joining parts. Keywords: vibration fatigue life, aluminum alloy, ABS polymer, ultrasonic lap welding, dissimilar joint, cantilever beam vibration system

Resume : Polymer composites are important materials for the development of products for high performance applications, lightweight constructions and smart textiles. The strength, fatigue and fracture resistance of the composites depend on the interfacial interactions between the polymers. Commonly, these factors are investigated by shear testing under static conditions. However, more information about the interface could be obtained by using methods having dynamic conditions. Dynamic mechanical thermal analysis (DMTA) and rheological measurements were performed to investigate the temperature-dependent flow behavior of polyamide (PA) laminated with polypropylene (PP) or modified PP. The results derived from DMTA showed that the flow behavior of the laminate depends on the type of PP used. Additionally, the mechanical strength correlates to the temperature at break of the bond between PA and PP. Moreover, the rheological measurements support these findings. This study indicates that DMTA is a technique to achieve more information of polymeric interfaces.

Resume : Ultra high molecular weight polyethylene（UHMWPE） fiber is a main material in coating layer of confused detonating fuse(CDF). In order to investigate its impact-resistance strength after high temperature process, microstructure of UHMWPE heated at 100℃、120℃、140℃、150℃ and detonating characteristics of CDF heated at 120℃、140℃、150℃ were carried out using SEM、DSC and detonating test of CDF (charged with HNS-II). And the SEM images show the smooth surface and multi-layer structure of untreated UHMWPE, while a few micro-pits were formed on the surface of fibers heating at 100℃、120℃. Moreover, sectional microstructure was still compactness heating at 100℃, but grain shape presented heating at 120℃. The fibers were melted mostly heating at 140℃ and 150℃, after separating the fiber heating at 140℃, multi-layer structure on the section was still existed, which disappeared heating at 150℃ and the molten material recrystallizes epitaxially after cooling。And DSC curves suggest two endothermic peaks heating at 100℃、120℃ and 140℃, which were the same with that unheated. However, there was only one endothermic peak heating at 150℃, and the peak was just 133℃, it is lower than the formers. The results of CDF detonating test indicate UHMWPE coating layers were still complete after heating at 120℃ and 140℃, but damage after heating at 150 ℃。Above all, for CDF charged with HNS-II, the impact-resistance strength of UHMWPE is still satisfied when the temperature is not more than 140℃。