Small scale mechanical behaviour of interfaces: bridging experimental and computational modelling methods

Resume : In next-generation high-strength/high-toughness steels, the austenite/martensite (fcc/bcc) interface is the dominant microstructural feature controlling important properties. In spite of decades of research, the fundamental structure and mechanism of motion of this interface have remained uncertain. Here, an atomistic fcc-bcc iron interface is constructed that has a structure that completely matches experimental observations. The interface reveals a defect structure differing from longstanding theory assumptions and violating conditions believed essential for a glissile interface. Based on the atomistically-observed interface defect structure, we (i) revise the conditions for achieving a glissile interface, (ii) revise the crystallographic double-shear theory of lath martensites, and (iii) develop a mesoscale model for lath martensite plasticity that accounts for retained austenite. The new parameter-free theory provides predictions in near-perfect agreement with both simulations and experiments and thus allows for guided design of materials with larger fcc to bcc transformation strain and, hence, higher toughening. The mesoscale model demonstrates how the nanoscale insights translate into understanding of macroscopic plasticity in these interface-controlled steels. The adroit application of atomistic modeling tools thus expands the possibilities for design and control of mesoscale architecture in high-performance steels.

Resume : Transformation induced plasticity is an essential feature of the class of martensitically transforming steels and its analysis is of great interest to material researchers as well as to industry. The commercial steel grade MarvalX12 - a maraging steel - has been chosen as a reference for characterizing the TRIP behavior. A meso-scale representative volume element (RVE) based on earlier work [1] has been set up and discretized where each element represents an initially austenitic domain that is subjected to martensitic transformation simulated by an instantaneous toggling of the element's properties. The evolution of the phase fraction rate, which is a function of temperature, stress and strain, determines the sequence of elements to be transformed. The transformation hardening relation determines the shape of the kinetics. The presented model is capable of reproducing the experimentally determined behavior of MarvalX12 for a certain range of loading cases including non-proportional loading paths. It can be used also to calibrate the macroscopic mean-field model developed in [1]. A closer look at a smaller scale is required to understand the material behavior within the austenitic grain just before martensitic transformation. To this end, another RVE has been developed on the micro-scale representing a sub-domain of a grain, where crystallographically different martensitic variants are nucleating and growing, thereby forming the block- and packet-structure, which can be observed using electron backscatter diffraction (EBSD). This model allows to investigate TRIP effects on the micro level, where the interaction of martensite variant formation with crystal plasticity comes into play.   [1] M. Fischlschweiger, G. Cailletaud and T. Antretter, International Journal of Plasticity, pp. 53-71, (2012)

Resume : We use a combination of atom-swap Monte-Carlo (MC) and molecular dynamics (MD) to study the equilibrium structure and mechanical properties of Cu(1-x)Ag(x)|Ni multilayers with 6nm layer width. We find that Cu|Ni multilayers form a semi-coherent interface with a network of partial dislocations arranged in a regular triangular pattern. Ag is then alloyed to the Cu layers in order to tune the lattice misfit in a controlled manner. A combination of MC and MD was used to equilibrate Cu(1-x)Ag(x)|Ni with x=0%, 5% and 10% towards their thermodynamic equilibrium. We find segregation of Ag within the Cu layers at 300K and 600K and alloying of Cu and Ni at 600K in good agreement with the experimental binary phase diagrams of the Cu-Ag and Cu-Ni systems. The resulting structures were then sheared parallel and perpendicular to the normal of the bilayer interface. We generally find initial sliding at the Cu|Ni interface within the misfit dislocation network, followed by emission of partial dislocations into the Cu layer, and finally an increase of flow stress with increasing Ag content. Additional calculations of biaxial tension that suppress sliding at the heterointerface show a similar trend. Hardening can be traced back to the formation of sessile stacking-fault tetrahedra whose density increases with the density of the interfacial misfit dislocation pattern that is controlled by the amount of Ag in the Cu layer.

Resume : A microelectronic component incorporates a variety of materials with very different properties. Following the trend of ongoing miniaturization, single films of such components have typically a thickness between a few nanometers and a micrometer. It is an important fact that materials arranged in a stack or composite behave different to homogeneous materials. Reasons for this behavior are sharp interfaces between the materials and a non-uniform residual stress state, which arises during film deposition. It is clear that physically correct predictions about the reliability of such components are a very demanding task. In this work, we present an important step into this direction. We are investigating the influence of interfaces and locally resolved residual stresses on the crack driving force for a crack extending through stacks consisting of tungsten (W) and copper (Cu) layers deposited on a silicon (Si) wafer. To calculate the crack driving forces in the material systems, we use the concept of configurational forces. With this approach, we are able to determine the impact of each material inhomogeneity and of the residual stresses on the crack driving force. The analysis shows that there are critical positions in each stack where the crack driving force is high or low, i.e. where crack growth can occur at low or high loads, respectively. Certain stack arrangements are preferable to others. For example, the introduction of a 500 nm Cu-layer between two W-layers decreases the crack driving force and reduces the propensity to fracture, as well as compressive residual stresses in the W-layers. The presented procedure is very helpful for the design of future thin film microelectronic components with high reliability and damage tolerance.

Resume : Nano-sized tubular structures have attracted considerable interest because they make possible the study of fundamental physical properties. In particular, the semiconductor zinc oxide (ZnO) has been widely studied due to its thermal stability, high mechanical strength and electron mobility, and wide application range. The high strength-to-weight ratio of interconnected tubular networks makes this a creative design strategy for reducing the linear decrease in strength and stiffness of low-density materials with increasing porosity. So we fabricate nanotubular ZnO with wall thickness of 45, 92, 123 nm using nanoporous gold (np-Au) with ligament diameter at necks of 1.43 μm as sacrificial template. Through micro-tensile and micro-compressive testing of nanotubular ZnO structures, we find that the exponent m in σ ̅∝ρ ̅^m, where σ ̅ is the relative strength and ρ ̅ is the relative density, for tension is 1.09 and for compression is 0.63. Both exponents are lower than the value of 1.5 in the Gibson-Ashby model that describes the relation between relative strength and relative density where the strength of constituent material is independent of external size, which indicates that strength of constituent ZnO increases as wall thickness decreases. We discuss that this wall-thickness-dependent strength of nanotubular ZnO is caused by strengthening of constituent ZnO by size reduction at the nanoscale or by the structure effect.

Resume : A novel oscillatory pressure-assisted sintering (OPS) process is reported to fabricate high-quality nanostructured ceramics. Compared with the nano-zirconia ceramics prepared by conventional pressureless sintering (PS) and hot-pressing (HP), the samples prepared by OPS process exhibited a higher density (99.9%), smaller grain size (120 nm), and more homogeneous microstructure. More remarkably, the OPS samples showing higher strength, hardness and elastic modulus compared with the samples prepared by other two techniques were obtained at relatively lower heating temperature and less holding time for 1300°C and 0.5 hour than other two techniques at 1450°C and 1 hour. The XRD patterns indicate that the samples with three preparation methods consist of single phase of tetragonal zirconia. The OPS samples were also analyzed by SEM, Raman and nanoindentation methods. In addition, the fracture model of OPS samples slightly changed from transgranular fracture to intergranular fracture. These results suggest that OPS is an effective technique for fabricating high-quality nanostructured zirconia ceramics with low heating temperature and less sintering time, which is beneficial to prolong working life of graphite molds and sintering devices, what’s more, it significantly reduces cost.

Resume : Recent TEM in situ mechanical experiments on single alumina nanoparticles have shown unexpected plasticity in room temperature alumina [1, 2]. These results push the theoretical boundaries of ceramics mechanical ductility towards comparable levels with metals. The important questions for materials science now are: (i) whether the plastic behaviour can be transferred into polycrystalline, continuous material; (ii) what is the microstructure of such plastic material and (iii) what is the mechanism behind the hypothetical plasticity of the material. Relatively cheap and abundantly available engineering ceramic, such as alumina, with room temperature plasticity would be a breakthrough in the engineering ceramics field. We report the findings of room temperature plasticity on our model material, alumina thin films produced by pulsed laser deposition. Alumina produced this way has an exotic, nanocrystalline microstructure, and is a strong candidate for having the capability for room temperature plasticity. We show the latest results of our In situ TEM studies and atomistic simulations that are focused on analysing and explaining the material’s mechanical response (strain, dislocation activity, fracture etc.) to compression and indentation forces and look for evidence of the mechanism behind the mechanical response. [1] E. Calvié et al. Journal of European ceramic society, Vol. 32, No. 10, p. 2067-2071, 2012 [2] E. Calvié, et al. Materials Letters, Vol. 119, p. 107-110, 2014

Resume : Perovskite has been widely used as active material of light-emitting diode (LED) due to its advantage such as high luminous efficiency and low cost of production. Current research of perovskite focused on applied in deformable electronics since demand for flexible and stretchable electronics increases. Since LED consists of component materials of thin films, stress distribution varies depending on position from neutral axis. It is important to analyze critical bending radius by stress concentration point of component materials because fracture will occur at this point. In this work, flexibility of perovskite LEDs was evaluated by mechanical properties of component materials. Hole-indentation was performed on thin films suspended over circular hole to measure elastic properties of component materials. We conducted uniaxial tensile test in SEM for perovskite samples, the weakest material of perovskite LEDs, to measure accuracy tensile properties. Critical bending radius was analyzed using stress distribution analysis of multilayer during bending with mechanical properties of component materials.

Resume : Recent investigations in biomaterials showed the advantages in the use of polymeric substrates coated with thin films in order to functionalize the surface to provide conductive, biocompatible and/or antibacterial activity. Ti-Ag thin films display, for example, excellent biocompatibility and reveal great potential to be used as conductive materials for prosthesis pressure sensors [1]. On the other hand, Zr-Cu-Ag thin film metallic glasses could combine promising antibacterial activity and high durability to be used as coating of medical instruments to fight hospital acquired-infections [2]. In both cases, the film deformation on flexible substrate needs to be better understood to optimize the mechanical durability of the devices. In this study, microscale characterization of Ti-Ag and Zr-Cu-Ag thin films was followed in situ using a tailored micro-tensile test machine implanted into a SEM chamber. In addition, to quantify the local deformation, we proposed an original approach involving the Digital Image Correlation. Finally, the influence of the thin film composition and structure was investigated. [1] C. Lopes et al. TiAgx thin films for lower limb prosthesis pressure sensors: effect of composition and structural changes on the electrical and thermal response of the films. Appl Surf Sci 2013; 285: 10-8. [2] A. Etiemble et al., Innovative Zr-Cu-Ag thin film metallic glass deposited by PVD magnetron sputtering for antibacterial applications, J. Alloys and Comp. (2016), doi: 10.1016/j.jallcom.2016.12.259

Resume : When a solid is immersed into a liquid solution, whatever their nature (dielectric or conductive), physicochemical phenomena lead to polarize a solid—liquid interface. Two zones with opposite sign of charge appear, one in the solid and the other in the liquid, forming thus the electric double layer (EDL). Various electrical and electrochemical methods have been combined in this work to determine the characteristics of the EDL at the interface between polycrystalline austenitic stainless steel 304L immersed in NaCl aqueous solution (0.01M, 0.1M and 1M). Its effective capacity Cdl was determined by both cyclic voltammetry (CV), for various electric fields and with different voltage scan speeds [1], and by electrochemical impedance spectroscopy (EIS) using an electrical equivalent circuit (EEC) [2]. Another electrical method by flow electrification allows to measure the space charge density ρw at the wall [3]. The results demonstrate that the double layer capacitance depends mainly on the concentration of the electrolyte and the applied potential and that the space charge density is also proportional to the concentration. The influence of the surface roughness (various preparations with SiC papers 240 or 800, or with diamond paste 0.25 µ) on the EDL was then studied by EIS and CV. It appeared that on a macroscopic scale, the roughness has a little influence on the capacity of the EDL at the 304L / NaCl (0.01M) interface. These results were discussed in regards with physical and chemical characterisations operated by XPS, MEB, AFM and WLI. Bibliography [1] Allen J. Bard, Electrochemical Methods. Fundamentals and Applications (1983) [2] G.J. Brug, A.L.G. van den Eeden, J. Electroanal. Chem 176, p275 (1984) [3] Thierry Paillat, Eric Moreau, Gérard Touchard, Journal of electrostatics 53, p171 (2001)

Resume : Modern electron microscopes provide an ideal platform for in situ mechanical testing and manipulation of materials at small length scales owing to high flexibility in realizing special sample geometries and loading scenarios without compromising microscope performance. This holds true for both, combined focused ion beam scanning electron microscopes (FIB-SEM) as well as transmission electron microscopes (TEM) with the former providing highest flexibility and the latter highest resolution. In this contribution we report on new approaches of in situ mechanical testing and manipulation in FIB-SEM and TEM that have been developed and applied to various materials systems within Erlangen's DFG Research Training Group GRK1896 “In situ Microscopy with Electrons, X-rays and Scanning Probes”. Examples include sliding friction experiments and defect manipulation in layered crystals and 2D materials, combined mechanical testing and 3D characterization of nanoporous metals as well as new preparation routines for in situ compression and tensile testing of glass structures. Wherever possible, modeling simulation aspects are also addressed.

Resume : A novel method was developed for the preparation of thin film microtensile specimens. Unlike other techniques, it does not rely on expensive cleanroom photolithographic equipment, but it is based on a combination of focused ion beam (FIB) milling and electron-beam-assisted etching with xenon difluoride precursor gas. The new method furthermore ensures that the area of interest is never exposed to ion beam irradiation so that a pristine microstructure is preserved. This is achieved by using a special shadow milling geometry with a thin silicon membrane simultaneously serving as a substrate and protective layer for the thin film of interest. A further advantage of the new method is that it enables the target preparation and mechanical testing of individual microstructural defects. The method was applied to nanotwinned Cu-Al as well as Au thin films. The fabricated tensile specimens were mounted on a push-to-pull mechanical conversion device and subsequently tested in-situ in the transmission electron microscope (TEM). [1] Liebig, J. P., Göken, M., Richter, G., Mačković, M., Przybilla, T., Spiecker, E., Pierron, O.N., Merle, B.: A flexible method for the preparation of thin film samples for in situ TEM characterization combining shadow-FIB milling and electron-beam-assisted etching. Ultramicroscopy, 171:82-88 (2016).

Resume : The size dependent plastic behavior of micro and nano-scaled non-organic materials has attracted enormous interest during the last decade. When the object size becomes comparable to intrinsic length scales, finite-size and quantum size effects occur influencing the physical properties. For complete understanding of the deformation process in-situ experiments are necessary. X-ray nanodiffraction in combination with nanoindeter provides unique information on stress-strain response during mechanical testing with strain resolution of 10^-5. In this presentation the setup for in-situ experiments at the Nanofocus Endstation of P03 beamline (PETRA III, DESY) will be presented with technical characteristic and examples of successful experiments will be shown.

Resume : Interfacial properties of thin films on compliant substrates are important to understand in order to design reliable flexible electronic devices. Due to the compliance of the substrates, measuring the adhesion energies of these systems is difficult with existing techniques and models. A new method and mechanics-based model have been developed to measure the adhesion energy of metal films on compliant substrates using an in situ uniaxial tensile test inside a scanning electron microscope. With this technique, the initial fracture and buckling of the film can be observed and correlated to the strain. Once buckling of the film has been achieved, the height and width of the buckles are then used in the new model to calculate the adhesion energy. To demonstrate the technique and model, titanium films on polyimide are studied before and after annealing. It will be shown that annealing decreases the adhesion because the interface structure changes.

Resume : The oxidation of Fe-based alloys in liquid Pb-Bi eutectic, with oxygen dissolved in liquid phase, leads to oxide formation on the surface. These structures are primarily determined by oxygen concentration and diffusion properties of cations from solid and oxygen anion from liquid phase. In this work, the influence of temperature and oxygen concentration on oxides growth dynamics and their diffusion properties is examined in oxidation of ferritic Fe-Cr-Al-alloys in liquid lead-bismuth at high temperatures. The order of oxide phases, and their structure, determined by XRD, EDS, TEM and Raman spectroscopy, are correlated with model predictions of dynamics of their formation and degradation. The ultimate goals have been to determine the oxide phases present in the scales, and to understand their evolution (as a function of temperature and oxygen concentration) and origins of strain induced in oxide and in bulk.

Resume : Layered materials present a promising route for achieving attractive combinations of properties, e.g., high hardness and toughness at the same time [1]. In this work, we evaluate the mechanical moduli of an Al2O3-TiO2 superlattice using effective elastic constants as well as a a more advanced direct ab initio approach which involves calculations of stress-strain curves of the superlattice. The effective elasticity is derived by means of the Grimsditch-Nizzoli [2] method, which allows to estimate the elasticity of a superlattice from the elasticity of the constituents. The ab initio calculations are performed in the framework of density functional theory, using the structural information (epitaxial relationships and interface structure) obtained in our previous work [3]. We compare the outcomes of the two approaches and discuss the performance of the Grimsditch-Nizzoli method when evaluating the bulk, Young's, and shear moduli. In particular, we demonstrate that the transverse Young's modulus obtained in the Grimsditch-Nizzoli method gives reasonable results, while a reliable description of the shear modulus is only obtained when applying the direct ab-initio approach. References: [1] Abadias, G. et al., Surface and Coatings Technology 202, 844 (2007) [2]Grimsditch, M. and Nizzoli, F., Phys. Rev. B 33, 5891 (1986) [3] Popov, M.N. et al., Phys. Rev. B 86, 205309 (2012)

Resume : Here we introduce an atomistic approach based on density functional theory and non-equilibrium Green’s function, which includes all the relevant ingredients required to model realistic metal-semiconductor interfaces and allows for a direct comparison between theory and experiments via I -V bias curve simulations. We apply this method to characterize an Ag/Si interface relevant for photovoltaic applications and study the rectifying-to-Ohmic transition as a function of the semiconductor doping. We also demonstrate that the standard “activation energy” method for the analysis of I -V bias data might be inaccurate for nonideal interfaces as it neglects electron tunneling, and that finite-size atomistic models have problems in describing these interfaces in the presence of doping due to a poor representation of space-charge effects. Finally, we will describe a novel surface Green’s function (SGF) method where the surface is described as a true semi-infinite system and present a number of examples to illustrate how the SGF method gives a number of benefits compared to the slab approach as well as enables new type of studies.

Resume : Titanium diboride (TiB2) has many desirable properties such as high hardness (Hv=3400), high melting point (3000K), low density, high electrical resistivity and good corrosion resistance. However, a high sintering temperature above 2000K is required to obtain dense TiB2 specimens. Grain growth occurs by the sintering at such high temperatures which causes the degradation of the mechanical properties. In order to obtain dense and hard TiB2 based materials by the sintering at lower temperatures, sintering of TiB2 with various intermetallics as sintering aid has been investigated. In the present study, it has been shown that AlTi-TiB2 composites in which sub-micron sized TiB2 particles are dispersed in AlTi matrix can be formed by the reaction sintering of Al, Ti and boron powders using the spark plasma sintering method at 1573K. The Vickers hardness of AlTi-TiB2 composites increases as the amount of TiB2 increases up to 85vol%. The highest Vickers hardness of 2200Hv has been obtained for AlTi-85vol%TiB2. No crack has been observed near the corners of the Vickers indentation indicating excellent fracture toughness of composites of sub-micron sized TiB2 particles. On the other hand, in the AlTi-TiB2 composites fabricated using micron sized TiB2 particles, cracking occurs in TiB2 particles and at the interface between TiB2 and AlTi matrix.

Resume : Epitaxial graphene on metallic surfaces can form a variety of moiré superstructures due to interfacial lattice parameter and/or symmetry mismatch [1][2]. The latter characterizes graphene on Ni(100), where the hexagonal carbon network accommodates onto a square surface lattice. The interfacial mismatch leads to moiré patterns with stripe-like or rhombic-network morphology and with periodicity depending on the relative misorientation angle between graphene and the Ni surface. Different moiré patterns are characterized by high resolution scanning tunneling microscopy (STM) images and successfully described by atomistic models which consider the geometric registry. Ab-initio density functional theory (DFT) simulations well reproduce the observed STM images and shed further light on the spatial corrugation of graphene and the interfacial interactions, indicating that, depending on the misorientation angle, graphene can be alternately physi- and chemisorbed or uniformly chemisorbed. The interaction is modulated periodically by the (sub)nanometer-sized moiré superstructures. [1] Vinogradov, N.A. et al., Phys. Rev. Lett., 109 (2012) 026101 [2] Murata, Y. et al., ACS Nano, 4 (2010) 6509-6514

Resume : we developed rubber material that is environment-friendly and superior in mechanical property using rubber-clay nanocomposites. Typical modifiers to enhance dynamical properties of rubber material were carbon black and silica. Recently, nano-clay was popular as modifiers. Many researches on nanocomposites are being actively carried out because they are excellent in modification even with a small quantity of them while nano-clay is difficult to diffuse. Fillers or modifiers used when manufacturing polymer nanocomposites include layered silicate, POSS nanoparticles, CNT and nanoparticles of metal or inorganic matters, among which layered silicate is now being most actively developed as polymer nanocomposites. The key technology of development of polymer nanocomposites is how to change layered clay so as to easily insert polymers into it. When organic matters are inserted using inorganic material like clay silicate that has a uniform structure with nano scale, in particular, nanocomposites are attracting great concerns in their application. The basic structure of clay, as it is well known, consists of silica tetrahedral and alumina octahedral sheets: it is classified into several groups including vermiculite and montmorillonite depending on its negative charge. In this study, acrylonitrile butadiene rubber (NBR) was used as rubber in combination; ZnO and stearic acid were used as vulcanization activators; and 3C was used as an additive; sulfur of purity 99.9% was used as a vulcanizing agent; TT and CZ were used as vulcanization accelerators; and carbon black, clay, and nano-clay were used as reinforced compound. Polymer layered silicate was made by the melted intercalation method in which polymers in the melted state were inserted between silicate layers: this method is advantageous in mass production and does not need to use solution. Thermal resistance was estimated through material tests of developed material at room temperature and aged condition. Fatigue durability was estimated after we developed a new method that could estimate fatigue lifetime of rubber parts in a short period in the initial stage. As results, fatigue lifetime evaluation by the fatigue lifetime prediction equation was exactly consistent with that obtained by fatigue tests of actual engine mounts. In addition, we verified that the developed material was superior in fatigue durability as well as mechanical properties because the lifetime of rubber component made by the developed material was longer than the existing material.

Resume : Duo to the unique and highly valuable properties, graphene sheets has attracted increasing attention. In this paper, the nonlocal elastic theory and classical plate theory (CLPT) are used to derive the governing equations. The element-free kp-Ritz method is employed to analyze the buckling behaviors of double layer circular graphene sheets (DLGSs) embedded in an elastic medium. Pasternak-type model is adopted to simulate the functioning of the elastic medium. Then, the effects of boundary conditions, aspect ratio, size of graphene sheets, nonlocal parameters on the buckling behavior of DLGSs are investigated.

Resume : Falls are dangerous for the elderly population; therefore, many fall detection systems have been developed. However, previous methods are bulky for elderly people or only use a single sensor to isolate falls from daily living activities, which makes a fall difficult to distinguish. We present a cost-effective and easy-to-use portable fall-detection sensor and algorithm. Specifically, to detect human falls, we used a three-axis accelerator and a three-axis gyroscope in a mobile phone. We used the Fourier descriptor-based frequency analysis method to classify both normal and falling status. To calculate the classification accuracy, the K-fold cross-validation method was used and K was set as 10. The performance indices were evaluated by five performance indices, including classification accuracy (TP+TN)/(TP+TN+FP+FN), sensitivity (TP/[TP+FN]), specificity (TN/[TN+FP]), positive predictive value (TP/[TP+FP]), and negative predictive value (TN/[TN+FN]), where TP is the number of true positive findings correctly classified as positive; TN = true negative; FP = false positive; and FN = false negative [8]. From the results, we noticed that the proposed method detects 384 data (falling or walking) out of 414 original data (falling or walking). The statistical analysis shows that the resulting accuracy was 96.14%, sensitivity was 100%, specificity was 92.08%, positive predictive value was 92.98%, and negative predictive value was 100%. In this paper, we propose a mobile phone-based falling detection sensor and algorithm. We used a three-axis accelerator and three-axis gyroscope to determine the direction and rotation of the subjects’ bodies. The support vector machine classifier was used to analyze the sensor’s signal. From the experimental results, the proposed method detects falling status with 96.14% accuracy, which indicates that human activity can be classified using a mobile phone. These detection methods can warn others of a user’s fall.

Resume : Single-grain mechanical properties have been measured with the aim of calibrating the material parameters of a crystal plasticity finite element model to a single-crystal response. Such simulation-oriented experiments allow the investigation of the deformation behaviour of individual grains in plastically deforming metals, which underpins the bridging of scales in multiscale modelling approaches to predict the mechanical behaviour of nuclear structural materials and support the elaboration of materials design rules for nuclear energy applications. Preliminary computations of 316L stainless steel pillars using macroscopically calibrated material properties indicated that applying a 100mN compressive force (the maximum allowed by the nanoindentation system where micropillar compression is performed) to pillars of diameter below 10 µm results in average compressive strains above 10 %, a sufficient amount of plastic deformation for an initial calibration of crystal plasticity material properties, while 20 µm pillars would result in mainly elastic deformation. On that basis, a series of micropillars have been fabricated by Focused Ion Beam inside single grains of polycrystalline steels, and compressed by a flat punch using a nanoindentation system. This work will present the results of the combination of compression tests and crystal plasticity finite element simulations.

Resume : Hydrophobicity of graphene has been investigated for the first time using multiscale simulation, i.e. combined quantum mechanics / molecular mechanics (QM/MM). Wettability of graphene was determined by using a concept of work of adhesion; free energy gap of a two-phase system (liquid and solid) between a sum of each solitary states and a neighbored state. Two phase thermodynamics (2PT) analysis was used for free energy calculations. Interfacial interactions from not only van der Waals, electrostatic potential but also perturbed or reorganized states were reproduced by potential feedback cycles in QM/MM simulation. Here, we investigate a consequence of various chemo-physical conditions in graphene such as layer thickness, corrugated structure, and nitrogen doping. The microscopic wettability of graphene and geometric alignment of waters on the surface were studied.

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 : The report is devoted to mechanisms that may operate influence of nanoparticles interaction with bulk matrix on phase transformations and in such way – on mechanical properties formation. By direct experimental control, it was shown that ferromagnetic nanoparticles, formed in Cu-Al-Mn alloys under 473 K for 3 h aging in a magnetic field of 1.5 kOe having different orientation, provide: 1) stress reduction, arising during phase (martensite) transformations; 2) superelasticity increasing. Oriented growth of precipitated nanoparticles in an applied field direction and their volume fraction increasing favor of induced martensitic transformation reversibility. The model of established effect has been proposed. When martensitic transformations take place the internal stresses arise as a result of austenite phase shear deformation. From other side, austenite aging before martensite transformation results in nanoparticles Cu2AlMn coherently connected with a matrix precipitation, that reduces internal stresses during transformation. This leads to thermal hysteresis of martensitic transformation narrowing and superelasticity increasing. The competition between these processes driving forces on bulk-, micro- and nanolevels is discussed. Thus, bridging scales (from nanoscale mechanisms to bulk behavior, mechanical properties formation due to intrinsic nanosize effects) is illustrated in this report through the role of ferromagnetic nanoparticles in Cu-Al-Mn alloy phase transformations.

Resume : The power conversion efficiency of perovskite solar cells has remarkably increased in the past few years. However, there are some drawbacks for perovskite solar cells to be commercialized. The main drawback is degradation of the perovskite structure by moisture. And spiro-OMeTAD, which is hole transporting layer (HTL) located between electrode and perovskite layer, is well known for hygroscopic characteristics. In this study, we investigated the behaviors of oxygen and water in spiro-OMeTAD using grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations. The spiro-OMeTAD with oxygen and water loaded models were used. Mean square displacements (MSD) and other physical properties were calculated to understand the behavior of oxygen and water in spiro-OMeTAD. Understanding hygroscopic characteristics of spiro-OMeTAD with computational simulations were attempted to offer an insight for developing high stability perovskite solar cells.

Resume : We have theoretically investigated the 1.55 µm p-type doped GaNAsBi-based Double Quantum Wells (DQWs) using the (16x16) BAC model combined with a self-consistent calculation. We have found that the coupling effect becomes more pronounced by reducing barriers width. The optical performance of the structure is enhanced when the DQWs are coupled and doped. Moreover, a p-i-n heterojunction based on GaNAsBi/GaAs (DQWs) designed for infrared photodetection was developed. The computed gain can reach the value 4.2 〖10〗^4 〖cm〗^(-1). The optimization of well parameters such as the Bismuth composition, the well width and the doping densities give rise to p-i-n heterojunction emitting at the wavelength 1.55 µm. The quantum confined stark effect on the optical properties of studied structures is also discussed.

Resume : We have experimentally studied the multicomponent responses of photoreflectance spectrum using selective phase analysis. After several experimental tests, the phase diagram of GaAs substrate in region of fundamental energy shows only one component. On the other hand, the PR spectrum of GaAsBi/GaAs structure reveals at least two contributions relative to fundamental band-band transition and Franz-Keldysh oscillations for GaAs and/or GaAsBi layers. A successful separation of different components is realized by the help of adequate phase angle. We seem that the separation of contributions combined with multilayers model is useful to extract the values of the physical parameters for each region of each structure. We have detailed the methodology and experimental procedure to identify each contribution.

Resume : Electronic band structure of strain-balanced GaAsN/GaAsBi type II double quantum wells (DQWs) was theoretically investigated by using the band anticrossing model combined with the envelope function approximation. The strain-balanced DQWs were coupled by the optimization of the well and barriers widths. We have discussed the coupling effect on the confined states energies and the oscillator strengths of inter-band transitions. The in-plane carrier effective masses and the optical absorption spectra of 1.3 and 1.55 µm GaAsN/GaAsBi type II DQWs are also examined.

Resume : Cu-Ag system has attracted significant attention in recent years because of their applications due to their unusual mechanical and electrical properties ; since it was used as contacts and interconnect layers in microelectronic industry [1] and as material magnets [2]. In the present work, we investigate the effect of silver inclusion on mechanical behaviors of copper nanomaterial during nanoindentation process by means of Molecular Dynamics Simulations, based on the embedded atomic method (EAM), using the LAMMPS code [3]. We investigate how Ag inclusion influences the copper nanoindentation. Simulation results show that the presence of silver inclusion has influenced the material strength. First, hardness of Cu matrix containing Ag inclusion is less larger than of pure Cu. Second, the composite Cu-Ag hardness increases by increasing the depth of the Ag inclusion. This result is in good agreement with previous simulations [4,5]. Keywords : Nanoindentation, MD simulation, Inclusion, Nanomaterial, Hardness. References [1] S. Strehle et al., Microelectron. Eng. 76 (2004) 205-211. [2] C. Zhao et al., Mater. Sci. & Engineer. A 652 (2016) 296–304. [3] S.J. Plimpton,  J. Comput. Phys. 117 (1995) 1–19. [4] C. Wei, C. Yu, Proc. of the Intl. Conf. on Future Trends in Structural, Civil, Environmental and Mechanical Engineering -- FTSCEM 2013 . [5] V.V. Pogorelko, A.E. Mayer, Mater. Scien. Eng. A642 (2015) 351-359.

Resume : The strong piezoelectric effect of III-V semiconductor nanostructures, renders them important candidates for micro- and opto-electronics applications. In heteroepitaxy, the property that mainly defines the efficiency of such nanostructures is the development of stress-strain fields due to lattice misfit. The present approach is a versatile route to solve elasticity problems at the nanoscale for various growth orientations and compositional profiles using the Finite Elements Method (FEM), in conjunction with High-Resolution Transmission Electron Microscopy (HRTEM) imaging. In our analysis, we modify the elastic characteristics of any (hkl)-oriented system with respect to the conventional <001> coordinate system. Thus, by determining the affine transformation from one coordinate system to another, we address the calculation of the new stiffness tensor C(hkl), in relation to the independent stiffness constants of the <001>-oriented system. This transformation comprises three successive rotations about the three axes of the <001> coordinate system. Simultaneously, the FEM mesh is chosen to be consistent with the bicrystal symmetry as expressed by a dichromatic pattern. Then, in the frame of thermo-elasticity theory and using the anisotropic elastic model, we calculate the elastic properties of any (hkl)-oriented nanostructures of various chemical compositions, thus enabling a direct comparison with quantitative HRTEM results obtained by geometric phase analysis or peak finding. Acknowledgements Work supported by the Research Projects for Excellence IKY/Siemens.

Resume : Fluoropolymers are very attractive compounds because of their versatility and their unique physical and chemical properties. In this work, a piezo-actuated testing device has been developed for measuring the elastic and plastic properties of FEP, ETFE, and PFA fluoropolymers films via uniaxial micro-tensile testing and finite element method simulations. The piezo-actuated micro-tensile setup was able to measure film elongation up to 210 μm with an accuracy of 50 nm. The tensile test results revealed a major difference in the material deformation mechanism as the stress was applied in the different direction of the polymer chains, and at the different measurement conditions (strain rate, temperature). The microstructure changes of the temperature affected fluoropolymers were investigated by XRD. The results confirmed the ability of the custom-built equipment for the measurements of very small load and displacement levels, which are a prerequisite for such type of investigations.

Resume : A new approach has been presented to find out optimal number of piezo-patches for the multimodal vibration reduction of smart plate using genetic algorithm. The length of the chromosomes/number of genes in genetic algorithm represents the number of piezopatches on the plate. The length of chromosome remains constant in general GA. Laying–up of varying length of chromosomes still remains a hard task as crossover implementation is more complex in this case. A variable length of chromosomes has been generated in adaptive GA to determine the optimal number of piezo-patches. The finite element model of smart plate is based on the Kirchoff’s plate theory has been utilized. The performance index based on spillover effects has been minimized while maintaining the degree of controllability to determine the optimal number of actuators. The results clearly indicate that present approach performs better in terms of computation effort and processing time.

Resume : The current work is focused on performance of sunflower biodiesel in a single cylinder direct injection diesel engine. Three engine input parameters are varied in order to predict optimum combinations of performance, combustion and emission parameters. Hybrid RSM-NSGA-II technique is applied to predict the optimal solutions. It is observed that design engineer can select any of the solutions depending upon the requirement of performance, combustion and emission parameters.

Resume : We explore numerically the morphological patterns of thermo-diffusive instabilities in combustion fronts with a continuum fuel source, within a range of Lewis numbers and ignition temperatures, focusing on the cellular regime. For this purpose, we generalize the recent model of Brailovsky et al. to include distinct process kinetics and reactant heterogeneity. The generalized model is derived analytically and validated with other established models in the limit of infinite Lewis number for zero-order and first-order kinetics. Cellular and dendritic instabilities are found at low Lewis numbers. These are studied using a dynamic adaptive mesh refinement technique that allows very large computational domains, thus allowing us to reduce finite-size effects that can affect or even preclude the emergence of these patterns. Our numerical linear stability analysis is consistent with the analytical results of Brailovsky et al. The distinct types of dynamics found in the vicinity of the critical Lewis number, ranging from steady-state cells to continued tip-splitting and cell-merging, are well described within the framework of thermo-diffusive instabilities and are consistent with previous numerical studies. These types of dynamics are classified as “quasi-linear” and characterized by low amplitude cells that may be strongly affected by the mode selection mechanism and growth prescribed by the linear theory. Below this range of Lewis number, highly non-linear effects become prominent and large amplitude, complex cellular and seaweed dendritic morphologies emerge. The cellular patterns simulated in this work are similar to those reported by Malchi et al. in experiments of flame propagation over a bed of nano-aluminum powder burning with a counter flowing oxidizer. These resemble the dendritic fingers observed in this study, in the limit of low-Lewis number. It is noteworthy that the physical dimension of our computational domain is roughly close to their experimental setup.

Resume : Nanoporous metals can take the form of macroscopic bodies that consist of a bicontinuous network of pores and solid ligaments. Owing to their large surface area to volume ratio nanoporous metals exhibit substantial surface induced property modifications. Amid these materials nanoporous gold (NPG) stands out as an ideal model material due to its adjustable surface area and high chemical stability. As a result of NPGs significant electromechanical coupling, electrical tuning of the strength and flow stress, elastic modulus, creep behavior and crack propagation has been reported. Yet, the impact of electrical modulation on the Poisson's ratio, an essential material parameter that links longitudinal and transverse strain, is still unknown. In our contribution we investigate the influence of electrical excitation on the transverse mechanical coupling of NPG through the analysis of a metal/electrolyte hybrid material. We present experiments of NPG under compression testing in 1 M perchloric acid while varying the applied electrical potential, E. Our results reveal a considerable dependence of the plastic Poisson's ratio, νP, on E. We link these findings to the surface tension, γ, a capillary force that promotes a tension/compression asymmetry in the nanostructure. Since γ depends on E, we demonstrate that plastic lateral expansion in NPG is governed by surface effects. In this line of argument we are able to interpret earlier reports showing a ligament size dependence of νP.

Resume : In this work we present results on in situ small scale testing of nanoporous gold (npg) in scanning electron microscopy (SEM) and transmission electron microscopy (TEM). By combining nano- and micromechanical testing of pillar structures with advanced tomographic imaging, a 3D characterization of the plastic deformation process in different states of deformation is achieved. For small strut sizes 360° electron tomography (ET) is applied enabling high quality reconstructions of the 3D morphology of npg without missing-wedge artefacts. Combining the geometric information with mechanical data from in situ testing in SEM and TEM the yield strength is precisely determined. Furthermore, the experimentally derived 3D data are used as input for large-scale molecular dynamics (MD) simulations in order to understand the role of strain localization and identify predominant defect processes. For larger strut sizes mechanical testing and 3D structure analysis of npg pillars are carried out by in situ SEM and high-resolution X-ray tomography (Nano-CT), respectively. Image correlation analysis applied to in situ SEM image series reveals the evolution of local strain gradients during deformation and, in particularly, local yielding in the very early stages of deformation. The yield strength strongly depends on strut size revealing a clear size effect. The scale-bridging approach is complemented by in situ nanomechanical testing of single struts in TEM.

Resume : A nanotribological evaluation of the electrochemical double layer at gold/electrolyte interfaces was undertaken by in-situ lateral force measurements. Chemisorption (specific adsorption) of sulphate, bromide and iodide caused a friction increase as has been reported on silver [1]. Chemisorbed bromide showed a greater friction than iodide due to a higher surface coverage, whereas fluoride exhibited no effect, as it did not chemisorb. Low normal forces (<11 nN) led to classical friction behaviour according to the Amonton’s law for non-specifically (in the Outer Helmholtz Plane) and specifically adsorbed anions (in the Inner Helmholtz Plane). At higher normal forces, the cantilever tip displaced the chemisorbed anions leading to a deviation from Amonton’s law. When a two-dimensional gold hydroxide layer was generated at high normal forces (>11 nN), the friction remained on a finite high level because the tip replaced the chemisorbed anions but could not penetrate the hydroxide layer. This study demonstrates that solvated anions in the Outer Helmholtz Plane do not contribute to friction significantly, whereas specifically adsorbed (chemisorbed) anions in the Inner Helmholtz Plane increase the mechanical resistance. Thus, nanotribology of a noble metal can be controlled by electrochemical surface charging conditions, e.g. by a potentiostat, and by the chemical nature of the anions. [1] W. Kautek, S. Dieluweit, and M. Sahre, J. Phys. Chem. 1997, 101, 2709.

Resume : A combined numerical-experimental analysis is applied in order to explain the complex behavior of a rare example of one-component dual actuator. The system of interest is a bilayer with aligned and randomly oriented poly(N-isopropyl acrylamide) fibers which displays irreversible change in shape by rolling in contact with water, and reversible size change in response to temperature changes. A combination of anisotropic Young’s modulus and temperature dependent swelling/shrinkage is shown to be responsible for the actuators controllable unique folding.

Resume : Fiber reinforced plastics (FRP) gained a strong interest in the last decades, finding applications in automotive, mass transportation, water management or consumer electronics. FRPs provide lightweight, design flexibility, cost-effectiveness and represent an attractive alternative for metal replacement. The use of FRP allows to overcome the mechanical limitations of base polymer, by enhancing the tensile strength, the elastic modulus and impact strength of the material. The fiber pull-out length, which is related to the adhesion and friction forces occurring at the interface between the surface of the fibers and the polymer matrix, provides critical information regarding the mechanical properties. However, the assessment of the pull-out length is challenging because SEM provides only a 2D projection of the 3D random orientation of the fibers on the fractured surface. Such projection introduces challenges and inaccuracy in the fiber pull-out length assessment. In order to improve the accuracy of the fiber pull-out length measurement, an algorithm is developed to perform the 3D surface reconstruction with SEM images acquired at different tilling angles. The common features on each image are detected, aligned, and used to estimate the projective geometry of SEM detectors. Based on the estimated geometry, the 3D surface is reconstructed via triangulation. The measured pull-out length with respect to the fractured surface is then correlated to the mechanical properties.

Resume : Highly annealed pyrolytic graphite (HAPG) is a high quality type of graphite with low angle (less than 0.1°) mosaic structure that usually is employed for the fabrication of X-ray optics. HAPG plate consist of large number of individual graphite crystals with size 100 μm, which have a small tilt angle to the plate surface and minimal distance between neighbor crystals. Understanding of HAPG internal crystal structure as initial as after mechanical processing (cutting, polishing) or its applying on substrate is meaning factor in HAPG future application. Investigations of HAPG internal microstructure were carried out by impulse acoustic microscopy. It is shown that high frequency ultrasound wave is sensitive radiation for characterization of crystal orientation, atomic interlayer delimitation inside HAPG plate, adhesive defects of HAPG films to substrate etc.

Resume : Composite materials fabricated with reinforcement of polymer matrix by regular package of carbon, glass or organic fibers are widely used today in aerospace engineering. Carbon fibers are very attractive as reinforcement due to their strength, stiffness, and low weight, high level of damping, good corrosion and heat resistance. Anisotropic properties and multiple boundaries in the bulk structure of polymer reinforced composites as well as technological imperfections result in more complex processes of material destruction. Studying mechanisms of defects formation at micro level are the basis for understanding destruction processes in composites to assess strength and reliability of such materials and constructions of them [1,2]. Optical, electron or probe microcopies are destructive techniques. In the paper the nondestructive high resolution ultrasonic technique for bulk visualization of the composite damaging under the mechanical loading and environment factors is proposed. Using short pulses of focused ultrasound probe (50-100MHz) makes it possible imaging with a lateral resolution of 30μm at a depth of several millimeters. Packaging of reinforced threads and defects of their adhesion to the composite matrix, pockets of polymer binding, interlayer delaminating, localization and distribution of the defects in the material bulk are well seen in acoustic images. Dynamics of microstructural changes under the tensile loading and low-speed impact is studied. Mechanisms of the material destruction are discussed. Research is supported by Russian Scientific Foundation No15-12-00057. [1] Greenhalgh E.S. Failure Analysis and Fractography of Polymer Composites. Woodhead Publishing, Cambridge, 2008; [2] Jollivet T., Walaszek H., Terrien N., JEC Composites Magazine, 81, 2013

Resume : Reliable performance under mechanical deformation is a central goal for flexible electronic sensors. The realization of mechanically rugged materials and device architectures depends crucially on the understanding of how strain affects electronic material properties and leads to defect formation. Scanning Kelvin-Probe Microscopy (SKPM) is a formidable technique for nanoelectronic investigations as it combines non-invasive measurement of surface topography and surface electrical potential. Here we show that SKPM becomes feasible on free-standing, deformed flexible samples when operated in the low-interaction regime of non-contact mode thereby providing the opportunity to study strain effects on nano-electronic properties. As an example we apply the technique to investigate strain effects and failure of flexible thin film transistors containing the organic semiconductor TIPS-pentacene during bending.[1] We find that the step-wise reduction of device performance at a critical bending radii is related to the formation of nano-cracks in the microcrystal morphology of the TIPS pentacene film. The cracks are easily identified due to the abrupt variation in SKPM surface potential caused by a local increase in resistance. Importantly, the strong surface adhesion of microcrystals to the elastic dielectric allows to maintain a conductive path also after fracture thus providing the opportunity to attenuate strain effects. We support our findings by numerical simulations of the bending mechanics of the hole transistor structure allowing to quantify the tensile strain exerted on the TIPS-pentacene micro-crystals as the fundamental origin of fracture. [1] T. Cramer, L. Travagli, S. Lai, L. Patruno, S. De Miranda, A. Bonfiglio, P. Cosseddu, B. Fraboni, Sci. Rep. 2016, 6, 38203.

Resume : Metal matrix composites (MMCs) come under advanced materials that can be used for a wide range of industrial applications. MMCs consist of a non-metallic reinforcement incorporated into a metallic matrix which can enhance advantageous properties over base metal alloys. Copper-Glass fiber reinforced aluminium based hybrid composites were prepared by compo casting method. 4 weight % copper was used as alloying element with Al because of its precipitation hardened properties. Different weight compositions of composites were developed and characterized by mechanical testing. A significant improvement in tensile strength and micro hardness were found, before and after heat treatment of the composite. The SEM analysis of the fractured surfaces showed dispersed and embedded glass fibers within the network leading to the enhanced strength.

Resume : High strength and light weight are the unique properties of Metal Matrix Composites which make them to be used in aerospace and automobile industries. Aluminium-carbon fibers hybrid composites were developed and studied the effect of. 2, 4, 6 and 8 wt.% ratio of carbon fibers with constant amount (2wt.%) of silica sand nanoparticles. Tensile testing and Micro Hardness Tester machines were used to measure the tensile and hardness properties of the developed composites. Changing the wt.% ratio of the reinforcements changed the mechanical as well as electrical properties of the developed hybrid composites. The fracture analysis of tensile tests was studied using Scanning Electron Microscope (SEM).

Resume : Huge scientific interest to novel nanostructures with superior properties and characteristics require appropriate experimental techniques with commensurate tools for detail investigations. P03 nanofocus endstation at PETRA III in DESY operated by Helmholtz Zentrum Geesthacht we provide high stability 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 X-ray beam with a size of only 250 x 350 nm². The strong focus on materials science at P03 is demonstrated by the wide range of experiments already performed with in situ sample environments: pressure, indentation force, tensile stress, fluid shear, magnetic fields - all of these parameters were successfully modified in situ and combined with the high spatial resolution provided by nanofocused beam.