Advanced small-scale mechanical characterization: strength, plasticity, fracture and fatigue

Resume : The lifetime of materials is a critical parameter for the reliability of devices. Further, rising requirements on materials lead to the necessity of a more distinct understanding of microstructural influences on the fatigue mechanisms. The progress of fatigue in alloys can be divided in several stages driven by cyclic irreversibilities, such as the occurrence of dislocation structures, formation of PSBs, growth of extrusions at the surface and crack initiation and growth. The lifetime of stress scenarios beyond the high cycle fatigue regime is mainly dominated by early stage fatigue mechanisms until crack initiation. Currently used in-situ characterization methods mostly operate either under vacuum or with complex stop and go practice. A combination of a micro fatigue setup designed for the detection of the early stages of fatigue occurrence and a novel microscopy technique would represent an alternative method allowing in-situ observation of formation of extrusions on the surface of samples. The ROCS-microscopy (Rotating Coherent Scattering Microscopy), up to now developed and used for the observation of biological cells, allows time and position dependent imaging independent of any atmospheric conditions. This method employs illumination of the sample’s surface with a collimated laser beam with a defined tilt and a variation of the azimuthal angle and integrates the back scattered light from different angles into an image. The aim was to implement an in-situ ROCS-microscopy based characterization method, to measure the formation of extrusions and performing first fatigue tests on martensitic steel, SAE 4150 subsequently. A preliminary study of the combined setup showed higher contrast for bright field images, while dark field images reproduced the topography of the sample surface. A comparison of obtained ROCS-microscopy images with alternative imaging methods showed higher image contrast for ROCS and resolvable structures of 190 nm distance. The formation of extrusions could be monitored by the evaluation of the accumulated peak intensity from the obtained in-situ ROCS images. Additionally, occurred extrusions and microcracks were characterized by scanning electron microscopy. Microstructural analysis showed ferritic areas, ascribed to high annealing temperatures, in the predominantly martensitic microstructure where a cumulated occurrence of extrusions could be observed. A link between the slip orientations of the extrusions with the slip plane traces of the damaged grain could be observed. Further, the observation of intragranular misorientations at extrusion borders, present already before the fatigue process, points to them acting as barriers against the growth of extrusions. The potential use of the in-situ ROCS fatigue setup allows a fast and simple characterization method to investigate the occurrence and propagation of extrusions. The influence of the microstructure under ambient conditions throughout early stages of fatigue can be investigated.

Resume : The recent development of in-situ mechanical testing and heating in SEM leads to linking the mechanical properties to the microstructures of materials by means of direct observation, strain analysis and EBSD misorientation measurement. An understanding of the connection between microstructures and mechanical properties helps researchers to design novel advanced materials in a highly effective way. Furthermore, such in-situ materials testing approaches deliver experimental datasets for the validation and the improvement of computational materials models, which is essential in designing novel high-performance materials by means of optimizing their microstructures and manufacture processes. The integration of in-situ testing accessories into a SEM up to now is, however, far from seamless and user friendly. The output is, on the other hand, very sobering in terms of the scale of sample area interrogated, throughput and reproducibility of results or datasets. For example, to validate and refine computational materials models, a large amount of highly resolved strain maps at precise mechanical loadings and temperatures of the sample are required which are, however, hardly available using the in-situ solutions at present. Another example is to determine grain misorientations through EBSD, defect formation and grain boundary migration caused by mechanical loading and subsequent tempering. There is, however, no real automated workflow for such long-term demanding experiments. To address these shortcomings, a well-integrated solution for demanding in-situ testing, combining high resolution surface sensitive SEM imaging and EDS/EBSD analytical methods with materials testing stages is under development. In this work, a solution which enables tailored in-situ automated workflows based on Python scripting is introduced. The automated workflow can generate meaningful data with utmost reproducibility and precision. On the other hand, such automated workflows make high throughput data acquisition at high image resolution and precise loading parameters in the SEM possible. The high quality of the acquired datasets facilitates post processing and data analysis, for e.g. high-resolution strain mapping by means of digital image correlation (DIC). By correlating the high-resolution images of the sample surface before and after its deformation during the in-situ experiment, the local surface strain can be evaluated by means of DIC. A selected area close to the center of the Cu sample is imaged with a high pixel density of 4000 x 3000 pixels at different nominal strain levels using the chamber SE detector. The DIC analysis performed by means of GOM Correlate is introduced. In this selected area, the local normal strain in the x direction is significantly larger than the nominal strain value, and the strain is mainly concentrated around the micrometer sized cracks which grow and propagate during the in-situ experiment. Further advancements such as automated feature tracking, autofocus and multiple regions of interest (ROIs) help to realize true one-button-start workflows and experiments. A heating element is integrated for high temperature in-situ testing up to 800°C. The temperature in the range of room temperature to 800°C can be kept constant for more than 24 hours for long-term in-situ testing combined with EBSD. Results such as grain boundary transition or grain deformation imaged using backscattered electron detection (BSD) at high temperatures will be shown. Examples of measuring grain misorientation during mechanical loading by means of high-resolution EBSD as well as high resolution strain mapping using DIC will also be discussed.

Resume : Hardness is one of the most complicated and important property of materials from a practical point of view. It refers to the property of a material to resist pressing-in or scratch of an indenter made of a much harder material than tested one. The determination of hardness is traditionally based on the experimental measurement procedure, during which the test material is indented with a small piece of another very hard material (usually diamond). In this case, the hardness value itself is calculated as the ratio of the maximum indentation force to the area of the imprint left on the test sample. However, the measured hardness depends not only on the shape of the indenter, but also on the depth of its penetration (or the scale of the measurements)[1,2]. In addition to complexity and high cost, the experimental determination of hardness does not allow it to be used in the computer design of new materials. Therefore, in computational materials science, hardness is calculated by empirical models based on various other available material characteristics [3–6]. Such models are typically constructed as explicit formulas approximating known experimental data and generally nit very accurate. We proposed a technique which is based on atomistic modeling of the indentation similarly to the experimental procedure. A crucial aspect in this case is the model of interatomic interaction used. The empirical potentials traditionally used for atomistic modeling, as a rule, do not have sufficient accuracy to ensure the predictive power of this method, and, in addition, are developed only for a limited class of materials. Quantum-mechanical models, on the other hand, having acceptable accuracy, do not allow calculating structures with more than 200 atoms without special efforts, which makes it practically impossible to directly use them for calculating hardness. In this situation, machine-learning interatomic potentials (MLIPs) have direct indications for use. Here we use moment tensor potentials (MTP) to solve this issue [7,8]. These models are trained on the results of quantum mechanical calculations and reproduce the behavior of ab-initio models after that. Performance of MLIPs is comparable with empirical potentials, and the accuracy is close to that of quantum-mechanical models. Thus, we calculated the hardness for several known covalent materials, namely diamond, SiC etc, and obtained nanohardness for different crystallographic surfaces. Obtained data is consistent with numerous experimental results. References 1. Fischer-Cripps A.C. Analysis of Nanoindentation Test Data // Nanoindentation. New York, NY: Springer New York, 2011. P. 39–75. 2. Broitman E. The nature of the frictional force at the macro-, micro-, and nano-scales // Friction. 2014. Vol. 2, № 1. P. 40–46. 3. Chen X.-Q. et al. Modeling hardness of polycrystalline materials and bulk metallic glasses // Intermetallics. 2011. Vol. 19. P. 1275–1281. 4. Mazhnik E., Oganov A.R. A model of hardness and fracture toughness of solids // J. Appl. Phys. 2019. Vol. 126, № 12. P. 125109. 5. Oganov A.R., Lyakhov A.O. Towards the theory of hardness of materials // J. Superhard Mater. 2010. Vol. 32, № 3. P. 143–147. 6. Gao F. et al. Hardness of Covalent Crystals // Phys Rev Lett. 2003. Vol. 91. P. 015502–015506. 7. Shapeev A. Moment Tensor Potentials: A Class of Systematically Improvable Interatomic Potentials // Multiscale Model. Simul. 2016. Vol. 14, № 3. P. 1153–1173. 8. Podryabinkin E.V., Shapeev A.V. Active learning of linearly parametrized interatomic potentials // Comput. Mater. Sci. 2017. Vol. 140. P. 171–180.

Resume : In recent years, nanoindentation systems have been developed which can operate at ever higher temperatures in order to characterize the local mechanical properties and thermally activated mechanisms of high temperature materials such as nickel-based superalloys at their operating temperature. In this work a new ultra-high temperature nanoindentation system for testing at up to 1100 °C will be presented. The system is capable to perform indents from small scale up to large indentation depths due to the combination of a 1 N actuator and a high frame stiffness of > 106 N/m even at 1100 °C. Dynamic testing allows a continuous determination of the contact stiffness which is essential for determining the depth dependent material properties, like hardness and modulus. Low drift rates can be achieved by an independent tip and sample heating, the active actuator cooling ensures that the machine properties will not change during ultra-high temperature or long term testing. Operating the nanoindenter inside a scanning electron microscope equipped with a high temperature backscattered electron detector opens the possibility of in-situ observations, as high vacuum prevents oxidation effects. Tests were performed on fused silica, molybdenum and single crystalline nickel using a constant strain rate, strain rate jump as well as a step-load and hold creep method to show the capability of the system at high temperatures.

Resume : Complex-phase (CP) steels attract more attention from the automobile industry due to their better stretch-flangeability compared with dual-phase steels in the same strength level. The microstructure of CP steels normally consists of ferrite, martensite and various forms of bainite. We investigate a commercial hot-rolled complex-phase steel (CP800), which is characterized by high strength and excellent hole expansion ratio. The metallographic preparation of these microstructures for high resolution investigations is limited to a very smooth polish of the sample. The microstructure of CP800 is then characterized as a ferritic-bainitic matrix with the dispersion of hard martensite islands through the EBSD and EPMA combined method. The local dispersion of mechanical properties is then observed through accelerated property mapping (XPM) of a Hysitron TI980 Triboindenter equipped with a cube-corner indenter. Over fifty thousand indents are located in an 60*60 µm2 area. The high-resolution nanohardness profile coupled with electron microscopic results shows that isolated high nanohardness peaks are caused by either hard constituents (martensite or precipitations) or extreme fine grain clusters. Besides these few peaks, the majority of the microstructure exhibits homogeneous nanohardness distribution with only smooth gradients. This hardness distribution leads to homogeneous plasticity distribution during deformation and consequently prevents the strain localization and early crack initiation during hole expansion test. Based on the results of this study, the XPM and high-resolution nanohardness map are effective in identifying microstructural constituents and presenting the gradient of hardness distributions in complex-phase microstructure.

Resume : For developing performant medical devices the biomechanical adaptation play a key role for both, the bulk material (at macro scale) and for surfaces (at micro/nano scale). As concerning the interaction of living cells with artificial surfaces, cellular mechanosensitivity has direct effects on tissue structure and functions. In this work, surfaces archirectures, similar of the cancellous bone, obtained by self-nano-crystallization, were realized on a gum-alloy as substrate, Ti-31.7Nb-6.21Zr-1.4Fe-0.16O (wt. %) (TNZFO), using silicone masks with micrometric patterns/perforations, on which was applied a Nano Surface-Severe Plastic Deformation (NS-SPD) process. For finding the 3D suitable geometries of the perforations and their distribution on the masks, the Generative Design algorithms were developed using dedicated software tools (Rhinoceros). The masks were obtained by 3D printing using the Fused Filament Fabrication method. The NS-SPD involves the multidirectional repeated mechanical impacts on the sample surface with high velocity balls at high strain rates of approximately 102-103 s-1. Deeper compressive residual stresses are generated beneath the NS-SPD surface, creating a superficial self-nanocrystallized structure, on un-covered areas due to the small volume-multidirectional local plastic deformation. NS-SPD parameters have a direct effect on the plastically deformed micro-volumes at the surface, affecting the local micro-topography and the microstructure evolution.

Resume : Nanocomposite coatings have attracted attention from industries due to their enhanced mechanical properties, tribological performance, thermal stability and oxidation resistance. However, it is challenging to tailor the structure of nanocomposite coatings and further improve the properties to meet some special needs in applications with extreme environments. In this study, (Ti0.6Al0.4)1-xYxN multilayer coatings were fabricated by the physical vapour deposition (PVD) method. The influence of yttrium on the crystal structures, mechanical properties and high temperature oxidation resistance were investigated in detail [1]. In the PVD process, the period and yttrium concentration were controlled by varying substrate rotation speed and power applied on the yttrium target respectively, which could modify the coating hardness, adhesion, wear resistance and durability against oxidation. Results showed that for a fixed period, doping 2.4 at% yttrium into Ti-Al-N coatings delivered enhanced hardness and improved the oxidation resistance without deterioration of adhesion and wear resistance compared to yttrium-free Ti-Al-N. Such type of hard protective coatings have potential applications in areas that exploit high-speed machining, casting and hot-forming. [1] Jingxian Wang et al, Metallurgical and Materials Transactions A, 2017, 48A, 4097 – 4110.

Resume : In this study, we perform a quasi-continuous simulation of nanoimprint on a rough inclined surface. The effects of crystal orientation, surface roughness, and inclined plane on the contact and imprinting mechanics are studied using quasi-continuum simulations. These effects are investigated in terms of strain distribution and the stress–strain curve. The results show that the contact interference leads to the formation of the defects and slip inside the material. The number of defects increases with increasing imprinting interference. The internal stress of the material become more intense, the more seriously affected as the surface roughness of the contact increase. When embossing an inclined surface, the smaller the angle of inclination, the deeper the imprinting depth when larger imprinting variation occurs. Here proposed method of rough contact behavior can help not only to significantly reduce the cost of experiments but also help to explain the contact behavior of imprinted inclined plane.

Resume : High-aspect-ratio polymer pillars have been of great interest in wetting and dry adhesion due to their unique surface topography. Specifically, tunable dry adhesion as manifested in gecko foot hairs has been attributed to millions of hierarchical fibrillar structures on gecko toe pads, allowing for adhesive attachment and rapid detachment. Here, we report on the preparation of tapered nanorods from epoxy with various cross-sections and study the shape effect on normal and shear adhesion strength. Cone-shaped, stepwise, and pencil-like structures (300 nm in diameter at the base of the pillars and 1.1 μm in height), are prepared from epoxy resin templated by nanoporous anodic aluminum oxide (AAO) membranes. The adhesion force was measured by nanoindentation. It showed that depending on the shapes of the nanorods, the adhesion force significantly changes and the adhesion in normal and shear direction are quite different. To understand the deformation mechanism of the tapered nanorods and the resulting adhesion behaviors, we performed quantitative in-situ indentation experiments in SEM. We confirmed that different nanorods exhibited different deformation, depending on the load applied in the normal and shear direction. The results from in-situ indentation imaging corroborated well with finite element analysis and nanoindentation measurements, suggesting that the shape of the nanorods played a key role in mechanical response, thus, dry adhesion strength.

Resume : One of the most common methods to measure the elastic modulus and hardness of thin films is to use nanoindentation. In order to remove the influence of a substrate the well-known “10% rule of thumb” is utilized. This “rule” states that the elastic modulus and hardness can be taken at 10% of the film thickness with no or little influence from the substrate. While this guideline may hold true for some film-substrate systems (hard-on-soft or soft-on-hard) and film thicknesses (greater than 100 nm), it cannot and should not, be applied universally. It will be shown on single crystalline copper films on sapphire, grown by thermal evaporation (50, 100, and 300 nm thick) that the hardness can be evaluated but the elastic modulus cannot be properly measured when compared to bulk single crystal copper. It will be demonstrated that the elastic modulus is a long range property that is substantially influenced by the substrate even at indentations of 10% of the thickness. For example, using the initial Hertzian elastic portion of the load-displacement curve before a pop-in occurs does not allow for the elastic modulus of copper to be measured. The findings reveal that the 10% rule should not be applied to evaluating the elastic modulus of thin films.

Resume : We developed rubber-clay nano-composites that are environment-friendly and excellent in mechanical properties and fatigue life. 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. Mechanical tests on the developed material were performed at room temperature and aging condition and we verified that mechanical properties of the developed material such as tensile strength, elongation, and modulus change were superior to the existing material. Fatigue durability was evaluated througth we developed a new method that could fatigue life prediction of rubber parts in a short period in the initial stage. Fatigue life of rubber material estimated by the fatigue life 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 engine mounts made by developed material was longer than the existing material. In this study, advance of related technologies was achieved by constructing complete technologies including design, analysis, and estimation of rubber parts. We expect that these results will contribute to enhance performance and reliability of rubber parts.

Resume : The density and distribution of sulfur crosslinks between the polymer chains promoted by the vulcanization process, combined with the use of reinforcing fillers, affect the mechanical properties of rubber nanocomposites (NCs). Activators and accelerators are used to improve the vulcanization rate and among them ZnO is the most used activator, due to its highly kinetic and crosslinking efficiency. Since the catalytic activity of zinc has a central role in the formation of crosslinks, this study focuses on the effect that the structure and the localization of the catalytic site inside the polymer matrix have on the mechanical behavior of rubber materials. To do this, a novel efficient activator, Zn@SiO2 based on zinc single sites anchored onto the silica surface, was used as double function filler, acting as both activator and reinforcing filler. The microscopic structure and the mechanical behavior of rubber NCs cured with Zn@SiO2 were studied by performing TEM analysis, DMA (Dynamic mechanical analysis), uniaxial tensile and fracture tests, and compared to those of rubber NCs conventionally cured with ZnO particles. Regarding rubber NCs cured with Zn@SiO2, TEM analysis showed a higher crosslink density close to the silica particles, while the mechanical tests showed enhanced mechanical properties. The results suggest that localizing the catalytic sites in the rubber matrix could be a promising tool to tune the crosslink distribution and the mechanical properties of rubber NCs.

Resume : In this study, new organic-inorganic binder conversion process, which can be applied in 3D printing technique, was introduced for preparing the ceramic mold and core with improved mechanical strength using two organic binders with different chemical properties, and one inorganic binder. Green body was fabricated with a 3D printer and ceramic slurry prepared by mixing the starting powder and two organic binders (non-aqueous and aqueous polymers). The green body was immersed in water at 60℃ for 5 min to remove the aqueous polymer, leaving space for penetrating an inorganic binder in the green body. After removing the aqueous polymer, the sample was immersed in the inorganic binder solution composed of tetraethyl orthosilicate (TEOS, SiO2 precursor) and sodium methoxide (NaOMe, Na2O precursor), and then dried at 80℃ for 1h. The prepared sample was heat-treated at 1000℃ for 1h for converting the inorganic binder to glass phase. The application of the dual polymer increased the penetrating amount of the inorganic binder in the green body, and finally the strength of the ceramic mold and core increased after the heat treatment. The novel organic-inorganic binder conversion process in the 3D printing process could confirm the feasibility of manufacturing a ceramic mold and core with effective strength for investment casting.

Resume : Calcium magnesium alumina-silicate (CMAS) attack on thermal barrier coatings (TBCs) is one of the reasons for the early delamination of TBCs in jet engine and gas turbine in high temperature environments. The increased operating temperature results that the finer CMAS debris tends to adhere to the coating surface and form molten CMAS which eliminates the void spaces within the TBC. In this study, we evaluated the effects of CMAS on the microstructure evolution in yttria-stabilized zirconia (YSZ), lanthanum zirconate (LZ), and composite coating with a 50:50 volume ratio of YSZ and LZ (LZ–YSZ). YSZ was easily to be deteriorated by CMAS due to the de-stabilization of t’-YSZ. LZ showed the mitigated penetration of CMAS by reaction products (Ca2La8(SiO4)6O2, CaAl2Si2O8, and MgAl2O4), which originated from high reactivity between LZ and molten CMAS. The LZ–YSZ composite showed a denser microstructure in the surface, resulting in retarding the penetration of CMAS. Therefore, the TBC with the LZ–YSZ composite will prolong the lifetime performance and protect the further corrosive degradation of TBCs in high temperature environments. The infiltration behavior and the reaction mechanism of molten CMAS for the YSZ, LZ, and LZ–YSZ composite were discussed after CMAS attack for 5, 10, and 20 h at 1250 °C.

Resume : In this study, the applicability of the superalloy fabricated by additive manufacturing (i.e. 3D printing) was investigated in comparison to the conventionally forged superalloy. The microstructure and mechanical properties of the 3D printed Inconel 718 alloy before and after hot isostatic pressing (HIP) were characterized and compared to the conventionally forged alloy. The as-printed alloy showed lower mechanical properties in hardness and elastic modulus than the HIP-treated and forged alloy due to its large amounts of defects, showing a lot of pores at the melt pool boundary on the fractured surface after tensile test. The HIP-treated alloy showed denser microstructure with less pores, implying lower fracture toughness. The yield strength was almost doubled after HIP-treatment, while the tensile strength showed a slight increase. However, the elongation property was evidently deteriorated after HIP treatment. The plastic deformation of the HIP-treated alloy after yielding was markedly low, resulting in a rapid fracture. Also, the surface properties for the superalloy prepared by 3D printing were improved by applying the diffusion coating.

Resume : A single-crystal casting has been performed to improve the thermomechanical properties of turbine blade, which is the key component of gas turbine. Ceramic core employed in the single-crystal casting process should endure for a long time at casting temperatures above 1500 C because of direct contact with a molten metal. In the fabrication of the core applied to a conventional casting, it takes a lot of time and energy because it requires dozens of hours at heat treatment to develop strength in the core. In this work, an inorganic binder was applied to a conventional injection-molding method to produce a core having enough strength and completely elution property during the casting process. The starting powders for the core used a mixture of three kinds of fused silica with different particle sizes, zircon flour, and silicon carbide. The mixed ceramic powder was coated with a silica precursor, and then dried at 80 °C for 1 h. The dried powder was mixed with wax and injection-molded. The injected core was heat-treated at 1200 and 1500 C for 1 h. The additional heat treatment at 1550 C for 3 h was conducted to investigate the applicability of the core coated with the inorganic binder in the casting process. The inorganic binder-coated core with a superior firing strength had no cracks or surface defects after the heat treatment, compared with the core without the inorganic binder. In addition, the turbine blade of 150 mm length was well-cast as a single crystal and the internal core was completely eluted.

Resume : Metastable β-Ti alloys are known for their high specific strength and deep hardenability. These remarkable properties are influenced by the size, shape and volume fraction of α-phase precipitates. The refinement of α-phase precipitates due to their heterogeneous nucleation from precursor nanoscale isothermal ω-phase (ωiso) precipitates has led to improved mechanical properties for these alloys. Therefore, it is important to understand the precipitation kinetics of ωiso precipitates to precisely control and optimise the mechanical properties of the alloy. The precipitation of ωiso precipitates is influenced by the cooling rate used during the solution treatment of the alloy. Hence, a small-angle neutron scattering study was done to understand the precise pathway of ωiso precipitates transformation from the β-phase matrix in Ti-5Al-5Mo-5V-3Cr (wt%) alloy during the ageing of the sample at 300 °C up to 8 h. For solution treatment of the alloy, two cooling rates, air cooling and water quenching, were used. The SANS data analysis showed the presence of ellipsoidal ωiso during the ageing period. The alloy samples after air cooling showed the delayed nucleation of ωiso precipitates. The precise pathway of transformation was clarified, and evolved microstructure was quantified.

Resume : Abstract: The current issue related to global warming and limited conventional energy resources has pushed the energy sector to renewable energy resources. The photovoltaic technology–low carbon energy resource is one of the most used renewable energy technology. The largest market share (over 80 %) photovoltaic technology is based on silicon photovoltaic modules (Si-PV). The Si-PV power plants are in continuous growth for covering the high energy demands. Therefore the Si-PV waste amounts in the future decades are expected to become an environmental and economically issue. The forecasts in this regard estimates the PV waste amount to reach 1.957.099 t by 2038, therefore the PV recycling represents an important targets worldwide. This paper investigates the durability of Si-PV composite materials especially regarding their stability in water, UV radiations and saline environment. The paper continues a previous research on Si-PV recycling in novel composite materials with waste rubber and high density polyethylene (HDPE). That work concluded the possibility of recycling up to 35-40 wt% Si-PV module wafers in all-waste polymer composites. The mechanical performance of the all-waste composites were measured in terms of tensile, compression and impact strength before and after ageing tests. The interface properties before and after ageing tests were determined by FTIR analysis, contact angle measurements and by SEM investigation. The crystalline structure was determined by X-ray diffraction (XRD). After water immersion the best combination of tensile, compression and impact strength was noticed for 35 wt% Si-PV composites 190 0C processed. The mechanical characteristics before water immersion were tensile strength of 2.65 MPa; compressive strength of 210.40 MPa; impact resistance of 44.70 kJ/m2 and after water immersion: tensile strength of 2.85 MPa; compression strength of 208.17 MPa and the impact strength of 42.97 kJ/m2. The best water stability for 40 wt% Si-PV composites was noticed for that 210 0C processed with the following mechanical characteristics-before water immersion: tensile strength of 2.86 MPa; compressive strength of 198.55 MPa and impact strength of 36.05 kJ/m2 and after water immersion: 2.89 MPa; 196.18 MPa and impact strength of 48.98 kJ/m2. Acknowledgements: This work was supported by a grant of Ministery of Research and Innovation, CNCS - UEFISCDI, project number PN-III-P1-1.1-PD-2016-0286, within PNCDI III, ctr. 101/02.05.2018.

Resume : Polyoxometalates (POMs), especially those containing vanadium are of great interest for a broad range of applications due to their structural diversity and tunable electronic properties. The opportunity of switching between multiple redox states without significant changes of the metal–oxo skeleton architecture makes them suitable for usage as molecular redox-active units in resistive switching memory cells. One prerequisite for the integration of POMs into nanoelectronic devices is the investigation of their adsorption behavior and the associated chemical processes on conductive substrates. Therefore we used the droplet deposition from organic solution of different organically derivatized Wells-Dawson-type POMs on HOPG and Au(111). We characterized the adsorption behavior with surface-sensitive methods such as scanning tunneling microscopy (STM), X-ray photoelectron microscopy (XPS) and grazing-incidence small-angle X-ray scattering (GISAXS). Additionally we investigated the influence of an increasing water amount in solution with SAXS and observed that it affects crucially the surface adsorption.

Resume : The stretchable strain sensor is an essential component in wearable electronics and demanded in recent years that can be used for personalized healthcare monitoring, movement detection, human-machine interfaces, soft robotics and other several areas. Therefore, strain sensors with tunable electromechanical performance in terms of stretchability and sensitivity are necessary to meet different mechanical deformations and various sensing ranges from such a broad application. Large research works have been done based on different mechanisms of stretchable strain sensors. Among them, crack-based strain sensors have attracted much attention. Although crack formation in material and structure is often regarded as undesired process to be avoided, researchers played with crack initiation and propagation and were able to fabricate stretchable strain sensors with different stretchability and sensitivity. However, previous works only investigated the crack patterns in the strain sensor with a uniform flat structure, we are curious about the crack pattern in a gradient structure. Herein, the purpose of this work is to study the crack pattern in the strain sensor with a thickness-gradient structure and evaluate its electromechanical performance. A thickness-gradient conductive thin film on the elastic substrate was fabricated via thermal evaporation at various tilted angles ranging from 0deg to 75deg. Thin films with different gradient-thickness were verified by AFM. The electromechanical performance of stretchable strain sensors were characterized by a semiconductor characterization system and mechanical tester. The crack patterns were observed by SEM and conformal microscopes. We obtained thickness-gradient strain sensor (gradient-thickness = 20%) with high sensitivity (GF=1400) at stain ( ɛ =20%) which was much higher than that of the uniform flat thickness (gradient-thickness= 0%, GF=100 at strain ɛ =20% ). We also found the electromechanical performance of strain sensors can be tuned by changing the percentage of thickness-gradient. Besides, the crack morphologies were also different by stretching the thickness-gradient strain sensors in the longitudinal and the transverse direction which cannot be observed in uniform-thickness strain sensors. The achievement of this work was to investigate the difference of crack patterns in thickness-gradient strain sensors and the uniform-thickness strain sensors, studied the mechanism and proposed a new strategy to fabricate strain sensors with high sensitivity and stretchability by using gradient structure through a simple method.

Resume : Indentation resulting in a permanent impression injects an amount of plastic strain that requires the creation of geometrically necessary dislocations. The applied indentation stress required is a function of the combination of both test and material length scales. Variation of indent size, therefore, can be used to determine the effective plastic zone size and dislocation density. Stacking Fault Energy is a lattice property dependent on the bond strength, coordination (lattice type) and lattice spacing (and so burgers vector). In high SFE materials, it is less probable for dislocations to move off the slip plane or form partials and so the probability of dislocations tangling and pinning is reduced leading to lower work hardening. It is expected, therefore that dislocations in high SFE materials will have longer mean free paths and larger plastic zones, and this will lead to lower dislocation densities and longer pinning length scales. Very little experimental validation of this exists. The success of equation 1 in describing hardness and uniaxial tensile tests and scratch test results suggests that this could be used to validate this expectation. We, therefore, have obtained a large number of different metal single crystals with stacking fault energies ranging from ~20 mJ/m2 to ~200 mJ/m2 . We performed Berkovich indentations into these single crystals to determine mean indentation pressure as a function of indent size and have determined the parameters k1 and k3√ρs in equation 1 (for a single crystal k2/d = zero). From this data, the relative size of plastic zone and the relative fraction of dislocations that are mobile were calculated after Hou & Jennett (2012). Preliminary results indicate that, as SFE increases, k1 does decrease, i.e. the plastic zone gets bigger and k3√ρs also decreases, I,e, separation of pinning dislocations increases. We intend to validate these estimates of plastic zone size using FIB cross-section and to investigate the effects of indenter geometry (strain field distribution), lattice type (FCC vs BCC) and crystal orientation (100) vs. (110) vs. (111) in selected materials.

Resume : High entropy alloys are a new class of alloys which consists of five or more major element in the range of 5-35 at.%. The ever increasing demands for new structural applications, high entropy alloys (HEA) are transpire as a new paradigm shift in material society. Nano-eutectic HEAs exhibit superior mechanical properties over the single-phase high entropy alloys. The optimum combination of strength and ductility are the key features of eutectic HEAs. In this work, the novel CoCrFeNiTax (0.2 ≤ x ≤ 0.5) HEAs with eutectic microstructure were designed and cast by arc melting. The phase composition, mechanical properties and stability of these alloys were investigated. The evolved microstructure in all the HEAs consist of eutectic lamellae with alternating α (γ-Ni)-FCC phase and β (Co2Ta)-type Laves phase. It was observed that x= 0.2, and 0.5 alloys contains eutectic microstructure along with proeutectic phase and at x= 0.4 homogenous eutectic microstructure was obtained. The CoCrFeNiTa0.4 HEA with the eutectic microstructure exhibited a relatively high combination of yield strength (~1820 MPa) and ductility (~21 %). Further, analysis regarding the eutectic phase stability in terms of different thermodynamic and electronic parameters considering the synergistic effect atomic size difference (δr), mixing enthalpy (ΔHmix) and valence electron concentration (VEC) were carried out. Also, from literature EHEAs comprising of FCC /BCC solid solution phases and intermetallic compound/topologically closed packed (TCP) phases have been surveyed thoroughly. The assessment predict the existence of stable eutectic/near eutectic phases in all surveyed and present EHEAs when −18 ≤ ΔHmix ≤ −6, 6 ≤ VEC ≤ 8.5 and δr > 3%. References: [1] B. Chanda, J. Das, Composition Dependence on the Evolution of Nanoeutectic in CoCrFeNiNbx (0.45≤ x ≤ 0.65) High Entropy Alloys, Adv. Eng. Mater., 1700908 (2017)1-9. [2] B. Chanda, J. Das, An assessment on the stability of the eutectic phases in high entropy alloys. J. Alloys Compd., 798 (2019) 163-173.

Resume : Physical properties of thin-film oxides, such as electrical transport and crystal structures, can be controlled by the strain induced by the lattice mismatch with a substrate. Therefore, it is an important issue in the fabrication of thin-film oxide-based devices to control the strained condition, for example, by changing a film thickness and crystallographic orientation of substrate. Meanwhile, microcracks in a film can be formed accompanied by a local strain relaxation and impede a current flow. In the case of VO2, electrical and possibly structural phases can be distributed inhomogeneously during an insulator-to-metal transition around room temperature. In particular, the VO2 film grown on TiO2(001) substrate can have microcracks and make position-dependent electrical and structural phases. In this presentation, we discuss the effect of the cracks in the strain relaxation along the lateral directions by using nano infrared imaging technique. The variations of infrared scattering amplitude and phase are clearly observed and they will be discussed with the distribution of structural and electrical properties.

Resume : Graphene aerogel is a three-dimensional porous form of graphene with a high surface-to-volume ratio. Graphene aerogels have attracted a significant amount of interest in recent years due to their unique mechanical and electrical properties that show great potential for numerous applications in engineering, electrochemistry, and biology. Here, we investigate the mechanical properties of graphene aerogels prepared via hydrothermal reduction of graphene oxide. The as-fabricated graphene aerogels, however, contain a lot of defects and oxygen groups which cause poor elastic properties. For this reason, we have thermally annealed the samples in a furnace in an inert gas atmosphere. The high-temperature annealing enabled a complete oxygen removal from the samples, resulting in a significant improvement of the elasticity of the graphene aerogels. The annealed graphene aerogels demonstrate superelastic compression up to a GPa pressure regime. To better understand the deformation mechanism in the samples we have performed in situ SEM measurements that visualized the deformation of individual graphene pores. Based on the observation we developed a model that is able to describe the stress-strain curves of the graphene aerogels. Our study provides new insights into the superelastic behavior and deformation mechanism of the porous graphene aerogels.

Resume : Mechanocromic polymers which produce an optical signal in response to a mechanical force, hold the promise to act as molecular mechanosensors, which can be used to image stress or strain. Most mechanocromic systems relie on homogeneous or heterogeneous bond scission and therefore show intrinsic limitations that are typically related to their two-state nature and the associated barrier between the two forms. A barrierless, simple and robust mechanochromic system still remains to be designed. Here we reported the new concept of donor-acceptor (D/A) torsional springs for force sensing and stress imaging. We investigated the force dependent D/A properties of the diketopyrrolopyrole (DPP) polymer with DFT and TDDFT. We found that these molecules should indeed be usable for force sensing and stress imaging in that their optical absorption properties change reversibly under the application of force. This can be traced back to change in the electronic properties due to changes in the dihedral angle and therefore in the coupling between the donor acceptor parts in the molecules. Application of external forces on DPP leads to force-induce planarization and a resulting shift of the optical spectra to longer wavelengths. Increased steric hindrance in ortho-DPP leads to a larger dihedral angle and larger optical shifts, making this polymer a more effective force sensor. Our mechanochromic system is barrierless could open opportunities and application of unprecedented extent.

Resume : We have constructed a graphene nanoelectromechanical resonator array systems and studied its basic electromechanical properties. Drum-like suspended graphene structures with various diameter were fabricated using micro contact transfer method combined with conventional lithography processes. Mechanical resonance frequencies of the suspended graphene radio were measured by laser interferometry technique. The resonance frequency of graphene radio has been shown in megahertz range, which can be tuned by applying electromechanical strain to the resonator. Duffing type mechanical oscillation behavior was observed as the driving force of graphene resonator was increased. Non-linear resonance behavior of graphene has been investigated for sensing a small variation of its mechanical resonance frequencies. Graphene based nanoelectromechanical radio was demonstrated as a proof of concept for measuring low frequency variation using non-linear behavior of electromechanical resonator. We will introduce our plan to estimate the mass change of single-molecule protein on graphene resonators with a sensitivity of less than a few daltons by measuring its resonance frequency variation.

Resume : Films, which are fabricated on metallic materials, exhibit a different kinds of function such as corrosion resistance, wear resistance and surface hardening. In this study, the novel anti-corrosive film was fabricated on Al alloy substrates using steam. The resultant film was characterized and evaluated by electrochemical measurements. In addition, the mechanical property of the thin-film was evaluated by tensile test and nanoindentation system. FE-SEM images of the film surfaces showed that plate-like or block type nanocrystals, which were densely formed over the entire surface. XRD patterns indicated that the film was composed mainly of AlO(OH) crystals. The potentiodynamic polarization curves revealed that the corrosion current density of the film-coated substrates significantly decreased, and that the pitting corrosion was completely suppressed. The film was also fabricated on the plate-like tensile test sample. The crack was initiated on the surface of the film when the introduced strain reached 2%, however, the delamination of the film was not observed even if the tensile test sample was fractured. The fracture strain of 2% was much higher compered to aluminum oxide, confirmed that the strong adhesion between the film and Al alloy substrate. Acknowledgement: This research was supported by JST under Program on Open Innovation Platform with Enterprises, Research Institute and Academia (OPERA) (No. 18072116), JSPS KAKENHI, Grant-in-Aid for Scientific Research(B) (No. 19H02482), and the light metal educational foundation, Inc.

Resume : Thin films of selected oxides deposited by Atomic Layer Deposition (ALD) are intensively studied for applications in electronics (including transparent electronics), optoelectronics (transparent electrodes), photovoltaics (mostly as transparent electrodes and passivation layers), and also as a barrier layers (organic photovoltaics, organic diodes). For some of the above mentioned applications thin films of ZnO and ZnO:Al (AZO) are deposited on transparent substrates, such as glass or foils. In this work we report on advantageous properties of ZnO-based films deposited on a glass substrate using low temperature ALD process. Hardness and Young modulus are determined and compared with the ones reported for similar films deposited by a sputtering. The results obtained - hardness H (8220 ± 786 GPa (for AZO); 9182 ± 815 GPa (for ZnO)) and Young modulus E (87 ± 7 GPa (for AZO); 93 ± 5 GPa (for ZnO)) confirm advantageous properties of the ALD-deposited films. For scratching tests we used MCT system by the CSEM Instruments, Switzerland. First scratches were observed when applying a 6N force for ZnO and 3,2 N force in the case of AZO. Even at larger forces, when first cracks occurred, no traces of delamination are found, confirming a very good adhesion of deposited films. New results for Al2O3 and HfO2 thin layers (used for gate oxides applications) will be discussed. These results indicate better mechanical properties of HfO2 films, as compared to Al2O3. This work was partially supported by the National Centre for Research and Development TECHMATSTRATEG1/347012/NCBR/2017, and AGH grant number 16.16.230.434, 16.16.130.942.

Resume : Aluminium foam is a promising material for light weight applications, because of its high specific strength and good damping capacity. In the present study, aluminium foam has been developed using cenosphere as space holder by spray forming route. The porous structure has been characterised by field emission electron microscopy (FESEM) to observe the microstructure, micro-computer tomography (µ-CT) to find the distribution of porosity throughout the matrix. The localised mechanical properties of the matrix and interface have been evaluated by Nano indentation (NI) study. The microstructure of the foam consists of presence of cenosphere dispersed in grain refined aluminium matrix. The density of the foam is 1.896 g/cm3 as compared to 2.7 gm/cm3 of commercially pure aluminium. The µ-CT results show that the spray formed foam introduces microporosity up to 8%, and the pore has spherical in nature. The nanoindentation results show that the addition of cenosphere enhanced nanohardness from 538.28±24.34 MPa (for as-received aluminium) to 672.86±54.5 MPa (for aluminium foam) and increases the energy absorption from 86.48 nJ (for as-received aluminium) to 93.70 nJ (for aluminium foam) of foam, however; it decreases elastic modulus of foam from 58 GPa (for as-received aluminium) to 46 GPa (for aluminium foam). Key words: Aluminium foam; porosity; microcomputer tomography (µ-CT); Nano-Indentation; spray forming.

Resume : There has been a vast effort towards the characterization of materials behavior at the nanoscale. Mechanical characterization of nanowires (NWs) is mainly carried out through tensile or bending tests. The interpretation of test data via comprehensive modeling is equally important to address the current challenge of understanding the size-dependent behavior of NWs. For instance, modeling of NW deformation at the atomistic level predicts size-dependent elastic properties for NWs only with cross-sectional dimensions less than ~10 nm. We undertake such a study by investigating the elastic modulus of Si NWs with a critical dimension of 28 nm via three-point bending test. Testing effort is accompanied by a comprehensive non-linear model of NW bending, including surface and intrinsic stress effects. The model considers all relevant effects including large deformations, intrinsic stresses and surface effect. The presence of intrinsic stresses is verified through scanning X-ray diffraction microscopy, while native oxide is characterized through high-resolution transmission electron microscopy. We examine the surface effect by implementing the surface stress and surface elastic modulus through atomistic simulations coupled with the continuum model. Native oxide covering the silicon NW is modeled and two independent surface parameters are quantified verifying that size effect is postulated to be primarily a result of differences in the NW surface and core elastic moduli. A deeper understanding of the surface effect on NW mechanical properties is critical for the control and correct utilization of such nanoscale phenomena.

Resume : Superelastic alloys are highly regarded for their unique ability to recover large deformations through reversible stress-induced phase transformations. Although NiTi alloy is the most well-known superelastic alloys, some Ni-free metastable ?-titanium alloys also exhibit interesting superelastic performances [1-2]. Due to their functional and mechanical properties, these alloys are attractive for a variety of applications such as actuators, sensors, dampers, sealing, biomedical tools and implantable devices. Their macroscopic deformation behavior including their bulk superelastic response has been extensively studied using macroscopic techniques (tensile and compression experiments) [3]. Much less work exists on the occurrence of a microscopic superelastic effect in a confined geometry during micro- and nano-scale deformations. With the increasing demand for small-scale applications, characterization of the local mechanical behavior is an important issue of extensive interest. Nanoindentation is conducted at the grain scale in Ni-free polycrystalline metastable ?-Ti alloys to study the effect of crystallographic orientations on elastic (elastic modulus), superelastic (strain recovery) and plastic (hardness) properties [4]. Results reflect the large scale mechanical anisotropy measured in bulk single crystals from macroscopic investigations [3]. Nanoindentation, especially using spherical indenters, is useful for probing superelastic response and mechanical anisotropy during nanoscale deformations. [1] Castany P., Ramarolahy A., Prima F., Laheurte P., Curfsd C., Gloriant T., In situ synchrotron X-ray diffraction study of the martensitic transformation in superelastic Ti-24Nb-0.5N and Ti-24Nb-0.5O alloys, Acta Mater. 88, 102?111 (2015). https://doi.org/10.1016/j.actamat.2015.01.014 [2] Ijaz M.F., Laillé D., Héraud L., Gordin D.M., Castany P., Gloriant T., Design of a novel superelastic Ti-23Hf-3Mo-4Sn biomedical alloy combining low modulus, high strength and large recovery strain, Materials Letters 177, 39-41 (2016). https://doi.org/10.1016/j.matlet.2016.04.184 [3] Zhang Y. W., Li S.J., Obbard E.G., Wang H., Wang S.C., Hao Y.L., Yang R., Elastic properties of Ti?24Nb?4Zr?8Sn single crystals with bcc crystal structure, Acta Mater., 59, 3081?3090 (2011). https://doi.org/10.1016/j.actamat.2011.01.048 [4] Jabir H., Fillon A., Castany P., Gloriant T., ?Crystallographic orientation dependence of mechanical properties in the superelastic Ti-24Nb-4Zr-8Sn alloy?, Phys. Rev. Mat. 3, 063608 (2019), DOI: 10.1103/PhysRevMaterials.3.063608

Resume : The precise characterization of micro- and nanomechanical material properties is up to now a field of material scientists with fundamental knowledge of contact mechanics. Failure to follow the device-related preparations for the actual measurement can have a major impact on the measurement results and make a quantitative statement often impossible. As digitalization progresses, the need for micro and nanomechanical sensors and actuators increases. This means that the need for quality-assuring analysis technology is growing in order to ensure that such components are of sufficient quality. This also increases the need to adapt the measurement method to the industrial environment. The expertise for the measuring process must pass from the user to the machine. Therefore, the entire chain of effects of the measuring process - the preparation of the measuring device, the sample preparation, the implementation of the measurement and the processing of the measured values ​​must be designed to be variable and error-free. SURFACE has taken with its sm@rt system the first step in this direction. Thanks to its modularity, the new sensor concept can be adapted to the application area - wide load range, large linearity range, high resolution, high measurement quality. All this is paired with very small sensor dimensions. Microcontroller systems with the latest signal processors and scalable software / hardware architecture are also used on the control side. The operating mode of the system is adapted to the operator's know-how by means of variable user levels and the measurement processing and preparation is carried out accordingly, so that the measurement process is finally carried out as a “one button action”. The evaluation of the current measurement results can then be compared and checked using stored material data. The deviations of the current data from the stored material data can then serve as a correction or evaluation of the local measurement. We will show examples to demonstrate the functionality of the system.

Resume : Despite the enormous potential of two-dimensional nanomaterials (2D-NMs) using them in 3D systems has proven challenging. This can be tackled by using the process of wet-spinning poly(vinyl alcohol) (PVA) fibres. 2D-NMs can be integrated into polymeric fibres to exploit their properties on a bulk scale. However, issues remain with these nanocomposite fibres such as controlling NM flake alignment and orientation. To overcome this the formation of striations in PVA fibres under strain has been investigated. Striations are microscopic fibrils that form perpendicular to the direction strain on the PVA fibre. Limited literature exists on PVA striations, so a model is proposed here where an extending amorphous region pulls lamella crystals into horizontal alignment within the semi-crystalline PVA. It is hypothesised that with control of striations it is possible to manipulate the 2D-NMs incorporated in the fibre. The nanomaterial used (graphene, graphene oxide, black phosphorous and molybdenum disulphide), weight fraction and NM flake size have been varied to determine the effects on mechanical properties, fibre morphology and nanomaterial alignment. Polarised Raman spectroscopy was used to investigate degree of NM alignment within fibres before and after stretching to increasing levels of strain. This work helps to determine the compatibility of various 2D-NMs in PVA fibres and whether the strain induced formation of striations can be used to impart control over the NMs within.

Resume : In the present work, the thermal shock resistance was carried out for Y2.5Nd0.5Al5O12 - x SiC composites (x = 10, 20, 30, 40 vol.%). The composites were previously cracked using a diamond pyramid of a hardness tester. To simulate the thermal shock, composites were heated in a muffle furnace and abruptly cooled in air. Heating was carried out in the temperature range from 200 to 1000 °C, in increments of 100 °C, for 20 minutes at each stage. The length of the cracks was measured using optical microscopy. It follows from the obtained data that the crack length practically did not change with increasing heating temperature and did not depend on the content of silicon carbide.

Resume : Musculoskeletal conditions (i.e. osteoarthritis) and trauma affect soft tissues, inducing for example articular cartilage deterioration and ligament/tendon injury. Therefore, the need of understanding their morphology and mechanics is of paramount importance to design new biomaterials and treatments. High-resolution X-ray computed tomography (XCT) offers accurate resolution to visualise and quantify morphology in mineralised tissues such as bone. However, XCT setups are typically unable to resolve weakly absorbing soft tissues (i.e. cartilage, ligaments) with sufficient contrast unless stained (i.e. PTA), which can significantly alter their morphology and mechanical properties if not sufficiently developed [1]. This study will report preliminary results on the use of less invasive staining [2] and phase-contrast XCT imaging to resolve features in soft tissues, specifically chondrocyte lacunae in articular cartilage to enable digital volume correlation (DVC) analysis. DVC was performed (zero-strain test) on a fresh cylindrical bone-cartilage plug extracted from a bovine medial tibial plateau in the longitudinal direction, as well as a whole stained mouse tibial plateau. XCT of the bone-cartilage interface in the bovine plug were obtained in phase contrast (Zeiss Versa 510, 2 micrometers voxel size) and stained mouse tibial plateau (GE Nanotom M, 1.12 micrometers voxel size) in absorption. A local multi-pass DVC approach (DaVis, LaVision) with a final sub-volume size of 40 voxels for the bovine and 80 voxels for the mouse specimens was used. A good DVC correlation pattern was found in the articular cartilage. For the phase-contrast images regions rich of chondrocyte lacunae enabled a better correlation, whereas in the stained tibial plateau high correlation in the articular cartilage was achieved. Standard deviation of the error (SDER) of the strain components for both bovine and mice specimens was always <100 microstrains. References: [1] Disney et al. 2017. Sci. Rep. DOI: 10.1038/s41598-017-16354-w; [2] Kerckhofs et al. 2018. Biomaterials. 159, 1–12

Resume : During the time, recycled nylon powder attracted the attention due to their multiple applications and can be used as raw material in different industries. Among these, it can be remarked textile, food, music industry, personal care etc. Since the physical properties (e.g. elastic constants or mechanical resistance) of the material are closely related to the applications, an accurate characterization of this become mandatory. Powder was collected from the 3D printer’s tank and the anti-ex vacuum cleaner. After decontamination, further processing is done by melting and solidifying the polymer. Firstly, this paper presents the characterization of the material by means of Fourier-Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscopy (SEM), energy dispersion X-ray spectroscopy (EDX), X-ray diffraction (XRD). Secondly, the mechanical characterization of recycled nylon powder (PA12) is discussed. In this sense, resistance tests of the material were performed for each melting- solidification cycle through two methods. After more cycles, it was observed that the mechanical resistance decreases, and the reusing become possible only in a few areas.

Resume : Deformation behavior of a new series of plasma sprayed Fe-based (Fe-10Cr-4B-4P-2C wt.%) amorphous/nanocrystalline composite coating was investigated by carrying out multi-scale indentation and nano-tribological tests. The coating with optimized spraying parameters exhibited excellent mechanical properties in terms of higher nanohardness, lower sliding friction coefficient (COF) and sliding wear. Decrease in both coefficient of friction and wear rate of the coatings with increasing plasma power was observed from dry sliding wear test, which also helped in understanding the wear mechanism in the coatings. Nanoindentation studies were carried out at various peak loads and loading rates to understand surface deformation behavior of the coating. The coating displayed loading rate dependent “pop-in” events on load-displacement curves and strain sensitivity model was applied to understand the effect of free volume concentration. Further investigation of coating deformation response in a tribological contact was evaluated by nano-scratch, which exhibited shear band formation along the nano-wear track.

Resume : Thermal barrier coatings (TBCs) consisting of top coat, bond coat and thermally grown oxide are used to provide thermal insulation for the hot sections of aerospace engines and other high temperature components. Mechanical properties of TBCs are highly influenced by the specific microstructural characteristics of its individual layers. Of all the components, the ceramic top coat layer is in direct exposure to the elevated temperatures of the engine and hence plays a major role in deciding the overall life of TBCs. The aim of the present study is to evaluate and comprehend the tensile behaviour of the most widely used top coat material, plasma sprayed ZrO2 7wt.% Y2O3 (7YSZ), of ~300µm thickness. An in situ uniaxial tensile setup was developed in conjunction with high resolution optical microscopy to characterize the YSZ freestanding thin films. Strains upto an accuracy of 0.01% were measured using digital image correlation technique which provided major insights into the deformation and failure of these coatings. The tensile stress vs. strain curves of the coatings exhibited a non-linear behavior, with an elastic modulus of 12±4 GPa and failure strength of ~16±4 MPa. Successive unloading-reloading experiments were performed at different stress levels to deconvolute the elastic behavior of these brittle ceramics. The role of microstructural features like crack density and porosity on the fracture of these films was observed using fractographs. Hence, a finite element based numerical model replicating the non-linear tensile behaviour including the influence of microcrack density and porosity on tensile stress was developed. The observations in this study experimentally strengthen the proposed theoretical micromechanisms for the failure of YSZs in literature. This study also enables the prediction of probability of failure of YSZ using Weibull parameters developed herein. Keywords: Yttria Stabilized Zirconia; Insitu tensile properties; Digital image correlation; Fractography; Finite element method.

Resume : Nanoindentation of ion-irradiated materials has received considerable attraction because of the potential capability to efficiently derive the effect of displacement damage on the hardness. In order to reach this goal, several assumptions have to be made within the analysis of the measured nanoindentation response. These include the shape and size of the indentation plastic zone, the separation of the irradiation effect from the indentation size effect and the type of superposition of individual hardness contributions. The aim of the study is to experimentally verify given sets of assumptions by way of variations of the ion energy. To this end, several Fe-Cr-based alloys were exposed to irradiations with Fe ions of energies between 1 and 8 MeV and the nanoindentation response was measured as function of the contact depth. The same procedure of analysis including assumptions was applied to all irradiations. The presentation will show, to what extent the results of the analysis, namely hardness versus displacement damage, depend on the ion energy. As a criterion, they should be invariant against variations of the ion energy.

Resume : Nanotwinned metals display a remarkable strength due to the high effectiveness of twin boundaries as obstacles to dislocation motion. In order to further characterize these interactions, micropillars containing single coherent twin boundaries with different orientations were compressed with a flat punch and subsequently investigated in the scanning electron microscope. The crystal orientations for compression were selected to activate different slip modes. The aim is to probe the different barrier effects that can act on gliding dislocations. The investigations concentrated on copper and α-brass. The latter is a low stacking-fault energy alloy exhibiting a high density of recrystallization twins. Coherent twin boundaries were selected from an EBSD orientation mapping of the sample and oriented by means of a custom sample holder. FIB-milling at these interfaces yielded micropillar samples containing a single twin boundary. Single crystal reference samples were obtained from the bulk of the grain located on both sides of the twin boundary. The microcompression tests enabled the quantification of the influence of the twin boundary barrier on the strength of each sample. The tests evidenced a strong dependency of the strength of the sample on crystal orientation and stacking-fault energy. The activated glide systems were subsequently identified from slip trace analysis and STEM mapping of lamellas obtained by FIB lift-out from the bulk of the tested micropillars.

Resume : Lamellar eutectic NiAl-Cr(Mo) alloys show an increase of the fracture toughness based on different toughening mechanisms. These mechanisms result from the fibrous or lamellar microstructure of the two constituting phases α-Cr(Mo) and β-NiAl. However, the influence of the fracture toughness of the individual phases and the evolution from early crack growth to the toughening mechanisms has not yet been systematically studied. Here bending tests on FIB-notched microcantilever beams are used to characterize the small scale fracture properties. The micromechanical investigations revealed that the fracture toughness of the α-Cr(Mo) phase (7.5-9.1 MPa√m) is much higher than the fracture toughness of β-NiAl (2.2-2.9 MPa√m). Larger cantilevers in the crack arresting orientation had an enhanced fracture toughness with up to 14.4 MPa√m, which is lower than that one of bulk lamellar samples. This is attributed to the small interaction volume which hinders the full exploitation of potential extrinsic toughening mechanisms. Further results of these alloys will be discussed in the paper.

Resume : This work shows the applicability of direct observation of metal cutting (down to sub-micron scales) combined with the measurement of the cutting force in-operando in an electron microscope (SEM). A prototypic device to perform linear cutting inside the vacuum chamber of an SEM has been designed, constructed and utilized to study intimate details of the machining process. Experiments on in-situ machining have shown that all the main structural features of deformation during cutting, such as the material’s shear zones, are preserved in the sub-micrometer cutting regime. A few peculiar features of the cutting process have been noticed by in-operando microscopic observation: It has been observed that before the nucleation of any shear band in front of the tool, the cutting force is ~50% higher as compared to the shearing steady state. Additionally it was observed that the local thickness of the area deformed under the tool showed a clear dependence of the underlying grain orientation. The process of machining at small scales typically leads to increasing cutting energies, well over the values found on the macro scale. In this work, availability of extremely small feeds and cutting tool apex radii in SEM micromachining allowed to analyze and discriminate the different sources for the size effect on the cutting energy. A model that discriminates between the different sources behind the increase of cutting energy has been developed which accurately predicts experimental values. It has been shown that at an industrially relevant range of feeds and tool radii, the size effect is determined primarily by geometrical factors, while at a sub-micrometer range the contribution of increasing geometrically necessary dislocation density becomes relevant.

Resume : High temperature applications require structural materials that can provide the necessary high temperature stability and corrosion resistance. While ceramics are highly suitable candidates in that regard, their use in structural components is often restricted due to their intrinsic brittleness. MAX phases, a group of ternary carbides that show unusually good damage tolerance, are a possible basis for the design of tougher ceramics. The origin of this toughness is believed to lie in the layered crystal structure of the hexagonal MAX phases, and it is believed that on this basis tougher ceramics can be designed using other structures also. The link between enhanced toughness and layered crystal structure can be explained by electron density shifts that occur between the atomic layers. These affect the local elastic properties, effectively easing the onset of plastic flow via dislocation movement parallel to the layers. The extent of these electron density shifts, and thus improved toughness, is linked to the electronegativity difference between the layers, and can therefore be adjusted via the layer composition. In the current study, we demonstrate how changes in layer composition can affect the flow behaviour in the form of basal plane slip in MAX phases. Testing of different MAX phases is carried out in the form of single crystal micropillar compression with digital image correlation, and a method is presented that allows for a more consistent extraction of the flow stresses. It is shown that the differences in flow stresses due to the above effect can vary significantly in MAX phases. Based on this outcome, nanoindentation was carried out in different components of the layered fcc Th6Mn23 structure type to demonstrate how the effect might take shape in other crystal systems.

Resume : The enhancement of the mechanical properties of metals by graphene additions has been demonstrated over the last decade for surface applications to mitigate the impact of tribological loadings or for strengthening purposes through bulk dispersion. This research is investigated the effect of the presence of a single layer graphene grown on Cu on the development of the contact plasticity for two configurations: one with graphene at the surface, the other with graphene sandwiched under a 100nm thick Cu cap layer. As a matter of fact, a film of copper (deposited on a quartz wafer) is the substrate used in the CVD process for graphene production, there is no need for transferring graphene which avoids any possible artifacts. Moreover, the adhesion between CVD-grown graphene and the underlying Cu film is larger than transferred graphene, since during transfer, wrinkles and ripples may form, thus weakening the interaction between graphene and the substrate. For the first configuration, nanoindentation under both displacement and load control conditions show earlier and shorter pop-in excursions compared to systems without graphene. The presence of graphene caused a marked effect on the indentation response in the second configuration as well, even larger than in the first one. Atomic force microscopy reveals much smoother pile-ups with no slip traces when graphene layer is present on the surface. In order to understand better the root causes of the observed effects on the plastic flow, transmission electron microscopy is used to compare samples after nanoindentation in terms of dislocation structures. Furthermore, 3D discrete dislocation dynamics (DDD) simulations are conducted to unravel more quantitatively how dislocations interact and multiply under the indent with and without the presence of graphene. These results provide a more quantitative understanding of the impact of graphene on dislocation mechanisms for both surface and bulk applications.

Resume : The nanolaminate (NL) systems, highly applauded in recent literature for their extreme strength, often suffer from a lack of ductility, with a negative impact on the transfer to engineering applications. There are only very few examples of NL with alternating crystalline layers, like the Cu/Nb system, in which the crystallography and the structural properties of the interfaces have been successfully tailored in order to improve the plastic deformability without significantly sacrificing the high strength. Efforts to ‘‘ductilize’’ another class of NLs made by alternating crystal/amorphous layers are even more rare, involving only attempts to tune the layer thickness in order to suppress catastrophic shear banding. However, several questions remain regarding the role of the crystal/amorphous interface and of the mechanisms controlling the interaction between dislocations and GBs with the amorphous phase. In this research, we explore the mechanical properties and nanoscale plasticity mechanisms in a tri-layer model system of Al/Al2O3/Al film deposited on Si substrate at room temperature by DC magnetron sputtering. The mechanical response of the films is investigated using nanoindentation as well as a UCLouvain lab-on-chip tensile method based on the relaxation of internal stresses in an actuator beam to pull on the specimen after chemical etching of the underlying sacrificial layers. The results of the nanoindentation confirm the occurrence of creep. Furthermore, by reducing the thicknesses of the alumina layer, a small decrease of theYoung’s modulus and hardness is observed. Preliminary in-situ TEM nanotensile tests show activation of intense GB processes in the Al layers, correlated with significant plastic deformation in the amorphous alumina layer.

Resume : It is now recognized that large earthquakes are preceded and followed by a wide range of events, e.g. bursts of low-frequency earthquakes (LFEs), tremors, which can release as much strain as megathrust earthquakes. These events are related to creeping parts of the fault interface for which some recent models have inferred the possibility of viscosities of the order of some 1017 Pa.s. However, the detailed mechanisms associated with these events are poorly known. In this study we adopt the mineral physics point of view and address the issue of the mechanical properties of antigorite which, among serpentines, is an important component of fault rheology. Given its particular crystal structure and its complex microstructure, the rheology of antigorite is still poorly understood. Standard ductile processes involving dislocation glide have been reported, but other mechanisms like kinking, delamination, which can lead to a semi-brittle behaviour are likely to contribute as well. Consequently, the type of rheological law that is adapted to this mineral is still a matter of debate. Recently, the development of a new generation of advanced instruments for quantitative in situ TEM nanomechanical testing has open a new field of investigation of mechanical properties in the transmission electron microscope (TEM). The major advantage of this technique is to provide a direct, visual access, to the deformation mechanisms as they operate. In the present work, it will be demonstrated that a step forward in the investigation of the plasticity mechanisms of antigorite can be made by using original sample preparation methods. Small beams (410.1 µm3) of antigorite are prepared by Focused Ion Beam and deformed in tension in the TEM. Cyclic deformation is applied in the load control mode using the PI-95 TEM picoindenter holder and the Push-to-Pull (PTP) device (Bruker.Inc). Automated crystal orientation mapping in TEM (ACOM-TEM) was used prior to the in situ TEM tests in order to obtain statistical information regarding the local orientation of the grains and the nature of the interfaces. Despite applied stresses close to 1 GPa, no dislocation activity could be observed. In contrast, significant plasticity, largely recoverable, resulted from the development of damage at interfaces. We propose that due to the crystal structure of antigorite, the interfaces represent a key factor in controlling its mechanical properties.

Resume : Olivine is a silicate with orthorhombic symmetry and (Mg, Fe)2SiO4 composition which is the dominant phase of the Earth’s mantle down to 410 km depth. There are evidence that in the convective mantle, olivine deformation involves dislocation glide and climb. However, due to low symmetry, this mineral does not possess enough slip systems to satisfy the Von Mises criterion. Several recent studies have focused on the possible contribution of grain boundaries (sliding, migration) to the deformation of olivine aggregates, but so far, the mechanisms at play are not clarified. The deformation of ultra-fine grains materials represents an opportunity to enhance grain boundary sliding (GBS) and in-situ nanomechanical testing in the transmission electron microscope (TEM) provides further opportunity to gain information on the acting mechanisms. In this study, we use the PI-95 TEM Picoindenter holder and the Push-to-Pull (PTP) device (Bruker.Inc) to perform quantitative in-situ tensile tests in the TEM. To carry on this study, olivine aggregates with very small grains are needed that are not available in nature. Here we start from amorphous thin films of Mg2SiO4 forsterite (the Mg-rich end member of the olivine solid solution) deposited by Pulsed laser deposition (PLD). First, small beams from this amorphous film ware heated in-situ in the TEM to crystallize olivine. Heating up to 900°C induces crystallization of the thin films with a final grain size of the order of 100 nm. These recrystallized beams were used to prepare the final nanotensile test specimens. We present the results of the tensile tests performed on these specimens with a special attention to the grain boundary activity.

Resume : Bulk metallic glasses (BMGs) exhibit some outstanding mechanical properties involving high fracture strength, and high elastic strain arising from the liquid-like atomic structure with no grain boundaries, dislocations, and phase segregations, contrary to crystalline materials. Recent reports have shown that thin film metallic glasses (TFMGs) are even more promising due to their mechanical properties with a unique combination of high strength (close to the theoretical limit) and ductility (> 10%). However, despite extensive research over recent years, the origin of the mechanical size effect in TMGFs is not fully unravelled yet and it can result either from geometric confinement or from a change of atomic arrangement in the films. In the present work, the fundamental mechanisms controlling the plastic deformation are investigated in Zr50Cu50 (%at.) TMGFs deposited by Pulsed Laser Deposition (PLD). The microstructure of the films is analysed using advanced TEM techniques including spatially resolved fluctuation electron microscopy (FEM) combined with state-of-the-art aberration corrected imaging and spectroscopy techniques. The deformation mechanisms are analysed by combining FEM and quantitative in-situ nanotensile testing using the Bruker PI 95 TEM PicoIndenter. The results reveal that the films exhibit a well-controlled nanoscale density/chemical heterogeneities and a very promising yield strength (~3 GPa) and ductility (>9%) product which depends on the nano-porosity content, with large/low plasticity and low/high yield strength obtained for nanoporous/compact metallic glass films.

Resume : Fatigue and Electrical Resistance of Silver Nanowire Networks Transparent conductive thin films (TCF) are widely used in electrical devices, especially for organic light-emitting diodes (OLED), screens & displays, solar cells and touch panels. TCFs must have low sheet resistance and high light transmittance. Generally, the materials used to make TCFs should behave low sheet resistance and high light transparency, typically the sheet resistance, Rs, should be ≤ 100 Ω/m2 with optical transmittance, T ≥ 90 % . Currently, indium tin oxide (ITO) films are the industry standard for TCF applications. ITO thin films show good electrical conductivity, high light transparency and good chemical stability, hence they have been used successfully for over 60 years in TCFs. However, the drawbacks of ITO, such as its brittleness and finite supply of In, have encouraged the exploration of new materials and methods to replace ITO and to extend TCFs to applications in bendable and stretchable electronics. Potential replacements include conductive polymers, carbon nanotubes, metallic grids, graphene, and metallic nanowires. Silver nanowire networks (AgNW) can show lower sheet resistance with higher light transmittance than ITO thin films; they also display a much greater strain to failure and greater resistance to mechanical damage. This project investigates the optoelectric properties and flexibility of AgNW networks with the objective of developing a fuller understanding of their behaviour, damage mechanisms and how these can be developed into predictive models for their properties and lifetime. Silver nanowires have been deposited onto flexible polymer substrates by spray coating to form a continuous stochastic network. Post-spraying treatment of either a low temperature anneal or a normal pressure is used to improve wire-wire electrical contact and reduce the network sheet resistance while maintaining optical transparency. The structural integrity of the film under flexible electronics service conditions has been assessed through repeated bending tests in a high cycle fatigue environment. At a specific bending cycles, the results showed a linear relationship between the square root of the sheet resistance and the inverse of the areal coverage of the network. Under cyclic bending fatigue testing, the gradient of this linear relationship which can be determined as the network resistance RN increases and is related to the number of strain cycles. According to the results, the increase of sheet resistance of networks is mainly caused by the loss of nanowire-nanowire junctions rather than a change of individual nanowires, which is further proved by SEM analysis. Keywords: Silver nanowire networks, transparent conductive films, flexible and stretchable electronics, bending fatigue.

Resume : Extracting mechanical property values from small sample volumes has tremendous benefits for nuclear materials evaluations. Reducing materials hazardous levels by reducing the sample volume, making ion beam irradiations accessible to mechanical properties or simply sample regions of specific interest are features one can take advantage of if SSMT is deployed. However, the values measured at small length scales are not the same as values measured at large length scales. Fundamental material science and some knowledge on the materials microstructure must be deployed to understand this translation between the scales. Further new infrastructure and technology today allows for a more rapid sample manufacturing allowing to probe the multiple length scales fast and efficient. This presentation will outline the scientific reasons for scaling effects associated with different testing techniques as well as mitigation strategies specifically for irradiated materials so one can quantify property changes at multiple length scales. Nanoindentation, micro-compression, micro-tensile and micro-bending on ion beam irradiated materials as well as neutron irradiated materials are presented while femto-second laser ablation based techniques for radioactive sample manufacturing is introduced.

Resume : X-750 Ni-based superalloy which is used as spacer material in the fuel channels of the CANada Deuterium Uranium (CANDU) reactor exhibits significant embrittlement after service due to neutron radiation damage effects. In this study an accelerated ion irradiation (Helium and Proton) was employed to simulate the after-service microstructure, coupling with advanced microstructural characterization methods to investigate the deformation, crack initiation\propagation and fracture mechanisms in X-750 superalloy. Small-specimen tensile tests have been carried out and indicate significant ductility reduction for He-implanted material compared to non-irradiated X-750. Also, fracture mode changed from the dimpled-ductile to the brittle fracture after He implantation. Cross-sectional TEM via focused ion beam (FIB) lift-out technique on the fracture surface was employed to trace initiated cracks for understanding the failure mechanisms after irradiation. Grain boundary cracks form preferentially at the interface of chromium carbide particles and propagate through shearing and coalescence of cavities decorate the grain boundary. Cracks also create within the grains that initiate at the interface of pre-existing nano-twins and matrix.

Resume : Hydrogen embrittlement, often leading to catastrophic failure, has been widely studied. However, there is still a lack of fundamental understanding and controversy about its specific effects and interaction mechanisms in steels. The investigated single-phase ferritic Fe-Cr binary alloys have high hydrogen diffusivity and low solubility, therefore ideal for in-situ studies. Hydrogen is electrochemically introduced to the specimens by an in-situ nanoindentation set-up developed in-house, which enables collecting mechanical properties and simultaneously charging the sample with hydrogen from the backside. As the surface of the specimen stays intact, it is possible to characterize specific microstructural changes before and after hydrogen charging employing, e.g., transmission electron microscopy, electron channeling contrast imaging, electron backscatter diffraction. During hydrogen charging, the hardness of the alloys increases with increased applied voltage, corresponding to a higher amount of hydrogen charged into the material, while the elastic modulus remains constant. The pop-in load decreases with increased applied voltage, indicating a reduction of the shear stress necessary for dislocation nucleation. Larger variations of both properties with higher Cr concentrations are also observed. The cross-section underneath indentation imprints was examined by TEM to identify the microstructure variations before and after hydrogen charging within specific grain orientations.

Resume : We study the response of body centered cubic (bcc) Mg under extreme conditions of pressure, temperature and strain rate. Bcc Mg was stabilized at ambient pressures in a Mg/Nb multilayer nanocomposite where the adjacent Mg/Nb interfaces are spaced within a few nanometers. We investigated the structure of the hitherto-unknown bcc Mg phase in the Mg/Nb multilayer nanocomposite under high pressures in a diamond anvil cell experiment using synchrotron radiation x-ray diffraction (XRD). Additionally, we performed high temperature micro-pillar compression tests and strain rate jump tests on Mg (bcc)/Nb 5nm/5nm and Mg (hcp)/Nb 50nm/50nm nanolaminates to compare the responses of hcp vs. bcc Mg. Results from these tests were analyzed in terms of the measured activation energies and activation volumes from sub-micrometer sized Mg/Nb multilayer nanocomposites.

Resume : In recent years, there have been important applications envisioned that would require small-scale devices to be implemented. An example of such are microelectromechanical systems (MEMS) used as energy harvesters. A significant challenge in the realisation of such devices is finding novel ways of lubrication, due to the high friction and wear present. In this work, Room Temperature Ionic Liquids (RTILs) are proposed as a solution to this problem, due to their numerous tuneable properties. The vapour pressure of ionic liquids is also very low, thus almost no liquid is lost due to evaporation. Additionally, their ionic behaviour is a topic with major scope for enhancing or controlling the compound’s properties through applied electric fields. Finally, these liquids are also able to be characterised through molecular simulations. In order to explore the friction properties at the nanoscale of RTILs, a MEMS tribometer has been developed. This consists of a miniature thrust pad bearing interface, manufactured using semiconductor fabrication techniques. Furthermore, this experimental rig is to be modified to apply charge on the system. In this way, the charged behaviour of RTILs can be studied. Thanks to previous molecular simulation work done at Imperial, the experimental results can be compared and validated. Following the conclusions of this work, applications will be explored in the nanoscale lubrication properties for MEMS devices and energy harvesters.

Resume : The aim of this study is to investigate whether Titanium-based alloys fabricated as films can display superelastic behaviors as their ingot-metallurgy counterparts. Superelastic response originates from the mechanical instability of the beta phase leading to a reversible phase transformation and a remarkable strain recovery. There is little information on the possibility to deposit Ni-free superelastic beta-type Ti-based films through physical vapor deposition methods. Quaternary Ti-Zr-Nb-Sn alloy films were deposited on silicon substrates by magnetron sputtering at room temperature. The morphological, crystallographic and microstructural characteristics of the films were studied by scanning electron microscopy (SEM), atomic force microscope (AFM), X-ray diffraction (XRD), and transmission electron microscopy (TEM). Superelastic responses and mechanical properties of films were investigated by nanoindentation using both spherical and Berkovich indenters. Results showed that metastable beta phase of Ti alloy films was stabilized at room temperature during magnetron sputtering. Superelastic effect exists in our nanograined alloy films and can be probed at the local scale by nanoindentation during nanoscale deformations under complex loading conditions.