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Abstract: Correlations between the grain orientations and elastic properties of plasma-nitrided polycrystalline 316L austenitic stainless steel are investigated. The grain orientations (h k l) in a delimited area were obtained from electron backscatter diffraction and related to hardness (Hhkl ) and elastic modulus (Ehkl ) maps obtained from large nanoindentation matrices. The influence of nitrogen concentration on the local mechanical properties has been studied by repeating these indentation matrices in the same area after successive partial removals of the nitrided layer. This nanoindentation tomography allowed the orientation, the shape and the surroundings of individual grains to be taken into account. The results show that plasma nitriding leads to a complete reversal of the elastic behaviour anisotropy: while the non-nitrided 316L austenitic stainless steel shows the typical elastic anisotropy of face-centred-cubic-type metals with a maximum of Ehkl for the oriented grains, the maximum of Ehkl is observed for the oriented grains in the nitrided layer. A similar anisotropy reversal is observed for the hardness Hhkl . These observations are discussed on the basis of the microstructural changes induced by the nitrogen incorporation. [Copyright &y& Elsevier]

Statistics on nanoindentation experiments performed on a fourth-generation Ni-based single-crystal superalloy reveal a bimodal hardness distribution, with two mean hardness values only 2.5% away from each other. Using a hardness cartography technique combined with the statistical analysis proved that the hardness values are not randomly distributed but correspond exactly to the dendrites and the interdendritic regions. In this context, hardness cartography is shown to push back the resolution limit of the nanoindentation test. [ABSTRACT FROM AUTHOR]

The changes in anisotropic hardness and indentation modulus induced by plasma nitriding at 400°C of a 316L polycrystalline austenitic stainless steel are analyzed. The dependence of hardness and elastic modulus modifications on the crystallographic orientation is investigated through instrumented indentation and electron backscattering diffraction. Both hardness and indentation modulus exhibit an inverted anisotropy compared to the untreated 316L, likely associated with the presence of the N atoms in interstitial sites. [ABSTRACT FROM AUTHOR]

Abstract: The changes in microstructure and mechanical properties of 4H-SiC implanted at room temperature with 50keV helium ions have been studied. Up to 1×1016 cm−2, no significant change in mechanical properties is observed. Nevertheless, the as-created damage that induce a normal strain profile of few percents, enhance the dislocation nucleation. The compressive in-plane stress impedes the crack formation. At medium fluences, when a buried amorphous layer is created, the hardness curve against penetration depth can be divided into three stages showing “constraining coating” effects. Indents are surrounded by large hillocks resulting from the flowing of the amorphous state. When the entire part of the implanted crystal is amorphous a large decrease is measured for both the hardness and elastic modulus. [Copyright &y& Elsevier]

Abstract: Incipient plasticity in magnesium oxide single crystals has been investigated by nanoindentation with a spherical indenter. For very low loads, the surface deformation can be limited to a single slip line, making modelling of the deformation easier. The recently developed nanoetching technique has been used to investigate the corresponding dislocation structure. Single pile-ups of a few dislocation half loops have been observed and fully characterised. A simple elastic model, based upon the calculation of the Peach Koehler force induced by the pile-ups and the spherical indenter stress field, has been used to determine the equilibrium positions of dislocations in these pile-ups. A comparison between the experimental and calculated structures has led to the determination of the lattice friction stress for edge and screw dislocations in magnesium oxide single crystals. This stress is found to be 65MPa for edge dislocations and 86MPa for screw dislocations. [Copyright &y& Elsevier]

In this paper, it is shown that combining AFM, chemical etching and controlled polishing, progressively removing thin layers of material, allows a three-dimensional reconstruction of the volume distribution of dislocations about and under nanoindentation imprints. To illustrate the method, results obtained in MgO, a material known for its simple plasticity, have been selected. It is shown, comparing surface deformation and etching pattern, that the entire dislocation distribution associated with small indents can be analysed in terms of individual dislocations, leading to a better understanding of the elementary mechanisms of plasticity associated with the early stages of indents formation in crystalline material. [Copyright &y& Elsevier]

Abstract The mechanical properties of materials usually depend on the size of the considered object. Silicon, for instance, undergoes between the macroscopic and nanometer scales, a brittle-to-ductile transition at room temperature. Although essential for the constantly developing Si-based nanotechnologies, the origin of this remarkable behaviour change remains undetermined for several years. The observation of the mechanisms responsible for plastic deformation in nano-objects is indeed highly challenging at the microscopic scale. One needs controlling the deformation while identifying induced individual plastic events at the smallest scale during the first stages of plasticity. This work describes nano-compression experiments on 100 nm-diameter Si nanopillars followed by post-mortem analysis of the deformed specimens through SEM and atomic resolution TEM imaging. The observed plastic deformation disrupts the usual description of low-temperature undissociated-dislocations-mediated plasticity and is comforted by molecular dynamics calculations. These results shed a new light on the transition between ductile and brittle regimes in silicon by introducing the missing link between plasticity and fracture. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Titanium alloys are widely employed in aerospace applications due to an outstanding combination of properties. The variety of loading conditions and microstructures encountered in industrial components is urging the development of microstructure sensitive modeling capabilities. In particular, reliable predictions require a good knowledge of operating deformation mechanisms. The present study aims at providing a thorough characterization of deformation mechanisms in the near-α Ti-6Al-2Sn-4Zr-2Mo alloy using experiments and simulations. The sequential activation of deformation mechanisms in the α phase was monitored in situ during a tensile test carried out in a scanning electron microscope using a combination of slip traces analysis and electron back-scattered diffraction. Basal slip is activated first, and prismatic slip activity, which proceeds at a higher macroscopic stress level, is needed for a significant creep/relaxation to occur. While little evidence of 〈 a 〉-type pyramidal slip was found, 〈 c + a 〉 pyramidal slip involved first-order pyramidal planes and operates at stress levels near or above the 0.2% proof stress. Atomic force microscopy characterization of the features of the different slip modes revealed that 〈 a 〉 slip is coarser and wavier than 〈 c + a 〉 slip. Twinning, which is usually neglected in such alloys within this strain regime, was observed to be slip stimulated at a plastic strain as low as 0.5%. Crystal plasticity parameters leading to an accurate simulation of the activation sequence of deformation mechanisms were then determined. For this purpose, critical resolved shear stress values were derived for the different slip and twinning modes using specific approaches and subsequently validated using crystal plasticity simulations based on fast-Fourier transforms. [ABSTRACT FROM AUTHOR]

Amorphous Fe0.55Zr0.45 films, having thickness of 400 nm, were grown on silicon substrates by co-deposition using ion beam sputtering. Limited surface roughness makes this system particularly suitable for fine-scale scanning force microscopy analysis and nano-indentation. The samples were irradiated with MeV C60 clusters, and the surface morphology of single impacts was found to have a “doughnut” shape, i.e. hillocks having a central crater. Quantitative evaluation of the deformation was achieved by measuring their height and diameter. When C60 projectiles deviate from normal incidence, a tail emerges along the direction of the incident beam. The height of the hillock and length of the tail are increasing with the incidence angle, and the magnitude of the deformation indicates that the damage mainly occurs due to a radial coherent mass transport outwards from the track core by a compression shockwave-like mechanism. The residual compressive in-plane stress, ∼−0.4 GPa for the as-deposited films, was found to notably influence the C60 induced plastic deformations. Indeed, stress relaxation results in a marked decrease in height combined with a significant widening of the surface features. This “flat” surface morphology is attributed to an enhanced radial efficiency of the pressure pulse, owing to a significant reduction of the hardness of the amorphous film after stress relaxation. The overall picture outlined from our observations suggests that the surface damage induced by single MeV C60 ions possibly is the signature of plastic deformation induced at large distances by an energetic radial pressure pulse. This unsteady shockwave allows the energy transfer outwards from the localised region along the ion path that experiences a sudden transient heating. [Copyright &y& Elsevier]

The role of topologically close packed (TCP) phases in deformation of superalloys and steels is still not fully resolved. In particular, the intrinsic deformation mechanisms of these phases are largely unknown including the active slip systems in most of these complex crystal structures. Here, we present a first detailed investigation of the mechanical properties of the Fe 7 Mo 6 μ-phase at room temperature using microcompression and nanoindentation with statistical EBSD-assisted slip trace analysis and TEM imaging. Slip occurs predominantly on the basal and prismatic planes, resulting also in decohesion on prismatic planes with high defect density. The correlation of the deformation structures and measured hardness reveals pronounced hardening where interaction of slip planes occurs and prevalent deformation at pre-existing defects. [ABSTRACT FROM AUTHOR]

Abstract: A thin cobalt layer is deposited by electron beam evaporation onto a germanium substrate after an in situ cleaning annealing at 400 or 700°C. The effect of these pre-treatments on the Co/Ge Schottky barrier properties and on the germanide formation is investigated by using different techniques. A strong influence of the pre-treatment is observed. The pre-treatment at 700°C removes the native oxide but enhances the diffusion of contaminants. After post-metal deposition annealing, the sample pre-treated at 700°C shows a double layer structure due to interdiffusion, whereas some large isolated islands are present in the sample pre-treated at 400°C. [Copyright &y& Elsevier]

Currently, kinematic field measurements for studying the mechanical behavior of materials and structures use common optical methods, such as mark tracking techniques, grid methods and correlation techniques. These techniques are employed over a region of interest ranging from micro to millimeter scale. However, when studies need to be conducted on even smaller scales such as sub-micrometric scale, the use of more complex means of observation is required. In this case, the work can be achieved using the scanning electron microscope (SEM), and the Focused-Ion-Beam (FIB) as marking technique. An adaptation of Digital Image Correlation (DIC) method Heaviside-based Digital Image Correlation (H-DIC) is chosen to investigate the mechanical behavior and taking into account local fractures. As the studied problem is to extract strains from a fractured material using a displacement field measurement method, derivative computation for strain determination is not well adapted. The proposed approach consists in extracting the residual strains from local first gradients of H-DIC, which are less perturbed by the fractures. Various tests were performed to evaluate the validity of this new proposed approach. An application to study the mechanical behavior of a metallic composite (Al/ ω -Al-Cu-Fe) is proposed. A particular exploitation of all field lies in the good separation of the strain fields and the cracked part. A discussion that focus on the comparison between a conventional DIC analysis and its extension was presented on the zones without or with fractures. Compression test on a two-phase material Al/ ω -Al-Cu-Fe, Maps after second loading/unloading cycle, and for tow zones (Z1, Z4). (a, d) SEM image, (b, e) equivalent strain H-DIC ε eq ∂ U ∂ X and (c, f) norme of displacement jump vector ║ U′ ║. Unlabelled Image • Coupling Scanning Electron Microscope, Focused-Ion-Beam, and Digital Image Correlation in a metrological process • Separation of strains and fracture properties using an extended DIC • Extracting residual strains from a fractured material for an ex-situ compression test • Application to study the microstructure and the mechanical behavior of a composite metallic material [ABSTRACT FROM AUTHOR]

We comparatively study the elastic properties of the two existing polytypes of tantalum, α-Ta (ground state bcc structure) and β-Ta (metastable tetragonal structure), using density functional theory (DFT) calculations and thin film growth experiments. The lattice parameter and the single-crystal elastic constants c ij for the two allotropic structures were computed using pseudo-potentials with generalized gradient approximation. Sound velocity measurements on single-phase, highly-textured (001) β-Ta and (110) α-Ta films deposited by magnetron sputtering were carried out using picosecond laser ultrasonic and Brillouin light scattering techniques, while hardness H and Young's modulus E were measured using nanoindentation. We found that the β-Ta phase is significantly harder (H ~18.0 GPa) than α-Ta (H ~10.3 GPa), while their elastic stiffness properties and Cauchy pressure p are similar, C 33 ~285–310 GPa, E ~ 188 GPa, p ~ 70–80 GPa, and agree well with the computed values. However, the out-of-plane shear modulus G of the β-Ta film is softer by 28% with respect to that of α-Ta film and DFT values (50 vs. 69 GPa). The present findings reveal that, contrarily to common claims, the β-Ta phase is not at all brittle but elastically and plastically deforms in a compliant way. Its toughness and elastic strain to failure ability is further comprehended from low G / B (0.205) and high H / E (≈0.1) ratios and the absence of radial cracks around indents, which suggests that β-Ta is also a suitable phase for use as wear-resistant coating in biomedical applications. • Sound velocities measured from picosecond acoustics and Brillouin spectroscopy. • Computed elastic constants agree with measured ones for α-Ta. • A significant softening (25%) of C 44 is measured for β-Ta. • Despite being harder, the β-Ta film is found to be more ductile. [ABSTRACT FROM AUTHOR]