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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 plasticity of Ti3SnC2, a recently synthesized MAX phase, was investigated. Localized deformation was induced by nanoindentation in a polycrystalline sample, and the resulting surface topography was observed by atomic force microscopy (AFM). For several grains, buckling around the indent was observed, in agreement with the kink band deformation process often reported for MAX-phase materials. For other grains, slip lines have been revealed by AFM. The corresponding slip systems have been identified through the determination of the local crystalline orientation by electron backscatter diffraction. First- and second-order pyramidal slip systems are shown to be active for some grain orientations, as well as dislocation interactions and cross slip from one system to the other. [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]

The nanoindentation test is commonly used for the local determination of mechanical properties (hardness, elastic modulus, etc.) and also to study the initial stages of plasticity (dislocation nucleation, dislocation interaction mechanisms) at the nanometre scale. In the latter case, the determination of the elastic stress field beneath the indenter is of primary interest. An analytical expression is derived for the elastic stress and strain fields beneath an axisymmetric punch. Most solutions, in the literature, are given for simple indenter shapes, such as flat, conical or spherical indenters. The complete solution proposed for an arbitrary indenter profile is described by a power law, in which the exponent can be an integer or not. The stress is given as the real part of complex analytical expressions. [ABSTRACT FROM AUTHOR]

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]

Grit blasting with alumina particles offers a conventional substrate surface preparation method intended to improve the adhesion of thermal spray coatings. However, the associated microstructural modifications to the substrate may, in fact, reduce the integrity of the interface. In this paper, we use nanoindentation cartography methods to characterize the local mechanical properties of the interface of a HVOF coating deposited on an alumina grit blasted 17-4PH stainless steel substrate. The results show a hardness gradient in the substrate, which we evaluate within the context of the refinement of the substrate microstructure. We show that nanohardness mapping, combined with Atomic Force Microscopy and Scanning Electron Microscopy analysis provides a viable approach to study this unique contribution of the microstructural relationship between the beneficial and detrimental impacts of grit blasting on the adhesion of thermal spray coatings. [ABSTRACT FROM AUTHOR]

We report on circular buckles experimentally observed by optical and atomic force microscopy on gold ductile thin films deposited by physical vapor deposition on silicon wafers. It is shown that, whatever the radius blister dimensions, their maximum deflections are higher than those expected by the elastic theory. It suggests that plastic events may take place in the film, impacting on the blister morphology as a result. Based on nanoindentation experiments carried out on our gold films, a proper plastic hardening law has been determined by calculations using the finite elements method. The influence of this elasto-plastic behavior on the buckled circular profiles has been then numerically studied, compared to the experimental observations and discussed. • Circular buckles on gold ductile thin films under high compressive stresses experimentally observed and numerically compared. • Proper elastic and elastic–plastic parameters numerically determined from nanoindentation process using FEM calculations. • Elastic plate theory not sufficient for deeper explanation of the buckled morphologies. • Impact of elastic–plastic behavior numerically studied and compared to experimental observations. • Plasticity well explains the experimental buckling structures by assuming a non-linear stress loading through a maximum stress value. [ABSTRACT FROM AUTHOR]

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]

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]