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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: Mechanical property changes of 4H–SiC implanted at room temperature with helium ions at fluences ranging from 7×1015 to 1×1017 cm−2 and at an ion energy of 50keV were investigated by using nano-indentation tests and subsequent atomic force microscopy investigations. Degradation of the mechanical properties of the near-surface due to helium implantation was analysed and correlated with the microstructure evolution examined through transmission electron microscopy cross-section and X-ray diffraction experiments. Up to 1×1016 He+ cm−2 no significant change in mechanical properties is seen whereas a normal strain profile of few percents is generated all along the ion path. Nevertheless, the as-created implantation damage enhance the dislocation nucleation. At intermediate fluences, when a buried amorphous layer is created, the hardness curve against penetration depth can be divided into three stages showing ‘constraining coating’ effects. When the entire part of the implanted crystal is amorphous a decrease of about 50% is measured for hardness value. [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]

Nanoindentations have been performed at very low load in (001) MgO single crystals. The surface deformation has been investigated by atomic force microscopy, and the rosette arms pattern, induced by the glide of only a few dislocations, has been observed. Moreover, surrounding steps, oriented along the <100> directions, have been identified and associated with the early stage of the formation of the square shaped slip lines pattern commonly observed in the case of microindentation performed under very high loads. The fine structure of these surrounding steps has been resolved clearly, and a mechanism has been proposed based upon interaction between dislocations belonging, respectively, to edge and screw rosette arms and upon the activation of a <011>(211) secondary glide system. [ABSTRACT FROM AUTHOR]

Berkovich nanoindentations have been performed at ambient temperature with a new apparatus on (001) cleaved surfaces of single-crystal Magnesium Oxide (MgO). Rosette slip lines pattern has been observed at a nanometric scale by Atomic Force Microscope. Moreover, the load-displacement curves present particular features such as pop-in, which has been linked to the AFM surface topography observations. Finally, we show that the combination of a new nanoindentation apparatus and an Atomic Force Microscope allows to study the very early stages of the plastic deformation during a nanoindentation in terms of individual dislocations. [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]