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In situ compression tests combined with neutron diffraction were performed on Ti2AlN MAX polycrystals with lamellar anisotropic microstructure: the diffraction peak evolution (position and profile) with applied stress reveals that lamellar grains parallel to compression axis remain elastic while lamellar grains perpendicular to compression plastify, both families being subjected to strong variations of heterogeneous strains (types II and III). We demonstrate that this behavior originates from the complex response of the very anisotropic lamellar microstructure and explains the observation of reversible hysteretic loops when cycling MAX polycrystals even in the elastic regime. [ABSTRACT FROM AUTHOR]

The HIPing of Ti-Al-1.9TiC and Ti-Al-0.2Sn-1.8TiC reactant mixtures yields Ti3AlC2 and Ti3Al0.8Sn0.2C2 solid solutions, respectively. Rietveld refinement gives a = 3.0786 ± 0.0001 Å and c = 18.589 ± 0.001 Å lattice parameters for Ti3AlC2 and a = 3.0837 ± 0.0001 Å and c = 18.621 ± 0.002 Å for Ti3Al0.8Sn0.2C2. Furthermore, about 2-3 vol% of Ti x Al y intermetallics is detected in Ti3Al0.8Sn0.2C2 solid solution. The intrinsic hardness, measured by nanoindention tests, is 11.4 ± 0.7 GPa for Ti3AlC2 and 10.2 ± 0.6 GPa for Ti3Al0.8Sn0.2C2. Furthermore, hardness values, measured by microindentation tests, are compared with the ones measured by nanoindentation. It is shown that microindentation and nanoindentation performed with large loads lead to underestimated hardness values as several grains are involved in the deformation process. Finally, Young's modulus of Ti3AlC2 and Ti3Al0.8Sn0.2C2 is 260 ± 10 and 250 ± 10 GPa, respectively. [ABSTRACT FROM AUTHOR]

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]

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]

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]