Evolution of Specimen Shape for Uniaxial Compression of Single Crystals (original) (raw)
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Plasticity size effects in tension and compression of single crystals
Journal of the Mechanics and Physics of Solids, 2005
The effect of size and loading conditions on the tension and compression stress-strain response of micron-sized planar crystals is investigated using discrete dislocation plasticity. The crystals are taken to have a single active slip system and both small-strain and finite-strain analyses are carried out. When rotation of the tensile axis is constrained, the build-up of geometrically necessary dislocations results in a weak size dependence but a strong Bauschinger effect. On the other hand, when rotation of the tensile axis is unconstrained, there is a strong size dependence, with the flow strength increasing with decreasing specimen size, and a negligible Bauschinger effect. Below a certain specimen size, the flow strength of the crystals is set by the nucleation strength of the initially present Frank-Read sources. The main features of the size dependence are the same for the small-strain and finite-strain analyses. However, the predicted hardening rates differ and the finite-strain analyses give rise to some tension-compression asymmetry. r
On the Deformation Heterogeneities Described By Crystal Plasticity
2015
The deformation fields within grains in polycrystalline materials are generally highly heterogeneous and can be the precursors to the nucleation of micro-cracks or cavities. Such behavior is conditioned by microstructural features, such as grain structure, texture, morphology, size, etc. The understanding of such complex phenomena is crucial to enable structural integrity assessments of engineering components, since it constitutes the physical bases on which to describe the local mechanisms of deformation and failure to be incorporated into structural integrity codes. This work provides a brief overview of the different continuum mechanics approaches used to describe the deformation behavior of either single crystals or individual grains in polycrystalline metallic materials. The crucial role played by physics based local and non-local crystal plasticity approaches in the prediction of heterogeneous deformation is discussed. Representative examples are given regarding the use of dis...
Acta Materialia 55 (2007) 4567–4583
The study presents crystal plasticity finite element simulations of cylindrical Cu single crystal micropillar compression tests. The aim is to study the influence of the stability of the initial crystal orientation, sample geometry (diameter-to-length ratio) and friction on the anisotropy and crystallographic orientation changes during such tests. Initial anisotropy (initial orientation) has a strong influence on the evolution of crystallographic orientation changes and also, to a minor extent, on the sample shape during compression. Pronounced orientation changes occur at an early stage of compression (at engineering strains of 0.2), entailing as a rule a large orientation spread within the initially uniformly oriented sample. A non-zero friction has a stabilizing effect on the course of the compression test even in cases where strong orientation changes occur. The evolution of orientation changes during compression is in part due to rigid body rotations (shape inclination due to buckling) rather than exclusively to crystallographic reorientation. Orientations that are crystallographically unstable and non-symmetric during compression tend to entail shape instability of the pillars at an earlier stage than observed for more stable cases.
polymer substrates. The type of conductive filler considerably determines the characteristics of nucleation, growth and the electroforming of the product microstructures.
Journal of Geophysical Research, 1984
A hydrothermally grown synthetic quartz crystal with 370 + 60 ppm hydroxyl impurity was cut into right rectangular prisms in eight crystallographic orientations. We compressed the prisms under constant axial force corresponding to a uniaxial stress of 140.0 + 0.5 MPa, and temperatures of 510 ø and 750øC. All but one of the samples sustained permanent axial strains of 2-3%. We established the operating slip systems from specimen shape change, slip bands and dislocation etch pits on polished surfaces, crystallographic orientation changes, stress optical features in thin sections, and transmission electron microscopy. The observed creep behavior and plasticity divided the samples into three groups'(1) Crystals compressed at 45 ø to [0001] and [2ii0] and those compressed 2_(01il) and 2_(0i11) deformed principally by slip parallel to [0001]. Creep rates were relatively high and were not strongly sensitive to test __ temperature. Dislocation arrays approximately parallel to (2110) are common. Dislocation loops are elongate parallel to [0001], indicating that the edge segments were more mobile than the screw segments. (2) The second g_roup of samples were loaded normal to [0001] in three orientations: 2_(2ii0), 2_(01i0), and at 45 ø to (1100). These samples deformed primarily by •10i0• (a) slip with some evidence for secondary slip on the other systems. They were more creep resistant than the first group and displayed a much higher sensitivity of creep rate to test temperature. Dislocation loops are very elongate parallel to [0001], indicating that the screw dislocation segments were much more mobile than the edge segments. (3) A sample compressed parallel to [0001] at 750øC crept at a barely detectable rate (~ 10-9 s-•) and no optical scale slip features were observed. These results confirm our earlier work on one orientation each from groups 1 and 2, which indicated a strong creep anisotropy for this same crystal. This creep anisotropy parallels a remarkably similar anisotropy in the diffusivity of impurities in quartz, suggesting a causal relationship between impurity diffusion and creep associated with hydrolytic weakening. INTRODUCTION Quartz crystals in nature can be remarkably pure. Total impurity concentrations in inclusion-free, colorless, and transparent vug-grown natural crystals rarely exceed 100 ppm [Frondel, 1962]. Natural transparent and colorless quartz has very high plastic yield strength even at elevated temperatures [Griggs and Blacic,