Crystal plasticity model of shear and kink bands–energetic approach (original) (raw)
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Some correlations between slip band emergence and dislocation pattern
IOP Conference Series: Materials Science and Engineering, 2009
Various forms of the plastic deformation in single crystals are studied on pure nickel and nickel alloys oriented for single slip [135] and multiple slip [001]. Particular attention is paid to the heterogeneity of deformation observed at two distinct scales: the slip bands and the dislocation organizations. The slip bands emerging at the surface can be studied using the atomic force microscopy (AFM). The height of extrusions and inter-band spacing depends on the orientation of tensile axis, the strain level and the nature of the alloy. At another scale, dislocation organizations typical of f.c.c. crystal have been observed, which depend on the orientation of tensile axis and on the stacking fault energy. A study by transmission electronic microscopy (TEM) has enabled us to approach the dimensional characteristics of these structures. In the case of mono-crystal oriented for single slip strained in stage III (γ ≈ 0.8) we observed a correlation between the inter-band spacing (d) and the inter-wall spacing (λ) of the type I dislocation pattern. This result suggests that this kind of walls act as a screen to the mobility of dislocations unlike equiaxed cells that would be only an obstacle to the dislocation mobility. This internal length is lower for Ni16%Cr alloy than for nickel. Consequently, stacking fault energy is probably a parameter which affects the internal length in relation with cross-slip capability. On the other hand, results, obtained of the [001] direction in nickel, are more complex due to multiple slip. Indeed, only equiaxed cells are observed for this orientation with cell size magnitude (λ) far lower than those observed for inter-band spacing (d). As in the case of samples oriented for single-slip, the equiaxed cells observed for samples oriented for multiple-slip seem to be only obstacles to the mobility of dislocations. However, there are probably walls associated with this kind of cells which act as barriers to the movement of dislocations.
Dense dislocation walls and microbands aligned with slip planes—theoretical considerations
Acta Materialia, 1997
of the dislocation boundaries in cold deformed f.c.c. metals at low and intermediate strains lie on crystallographic slip planes and others have a macroscopic direction with respect to the sample axes (i.e. they are non-crystallographic). A model for the occurrence of the former type of dislocation boundaries is proposed. The model combines slip pattern analysis and dislocation theory. It is assumed (i) that the dislocations in the boundaries are generated by slip, (ii) that the deformation temperature is low enough to exclude dislocation climb and (iii) that the driving force for formation of boundaries is minimisation of the energy stored in the boundaries. Formation of crystallographic boundaries is predicted if two active slip systems in the same slip plane account for a large fraction of the total slip. For single crystals the agreement between predicted and experimentally observed crystallographic and non-crystallographic boundaries is excellent. For different grain orientations in poly crystalline aluminium specimens, the agreement between prediction and experiment is satisfactory in view of the complexity of polycrystal studies compared to studies of single crystals. 0 1997 Acta Metallurgica Inc.
Slip systems, lattice rotations and dislocation boundaries
Materials Science and Engineering: A, 2008
Plastic deformation by slip induces rotations of the crystallographic lattice and evolution of dislocation structures. Both lattice rotations and dislocation structures exhibit a dependence on the grain orientation, which reflects underlying relations to the slip pattern. Relations between the type of dislocation structure formed, in particular the crystallographic alignment of dislocation boundaries, and the slip pattern are demonstrated. These relations are applied to polycrystals deformed in tension and rolling, producing good agreement with experiment for rolling but less good agreement for tension. The grain orientations, for which the relations do not hold in tension, are also the grain orientations where the deviation between lattice rotations observed by three-dimensional X-ray diffraction (3DXRD) during tension deviate the most from those predicted with the Taylor model. The origin of these discrepancies is discussed. Finally, the implications of the relations between slip and dislocation structures for the modelling of mechanical properties are discussed.
The formation of a quasi-periodic surface profile by means of dislocation slip
Journal of Physics: Conference Series, 2017
This work presents the experimental results concerning the research of the morphology of the face-centered cubic (FCC) single crystals surface after compression deformation. Our aim is to identify the method of forming a quasiperiodic profile of single crystals with different crystal geometrical orientation. A set of modern methods as optical and confocal microscopy is applied. It is done to determine the morphology of surface parameters. The results show that the octahedral slip is an integral part of the formation of quasiperiodic profile surface and is starting with the initial strain. We present values of shear quantity directly in shear traces, meso-and macrobands and in shear tracks shift within them. The similarity of the process of surface profile forming at different levels of scale is given. Finally, we suppose that octahedral slip is the main way to form quasiperiodic deformation relief. We compare our results to published data and identify common regularities for various metal FCC single crystals with different deformation methods. The experimental results presented can be used for mathematical modeling of plastic deformation. Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
Physical Review B, 1990
The dynamic organization of dislocations into spatially heterogeneous substructures is demonstrated by applying the principles of dislocation dynamics that were outlined in the preceding paper. Here it is shown that the formation of persistent slip bands is a consequence of the competition between dipole formation and annihilation of dislocations of opposite Burgers vectors in the absence of temperature-enhanced climb recovery under cyclic stress conditions. Planar arrays, which are also uniaxial structures, are shown to arise from enhanced dislocation multiplication and the formation of stable dipole configurations along a slip plane at lower temperatures where climb is unimportant. Biaxial dislocation systems experience additional degrees of freedom compared with uniaxial systems because of available motion along additional slip systems. It is demonstrated that for a systern of orthogonal slip directions at high temperatures in which climb and glide motion are competitive, dislocation cellular structures form as a result of immobile dipole and junction formation and by the internal elastic strain energy minimization caused by long-range self-shielding. It is shown that the internal elastic strain energy is reduced by the self-organization process. However, the short-range nonlinear processes (i.e. , dipole and junction formation) are shown not to allow absolute elastic energy minimization.
Letters on Materials, 2019
Rotational-type mesodefects-strain induced junction disclinations, are formed at joints and ledges of high-angle grain boundaries during plastic deformation of polycrystals. The strength of these mesodefects increases with deformation and, as a result, they create non-uniform elastic stress fields, which significantly affect the processes of structure formation, strain hardening and material's fracture. In the present work, the interaction of a slip band propagating in a grain body under the action of an external stress field with an elastic field of wedge disclination is investigated by the discrete dislocation dynamics simulation method. The simulation results show that the behavior of the front of the band has common regularities. For a given distance between the slip band and the disclination y 0 , the behavior of the dislocation cluster in the front of the band essentially depends on the distance between the slip planes of the dislocations h. For small h, it is similar to the behavior of a dislocation pileup. However, the maximum density of dislocations occurs not in the head of the cluster, as in the classical blocked dislocation pileup , but in its central part. An increase in the distance h between dislocations slip planes leads to a "splitting" of the front of the band. For larger h the front of the band transforms into a broken from both sides dislocation wall. The minimal distance h c , above which such dislocation wall is formed, depends on the disclination strength ω 0 and the distance y 0. The maximal misorientation of the wall is equal to the disclination strength ω 0 at y 0 = 0. With an increase in the external stress, the wall overcomes the force barrier of the disclination and moves as a whole to the lateral surface of the crystal. Multiple reiteration of this process leads to the possible appearance of the system of broken from both sides dislocation walls in the body of grain.
Continuum dislocation theory accounting for redundant dislocations and Taylor hardening
International Journal of Engineering Science, 2016
This paper develops the phenomenological continuum dislocation theory accounting for the density of redundant dislocations and Taylor hardening for single crystals. As illustration, the problem of anti-plane constrained shear of single crystal deforming in single slip is solved within the proposed theory. The distribution of excess dislocations in the final state of equilibrium as well as the stress-strain curve exhibiting the Bauschinger translational work hardening and the size effect are found. Comparison with the stress-strain curve obtained from the continuum dislocation theory without the density of redundant dislocations and Taylor hardening is provided.
Strain path change effect on dislocation microstructure of multicrystalline copper sheets
Materials Chemistry and Physics, 2006
In this study, coarse-grained copper sheets were subjected to tension-rolling and rolling-tension strain path sequences. In both cases, two different types of strain path change were studied: the tensile and rolling directions were parallel and normal to each other. TEM observations of deformed samples showed the typical dislocation structures for the prestraining paths in tension and rolling. Special microband features, not observed during prestrain, were found during the second strain path, whatever the sequence and type of strain path change. The microstructure observed during reloading is discussed in terms of the sequence and type of strain path change, parallel or normal. The frequency of appearance of microbands is discussed in terms of the activity of new slip systems, i.e. not active during the prestrain path and connected with the number of the active slip systems after reloading. The results from this study, obtained for coarse-grained multicrystalline copper sheets, are compared with previous ones for fine and medium-grained copper.