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Mechanical Properties
Published in Alan Cottrell, An Introduction to Metallurgy, 2019
The general effect of the extra kinematic freedom allowed the dislocations by these thermally-assisted movements is to ‘tidy up’ the cold worked structure. Dislocations of opposite signs come together and annihilate each other. Those of the same sign polygonize into well-defined cell walls. These recovery processes limit the amount of hardening attainable by cold working and also cause cold worked metals to soften when heated. The cross-slip process, being a form of glide, depends on shear stress in the material for its occurrence, irrespective of whether this stress is produced by the applied forces or by the dislocation structure. It thus tends to set in characteristically at a certain level of work hardening and can happen even at very low temperatures, e.g. 0.1 Tm, at sufficiently high stresses, e.g. 10−2μ. By contrast, the climb process depends on the movements of vacancies (some of which may have been produced by the plastic deformation itself) and so tends to set in characteristically at a certain temperature level, about 0.3 Tm. Because it is less sensitive to the level of stress in the material it can produce much more complete softening than the cross-slip process.
Structural Description of Materials
Published in Snehanshu Pal, Bankim Chandra Ray, Molecular Dynamics Simulation of Nanostructured Materials, 2020
Snehanshu Pal, Bankim Chandra Ray
Cross-slip is a sort of dislocation movement whereby the slip planes move past one another. It is the procedure of the slip planes moving. Initially, a screw dislocation can move with one slip plane and then onto the next by virtue of the dislocation. It additionally encourages the screw disengagement to maintain a strategic distance from an obstruction on its underlying shape.
Creep properties and deformation mechanisms of single-crystalline γ′-strengthened superalloys in dependence of the Co/Ni ratio
Published in Philosophical Magazine, 2022
N. Volz, C.H. Zenk, N. Karpstein, M. Lenz, E. Spiecker, M. Göken, S. Neumeier
As a further strengthening contribution, we considered the stacking fault energy of the γ matrix, suggesting that a high stacking fault energy and therefore a small dissociation distance of partial dislocations promotes recombination and cross-slip. As a consequence, a high stacking fault energy is assumed to be disadvantageous for the creep properties compared to low . The stacking fault energies of the matrix compositions (given in Table 3) of the NCX alloys were calculated using Thermo-Calc. The results are illustrated in Figure 5e. The graph shows a steady decrease of the stacking fault energy with increasing Co-content. This would suggest easier cross-slip of dislocations on the Ni-rich side due to smaller splitting distances of the partial dislocations. Enhanced cross-slip would result in lower creep strength. However, the trend in the investigated alloy series is exactly the opposite. The creep properties are actually better in the Ni-rich alloys, although TEM investigations revealed dominant deformation in the matrix phase and TC predict the stacking fault energy to be higher. This is now also contradicting to the assumption that the strengthening contribution of the matrix phase is more pronounced in the Ni-rich alloys. Maybe the high solid solution strengthening contribution is the key factor for the better creep properties in the Ni-rich alloys. Nevertheless, also the changing deformation mechanisms could result in changing creep properties.
3D DDD modelling of dislocation–precipitate interaction in a nickel-based single crystal superalloy under cyclic deformation
Published in Philosophical Magazine, 2018
Bing Lin, Minsheng Huang, Liguo Zhao, Anish Roy, Vadim Silberschmidt, Nick Barnard, Mark Whittaker, Gordon McColvin
Discrete dislocation dynamics and its computer simulation have advanced significantly over the past two decades, where such important features as dislocation intersection, slip geometry, multiplication, line tension effects and cross-slip have been successfully modelled to simulate dislocation patterning observed in experiments. However, almost all studies are limited to isotropic and homogeneous media, and the interactions between dislocations and material microstructure, which is the major source for heterogeneous dislocation arrangements and the generation of internal stress concentration and initiation of cracks, is generally excluded for simplicity. One of our aims is to understand how the dislocation–microstructure interaction affects the global stress–strain behaviour during plastic deformation, which cannot be captured by the classical continuum model (e.g. crystal plasticity).
Hydrogen-induced strain localisation in oxygen-free copper in the initial stage of plastic deformation
Published in Philosophical Magazine, 2018
Yuriy Yagodzinskyy, Evgenii Malitckii, Filip Tuomisto, Hannu Hänninen
Markedly higher density of the shallow dislocation slip line offsets forming bunches with typical width of about 20 μm (see Figure 3(f)) are characteristic features of the Stage I of plastic deformation of copper with hydrogen. The observed refinement of the dislocation slip may originate from different reasons. It can be concluded that in the presence of hydrogen a higher number of dislocation sources is activated in the closely adjacent slip planes compared to that in the H-free specimen at the same amount of plastic strain. On the other hand, in the presence of hydrogen the refinement may result from the double cross-slip of screw dislocations. The two pieces of edge dislocation on the cross-slip plane act as the anchoring points for a new dislocation source. The loop expanding on the slip plane parallel to the original plane may in turn cross-slip and become further a new source, which results in the multiplication of dislocation sources in the closely adjacent slip planes. This process not only increases the number of dislocations on the original slip plane, but also causes the slip band to widen.