Dislocations mobility

Mechanical engineering Physical metallurgy, crystal dislocation mobility... 

Thus, in simple metals, interactions between dislocations rather than interactions between atoms, are most important. The hardnesses of metals depend on deformation hardening (dislocation interactions) rather than individual mobilities. The elastic resistance to shear plays a dominant role because it is directly involved with dislocation mobility. 

Since covalent bonding is localized, and forms open crystal structures (diamond, zincblende, wurtzite, and the like) dislocation mobility is very different than in pure metals. In these crystals, discrete electron-pair bonds must be disrupted in order for dislocations to move. 

The next two figures show that crystal structure type and ionicity also play a role in determining dislocation mobility, and therefore hardness. First, if data for the III-N compounds are plotted on Figure 5.2 they do not fall on the regression line. The reason is that they have hexagonal rather than cubic crystal structures. However, when plotted by themselves as in Figure 5.3 their hardnesses are proportional to their bond moduli. 

A plot of them (Figure 5.6) shows that they are proportional to the bond moduli. Thus the bond moduli are fundamental physical parameters which measure shear stiffness, and vice versa. Also, it may be concluded that hardness (and dislocation mobility) depends on the octahedral shear stiffnesses of this class of crystals (see also Gilman, 1973). 

J. J. Gilman, Quantized Dislocation Mobility, in Micromechanics of Advanced Materials A Symposium in Honor of Professor James Li s 70th Birthday, Ed. by Chu, Liaw, Arsenault, Sadananda, Chan, Gerberich, Chau, and Kung, The Mater. Soc, Warrendale, PA, USA (1995). 

V. R. Parameswaran and R. J. Arsenault, Dislocation Mobility Studies in Crystals— Four Decades in Retrospect, in The Johannes Weertman Symposium, Edited by R. J. Arsenault, D. Cole, T. Gross, G. Kostorz, P. K. Liaw, S. Parameswaran, and H. Sizek,The Materials Society, Warrendale, PA, USA (1996). 

Two outstanding properties of FeB metallic glasses are their low magnetic permeabilities and their low acoustic attenuations. The former results from their lack of magnetic anisotropy and has led to their use in power transformers, theft detectors, and various electronic devices. The latter results from the very low dislocation mobility in them. 

The presence of grain boundaries also affect slip in the material even after plastic deformation has commenced. A phenomenon known as strain hardening, or work hardening or cold working, is the result of constrained dislocation mobility and increased... 

The yield point, work-hardening, and recovery. The yield stress, whether in a creep or a constant strain-rate experiment, is determined by the onset of dislocation mobility, usually glide. The subsequent deformation depends on the density mobile dislocations and their speed v. Provided the dislocations are distributed reasonably homogeneously in the specimen, the deformation is described by the Orowan equation... 

The water-weakening effect observed in the stress-strain curves of specimens deformed in either regime is not reflected in the observed microstructures. This may be because the small difference in strength of wet and dry specimens (only a factor of about 2) is a consequence of only a small change in the relative activities of different deformation mechanisms, such as glide versus climb or dislocation mobility versus diffusion-controlled creep. However, these observations are not consistent with the assertion of Mackwell et al. (1985) that recovery due to climb is significant... 

On both Crl8Re and Cr35Re alloys, the main contribution to deformation is nevertheless by dislocation slip. Even at room temperature dislocation mobility in Cr-35Re alloys is enough form localized cellular... 

In materials with high dislocation mobility such as copper, dislocation patterns proceed through the rapid motion of dislocations in a very small volume of the specimen [36]. Under high strain rate deformation conditions, it is expected that the dislocations move at subsonic speed or even as fast as the shear wave velocity. The random motion of dislocations on their slip planes causes random changes not only in the local dislocation densities, but also in the dislocation velocities. 

J. Lothe, Elastic Strain Fields and Dislocation Mobility, V. L. Indenbom, J. Lothe., Eds. ( Amsterdam north-Holland, 1992) p.447. 

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