Dislocation processes involved in deformation

In this section, the basic dislocation processes involved in the progressive deformation of a crystalline solid are discussed briefly to provide background for the detailed discussion of the deformation microstructures observed by TEM in specific minerals to follow. Particular attention is given to relating the nucleation, glide, climb, multiplication, and interaction of dislocations to the various stages of the creep and stress-strain curves. More discussion can be found in the texts referred to in Section 9.1. 

The dislocation nucleation just discussed is a preyield phenomenon in any deformation experiment, it may occur (i) during any preconditioning treatment at temperature and pressure before the shear stress is applied, (ii) during the incubation period in a creep test, or (iii) during the nominally elastic region in a constant strain-rate experiment. Thus, the microstructure of the crystal immediately prior to the onset of deformation may not be the same as the microstructure of the as-grown crystal. 

2 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 

In a constant strain-rate experiment, the rapid multiplication of dislocations following the yield point can produce more mobile dislocations than are necessary to maintain the imposed strain-rate and consequently the stress drops. The deformation will continue at a constant stress provided any decrease in u is compensated by an increase in iom, or vice versa. However, in general, the stress rises with increasing strain. The slope (dajdt) of the stress-strain curve is determined by the competition between two dislocation processes namely, work-hardening and recovery, which we now consider briefly. 

Although there is as yet no unified theory of work-hardening, the process is essentially due to the mutual interaction between the dislocations, which produces barriers to continuing dislocation motion. Some of the important interaction processes include  

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