The usual method of introducing engineers to the concept of dislocations and their role in plastic flow is to compare an estimate of the theoretical shear strength of solids (of order μ/30 where (I is the shear modulus) and the observed critical resolved shear stress of either single crystals (μ/10~4) or practical engineering material such as structural steels where the yield stress in shear is of order μ/5Xl0~2. However, if we consider the problem in reverse, we can consider the accumulation of dislocations as one important mechanism by which we can produce engineering materials in which the strength level approaches the theoretical strength. If we assume that the flow stress can be expressed in terms of the mean free path between stored dislocations or as the square root of the global dislocation density, then we can see the influence of dislocation density in a diagrammatic form as in Figure 1. We are interested both in the understanding of the physics of deformation in materials with a high density of dislocations and in the development of ultra high strength materials with useful combinations of properties such as strength and ductility, strength and toughness, strength and electrical conductivity. The salient factor to be considered in this regard is that in a number of systems with increasing imposed strain both the tensile strength and the ductility It is clear that the strengthening by dislocation accumulation due to large imposed plastic strains represents an important approach both to the development of new, potentially valuable, engineering materials and an important area of basic understanding in terms of the mechanical response of materials close to their theoretical strength. Thus, this article will survey some of the factors which influence dislocation accumulation at large strains and the consequences of such accumulation processes.
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