DISLOCATION MECHANICS ASPECTS OF PLASTIC INSTABILITY AND SHEAR BANDING

A dislocation mechanics based constitutive equation explanation is given for the observation that face-centered-cubic (fcc) metals are less shear band prone than body-centered-cubic (bcc) ones. Factors relating to plastic flow mechanisms within the polycrystal grains of a material as well as at the boundaries between the grains are important: (1) In the fcc case, dislocation ''forest'' intersections that accumulate with increased straining produce a thermally-activated component of the polycrystal flow stress. As a consequence, the dislocation forest build-up gives a positive strain rate dependence of strain hardening that leads to more stable plastic flow. (2) In bcc metals and alloys, more so than fcc ones, dislocation pile-up stress concentrations occur at the polycrystal grain boundaries to produce a strong (Hall-Petch) grain size dependence of the plastic flow stress. The localized heating at such pile-up breakthroughs provides a fundamental mechanism for the promotion of shear banding.