On rate sensitivity of f.c.c. metals, instantaneous rate sensitivity and rate sensitivity of strain hardening

Abstract

A n up-to-date approach to the phenomenon of rate sensitivity observed in f.c.c. metals is discussed. It is shown that the rate sensitivity of strain hardening, which so far has been neglected, plays an equal or even dominant role in an estimation of the total rate sensitivity. It is suggested that the presence of the rate sensitivity of strain hardening is developed by the athermal generation of structural defects (dislocations), while at the same time collision and partial annihilation of dislocations occurs with the assistance of thermal activation. These micromechanisms of plastic deformation are capable of developing so-called strain rate and temperature history effects. Some experimental evidence of strain rate history effects, for both polycrystals and monocrystals, are provided in this paper; they are discussed within the framework of instantaneous rate sensitivity versus rate sensitivity of strain hardening. Both rate sensitivities are reviewed within the framework of thermal activation strain rate analysis. Experimental data are provided for aluminium, copper and lead. They conclusively demonstrate the importance of rate sensitivity of strain hardening which is developed by dynamic recovery (annihilation of defects during plastic deformation). Some fundamentals of how to construct constitutive relations have been discussed on the basis that the total flow stress τ is the sum of the effective stress τ∗ and the internal stress τ μ. General relations for structural evolution have been analysed, which are able to describe strain rate and temperature history effects. On the basis of earlier results a general relationship for structural evolution has been proposed using the concept of the effective dislocation multiplication coefficient M eff. It is shown that an evolutionary relationship should be of differential type with generation and annihilation terms. Finally, some recommendations are provided as to constitutive modeling and future studies of both instantaneous rate sensitivity and rate sensitivity of strain hardening.