Load relaxation in aluminum: I. Theory of plastic deformation. II. Plastic equation of state

Abstract

According to the theory of thermally-activated deformation, the plastic strain rate equality\((\dot \in _p )_t = _{ - dt} = (\dot \in _p )_t = _{dt} \) will hold in a load relaxation experiment, wheret = 0 is de-fined as the time at which the crosshead stops. In this theory, plastic flow is intrinsically time dependent and its rate is controlled by interaction of glide dislocations with thermal obstacles(e.g. forest dislocations). The strain rate equation is of the form\(\dot \in _p = \dot \in _p (\sigma ,S,T)\) and att = 0 none of these variables changes instantaneously. Measurements reported here for [111] aluminum single crystals indicate that this prediction is wrong. The ratio\((\dot \in _p )_t = _{dt} /(\dot \in _p )_t = _{ - dt} \) is near zero at low stress and approaches unity only at high stress. This result is predicted if plastic strain itself is time-independent (athermal), as in the author’s recent theory. Time-dependent strain is then the result of thermal changes in structure, namely loss (recovery) and rearrangement of obstacle dislocations. Experi-ments were also done to test further the essential hypothesis of Hart’s recent formula-tion of an equation of state for plastic deformation-namely that each distinct σ-@#@ \(\sigma - \dot \in \) curve derived from load relaxation data corresponds to a unique “hardness” state and that re-covery does not occur. Significant differences were observed in the 77 K strsss-strain curves for 295 K relaxed and unrelaxed samples which indicate that substantial loss and some rearrangement of dislocations has occurred during the relaxation. It is concluded from both experiments that load relaxation in aluminum is a manifestation of recovery creep and cannot be taken as evidence for a plastic equation of state.