Effects of temperature on the plastic properties of aluminium crystals

Abstract

By changing the temperature of deformation, during tensile experiments on single crystals of aluminium , it has proved possible to separate reversible changes of flow stress with temperature from irreversible ones. Below 130° K the reversible change of flow stress is large; above room temperature it is hardly more than the change of elastic constants with temperature. These reversible changes are highly reproducible and, over a wide range of cold-worked states, proportional to the flow stress itself. The principal irreversible effect is a sharp yield drop, accompanied by a Lüders band, which appears when a crystal is strained plastically at a high temperature (e.g. 300° K) immediately after being heavily cold-worked at a low temperature (e.g. 90° K ). Intermediate annealing treatments reduce or eliminate this yield drop. Other experiments have proved that the yield drop is not a strain-ageing phenomenon but an extreme example of work softening, and that work hardening induced by preliminary deformation at the low temperature becomes unstable during plastic deformation at the high temperature and is rapidly removed. An explanation of the yield drop is suggested, based on recent theories of work hardening. It is proposed that, under the combined influence of stress and temperature, sessile dislocations at the heads of piled-up groups of dislocations become unlocked; the piled-up groups then partly collapse and many dislocations are released for slip. The reversible changes of flow stress are interpreted in terms of the intersection of dislocation lines.