Thermally activated glide in face-centred cubic metals and its application to the theory of strain hardening

Abstract

Abstract The influence of temperature and strain rate on the flow stress of polycrystalline aluminium, and single crystals of aluminium, copper and silver was studied. Tensile tests were carried out at various temperatures down to 1.7°K. The results indicate that the reversible change of flow stress (after correction for variation in elastic modulus in experiments involving temperature change) results from the thermally activated surmounting of barriers by glide dislocations. The main barriers are believed to be forest dislocations cutting the glide plane. The variation of the force between a dislocation and an obstacle with their separation was derived for the three metals; the values agreed well with the expected forms. The maximum force can be computed if the distance at which the force rises steeply is assumed to be equal to the Burgers vector; the values are, for aluminium, 3.5 × 10−5 dynes, and for copper, 6.3 × 10−5 dynes. It has been shown that the internal stress varying over distances larger than the separation between successive dislocations emitted from the same source will not contribute appreciably to the hardening, and that the most important source of hardening is the dislocation forest. This is borne out by the fact that the flow stress ratio is approximately the same for polycrystalline and single crystal specimens of aluminium deformed both in tension and torsion, and also for commercially pure aluminium. The rate of strain hardening in the linear region has been calculated using the above assumptions, and agrees well with experimental data.