On the mechanism of work hardening in face-centred cubic metals, with special reference to polycrystalline copper

Abstract

Tensile specimens of oxygen-free polycrystalline copper (99.9911%) were annealed for various periods in vacuo, in the range 500–700°C so as to obtain variations in the grain sizes and yield points. The initial rate of work hardening of all specimens was linear up to strains of about 1.5%, and independent of the magnitude of the yield stress. This hardening appeared to be of the type responsible for the linear hardening which follows easy-glide in single crystals, the theory of which has recently been developed by Friedel. At higher strains parabolic hardening occurred. In this stage the coefficient of work hardening, χ _p , defined as the ratio d[sgrave]²/dε, where [sgrave] and ε are the tensile stress and strain respectively, was found to obey the relation χ _p ≃G[sgrave]_Y/π, i.e. it was proportional to the product of the shear modulus G, and the tensile yield stress [sgrave]_Y. This equation is derivable from Mott's theory, in which parabolic hardening is regarded as resulting from the action of the long range stress fields of piled up dislocation arrays on Frank-Read sources in the grains.