Softening of high-purity body-centred cubic metals by alloying

The softening of high-purity body-centred cubic metals by interstitial or substitutional foreign atoms, a widespread, extensively studied phenomenon, withstood all attempts of convincing explanation over several decades, in spite of a striking simplicity of its main features. The proposal made by one of the authors in 1995 is shown to account for all principal features of alloy softening, e.g. that it occurs in bcc mono-and polycrystals but not in face-centred cubic metals, that it affects only the strongly temperature-dependent contribution to the flow stress but not below a fairly well-defined deformation temperature, and that in a given metal the main characteristics of the softening are independent of the nature and the concentration of the solute, the only major exception being the temperature up to which the softening extends. The observations are explained by a single hypothesis, viz. that the incorporation of foreign atoms into the cores of a0/2 screw dislocations shifts their transformation from a configuration capable of slipping on one of its {110} slip planes to a configuration with a definite {112} slip plane from the so-called lower-bend temperature T = Ť of the pure metal to a higher temperature in the alloy-softened material. Examples are given for the quantitative verification of several predictions of this hypothesis. General consequences for the study of ultra- pure metals are discussed