Diffusion of chromium and palladium in β-titanium

Abstract

Interdiffusion coefficients in the β phase of Ti-Cr and Ti-Pd alloys have been determined by Matano's method in the temperature range between 1173 and 1823 K with (pure Ti)-(Ti-4.62 at% Cr alloy) and (pure Ti)-(Ti-1.84 at% Pd alloy) couples. The impurity diffusion coefficients of Cr and Pd in β-Ti have been determined by extrapolating concentration dependence of the interdiffusion coefficient to the infinite dilution of solute, and the following results have been obtained. In the whole temperature range, the diffusion coefficient of Cr, DCr, in β-ti is about twice as large as the self-diffusion coefficient in β-Ti, DTi. The magnitude of diffusion coefficient of Pd, DPd, in β-Ti lies in the range between DCr and DTi, that is, at higher temperatures DPd is nearly of the same magnitude as DCr and at lower temperatures it is nearly equal to DTi. Both the Arrhenius plots of DCr and DPd indicate an upward curvature showing the behavior of so-called anomalous diffusion in b.c.c. metals, but the curvature of DCr is much looser than those of DPd and DTi. The curved Arrhenius plots have been explained by the monovacancy mechanism taking into account a temperature dependent migration energy of a vacancy due to the softening of the longitudinal acoustic LA 2/3 〈111〉 phonon in β-Ti. It has been recognized that the activation energy estimated for the monovacancy mechanism in the impurity diffusion of Cr and Pd and self-diffusion in β-Ti is proportional to the square of radius of the diffusing atom. This suggests that the size effect is dominant in the impurity diffusion in β-Ti.