Pressure dependence of self- and solute diffusion in bcc zirconium

The pressure effect of self-diffusion in the high-temperature bcc phase of zirconium has been studied with high accuracy using the radiotracer technique. Activation volumes of 0.184+/-0.016 Omega (Omega: atomic volume) at 1423 K and 0.213+/-0.014 Omega at 1273 K were obtained. Simultaneously, the activation volumes of the Nb-95 solute diffusion were measured which amount to 0.161+/-0.014 Omega at 1423 K and 0.193+/-0.023 Omega at 1273 K. The small activation volumes indicate a strong relaxation of the vacancy. The large relaxation volume reflects the inherent weakness of the bcc lattice towards a shear in (111) direction. The results provide an explanation for the absence of positron trapping in group-IV transition metals.