Tracer diffusion of hafnium, niobium and zirconium in Hf-Nb alloys

Abstract

The tracer diffusion of 181Hf, 95Nb and 95Zr was measured in the BCC structure of a Hf-Nb(10 at.%) and a Hf-Nb(18 at.%) alloy. It was the primary aim of the measurements to test whether self-diffusion in the two alloys investigated is in accordance with the Arrhenius law of diffusion or reveals any curvature, as was previously found for BCC Zr-Nb alloys of different composition. For Zr-Nb alloys a continuous change in the diffusion anomaly with respect to self-diffusion in pure BCC Zr was found, reflecting the systematic variation in the electronic structure when alloying group V to group VI BCC transition metals. The classification of self-diffusion in BCC hafnium as normal or 'anomalous' has not been possible so far because of the rather limited existence range of its BCC phase of only 485 K. Alloying Hf with the group V metal Nb stabilizes the BCC structure down to lower temperatures and extends its existence range considerably. Since the model of phonon-related diffusion predicts self-diffusion in BCC Hf to show in principle a curved Arrhenius plot, one could expect this curvature to be revealed in the two Hf-Nb alloys. However, the measurements yielded straight Arrhenius plots for the Hf-Nb(10 at.%) and the Hf-Nb(18 at.%) alloy. Since the tracer diffusion of the alloying component Nb was studied as well, the random alloy model for concentrated solid solutions allowed the calculation of the partial correlation factors fHf and fNb. The values of f extrapolated to pure BCC hafnium are in good agreement with the theoretical value for monovacancy diffusion.