Tracer diffusion and mechanism of non-Arrhenius diffusion behavior of Zr and Nb in body-centered cubic Zr–Nb alloys

Abstract

Tracer diffusion of Zr95 and Nb95 has been measured in a set of β-Zr-Nb alloys (Zr-5.5 at. % Nb, Zr-16.3 at. % Nb, and Zr-28.1 at. % Nb) in a wide temperature range from 1035 to 1871 K. All experimental concentration profiles revealed a Gaussian-like behavior over two to three decades in logarithm of concentrations. Experimental data were analyzed in terms of the Manning’s random alloy model and the partial correlation factors were calculated as functions of temperature. The partial correlation factors vary smoothly over the whole temperature range, whereas the diffusivity shows a pronounced curvature of the Arrhenius plots. A correlation was found between the degree of deviation from the Arrhenius behavior of the diffusion coefficients of both components and the lattice dynamics features of the measured phonon dispersions in their dependence on the Nb content in these alloys. The temperature dependence of the isotope effect parameter E of Zr diffusion was measured in the Zr-5.5 at.% Nb alloy. E essentially increases with temperature and the energy transfer factor ΔK deduced for the alloy was found to be somewhat larger than the corresponding values for pure β-Zr, especially at low temperatures. The whole set of experimental data leads to the conclusion that only one diffusion mechanism—the vacancy mechanism—is operating in the whole investigated temperature range.