HAFNIUM DIFFUSION IN ZIRCALOY-2 AND ZR-2.5 WT-PERCENT NB - A RUTHERFORD BACKSCATTERING STUDY

Rutherford backscattering has been applied to the determination of the temperature dependence of Hf diffusion in the commercial alloys Zr-2.5 wt.% Nb and Zircaloy-2. The diffusion anneals were made, under vacuum, in the temperature interval 830-1101 K; the specimens were well annealed with large (7-20-mu-m) grains. The diffusion coefficients, D, are of the same order of magnitude as self-diffusion values (10(-17) to 10(-21) m2/s) over the comparable temperature range; also they display broadly similar temperature dependences. These results indicate that diffusion in the alloys, like that in pure Zr, is controlled by the presence of trace Fe in solid solution. The D values for Zircaloy-2 are generally higher than those for Zr-2.5 wt.% Nb, but the difference is relatively small. The small differences in substitutional diffusion among Zircaloy-2, Zr-2.5 wt.% Nb and Zr may be attributable to "compositional and microstructural effects".