SOLUTE DIFFUSION IN ALPHA-ZR - RUTHERFORD BACKSCATTERING AND SECONDARY-ION MASS-SPECTROMETRY STUDY

Substitutional diffusion coefficients D of Hf, Ta and Pb have been measured in alpha-Zr single crystals and the diffusion profiles analysed by Rutherford backscattering (Hf, Ta and Pb) and secondary-ion mass spectrometry (Hf). Measurements were made either parallel (D parallel-to) or perpendicular (D perpendicular-to) to the specimen c axis. Although the main study is concerned with the influence of Fe on the diffusion of Hf, Ta and Pb at 1100 K, some preliminary data are also reported for Pb and Ta at 872 and 1000 K. The initial measurements at 1100 K show that D(Ta) > D(Hf) > D(Pb); the Zr specimens used for these measurements were then treated to reduce the Fe content (65-30 at.p.p.m.) and the diffusion coefficients remeasured. With one exception, D parallel-to for Hf, the diffusion coefficients fell by a factor of about two with the decrease in Fe concentration; D parallel-to for Hf fell by about an order of magnitude. All the D values measured at 1100 K for Hf, Ta and Pb lie in the range 4 x 10(-19) -3 x 10(-17) m2 s-1: those for Hf, and their dependence on the Fe concentration (with the exception of the low-Fe D parallel-to datum), are notably similar to alpha-Zr self-diffusion values presented in a very recent study. Together, the results are considered in terms of intrinsically normal diffusion with enhancement of the more slowly diffusing species by an Fe-associated defect. A comparison of solute D values measured in alpha-Zr, where Fe levels have not been deliberately reduced, shows that substitutional solute diffusion increases with decreasing size and valence.