Effect of nonequilibrium defects due to a phase transformation on the diffusion ofβ-zirconium

Abstract

The effect of the hcp\ensuremath{\rightarrow}bcc phase transformation on the anomalous diffusion characteristics of bcc zirconium has been investigated in this study. $^{113}\mathrm{Sn}$ and $^{59}\mathrm{Fe}$ tracers were used to typify the diffusing nature of self- and fast-diffusing atoms. A differential measurement of diffusion in two samples was carried out under very different preannealing conditions but otherwise identical states of diffusion time and temperature. In the first set of experiments, one of the samples was annealed for lengthy periods of time at premelting temperatures prior to diffusion in an attempt to decrease the defect concentration produced by the phase transformation. During the second set of experiments, the sample was cycled three times through the phase transformation temperature (\ensuremath{\sim}890\ifmmode^\circ\else\textdegree\fi{}C) (hcp\ensuremath{\rightarrow}bcc\ensuremath{\rightarrow}hcp\ensuremath{\rightarrow}bcc), with a view to enhance the defect density due to phase changes. In both sets of experiments, the second sample was raised to the diffusion temperature directly. The relative diffusivities of the two samples were measured by serial sectioning and counting techniques. Appropriate correction methods were derived, for each set, to identify the proper interface because of the unique sectioning problems encountered. Within errors, no significant difference in diffusivity was found for $^{113}\mathrm{Sn}$ diffusion for differing preannealing conditions. A small difference (\ensuremath{\sim}30%) was found for $^{59}\mathrm{Fe}$ diffusion, but this result was somewhat obscured by the non-Gaussian tracer profiles measured for this case. The results show that the anomalous diffusion behavior of bcc zirconium is not a result of excess defects introduced by the hcp\ensuremath{\rightarrow}bcc phase transformation.