The observation of enrichment of O and Zr and depletion of Nb in the near surface region of a β-(Zr-20%Nb) alloy

Thermal treatment of β-(Zr-20%Nb) alloy results in the redistribution of Zr, Nb and dissolved oxygen impurity at the surface and eventually over depths of 100's of nm. Initially at 200 < T < 300 C, only the segregation of oxygen occurs to the top 2 or 3 atomic layers (<few nm). At higher temperatures the surface concentration of Zr also increases, while the Nb concentration decreases. Depth profiling shows that beneath the segregated region, there exists a Nb-depleted and Zr-enriched region that extends over ∝100 nm. The depth over which this region extends increases with annealing time and temperature. The driving force for the observed changes of concentration in the surface region during annealing is discussed. The diffusion coefficient of oxygen in the β-(Zr-20%Nb) alloy at 175-300 C is obtained from the kinetics of segregation which yielded a value of 6.9 x 10-5 exp(-35720 cal/RT) m2/s. (orig.)