Thermal etching of α-Zr single-crystal surfaces

Extensive thermal etching of α-Zr single crystals has been found to occur during high-temperature annealing (820°C) under ultra-high vacuum (< 1.0 × 10−7Pa). Two grades of material were examined, Z1, high purity; and Z2, nominally pure: levels of the “surface active” element, Fe, were about 1 and 50 ppma, in Z1 and Z2, respectively. In Z1, strong faceting occurred on a high-index surface (8° off the (1010) plane) and weak linear facets appeared on the (1010) plane. In Z2, etch pits and linear groove defects formed on the (1010) plane. Etch-pit formation may be promoted by Fe segregation to dislocations, Fe-rich precipitates were found at the bases of clusters of pits. Etch pit counts were consistent with dislocation densities of about 1.0 × 1011/m2. The (0002) plane remained comparatively flat, but Fe-rich, needle-shaped precipitates at 60° angles with each other were formed. An analysis of the results implies that the surface energies increase in the order (0002) < (1011) < (1010), and that the etching mechanism is surface diffusion.