THERMAL ETCHING OF ALPHA-ZR SINGLE-CRYSTAL SURFACES

Extensive thermal etching of alpha-Zr single crystals has been found to occur during high-temperature annealing (820 degrees C) under ultra-high vacuum (<1.0 X 10(-7) Pa). 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 22, respectively. In Z1, strong faceting occurred on a high-index surface (8 degrees off the (10 (1) over bar 0) plane) and weak linear facets appeared on the (10 (1) over bar 0) plane. In Z2, etch pits and linear groove defects formed on the (10 (1) over bar 0) 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 X 10(11)/m(2). The (0002) plane remained comparatively flat, but Fe-rich, needle-shaped precipitates at 60 degrees angles with each other were formed. An analysis of the results implies that the surface energies increase in the order (0002) < (10 (1) over bar 1) < (10 (1) over bar 0), and that the etching mechanism is surface diffusion.