THERMAL ETCHING OF ALPHA-ZR SINGLE-CRYSTAL SURFACES

Extensive thermal etching of alpha-Zr single crystals has been found t o 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 activ e'' 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 of f the (10 (1) over bar 0) plane) and weak linear facets appeared on th e (10 (1) over bar 0) plane. In Z2, etch pits and linear groove defect s formed on the (10 (1) over bar 0) plane. Etch-pit formation may be p romoted by Fe segregation to dislocations, Fe-rich precipitates were f ound 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) pla ne remained comparatively flat, but Fe-rich, needle-shaped precipitate s 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 me chanism is surface diffusion.