Abstract

Examination of small euhedral crystals of α-Al2O3 (sapphire) that were observed following oxidation of aluminum and an aluminum alloy in wet hydrogen at high temperatures showed that growth probably occurs by decomposition of AlO on an Al2O3 surface. Rapid growth occurs on the tips of needles and the edges of platelets at lattice steps formed by intersection of hollow screw dislocations with these surfaces. Screw dislocations were detected by x-ray measurement of the lattice twist they produce in the needle-like crystals or ``whiskers.'' Needles were bent to elastic strains of over 2% corresponding to a strength of 1011 dyne cm−2.