Changes in dislocation mobility and micro-hardness, on the (111) faces of crystals which were exposed to the atmosphere, light irradiation and heat, were investigated. A marked increase (up to 300%) in hardness was observed after irradiation-assisted aging in air. Its cause was suggested to be photo-oxidation of the near-surface (1 to 5μ) layer. The intercalation of absorbed O during aging in air (in the dark) resulted in a slight (about 11%) increase in the hardness, while the dislocation mobility in the oxygenated crystals was reduced by a factor of 3. Abrupt softening of the photo-oxidized layer upon heating to 310 or 480K was observed; thus indicating a 2-stage structural transformation. An anomaly was found in the temperature-dependence of the hardness of pristine samples at temperatures above 470K.

I.Manika, J.Maniks, J.Kalnacs: Philosophical Magazine Letters, 1998, 77[6], 321-6