Oxidation Subscale of γ-Titanium Aluminide

The subscale formed during high temperature rapid oxidation of γ-titanium aluminum is revealed by transmission electron microscopy and microanalysis to consist of two phases: one hexagonal with unit cell dimensions a = 0.58nm, c = 0.47nm (±0.005nm), and a composition close to Ti6Al3O4; the other simple cubic with a = 0.69nm (±0.005nm) and composition Ti3Al2O3, with oxygen concentration known to about 10% accuracy. The results strongly suggest that the hexagonal phase is a solid solution of O in Ti3Al, while the cubic phase is a novel ternary compound with space group P4232. These subscale phases exhibit some tendency for a crystallographic orientation relationship with (110)cubic//(011¯0)hexagonal and [01¯1]cubic//[2¯112]hexagonal, and the hexagonal phase is less stable than the cubic phase as the distance to the sample surface decreases and oxygen level increases.