Oxidation subscale of gamma-titanium aluminide

The subscale formed during high temperature rapid oxidation of gamma-titanium aluminum is revealed by transmission electron microscopy and microanalysis to consist of two phases: one hexagonal with unit cell dimensions a = 0.58 nm, c = 0.47 nm (+/- 0.005 nm), and a composition close to Ti6Al3O4; the other simple cubic with a = 0.69 nm (+/-0.005 nm) 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 P4(2)32. These subscale phases exhibit some tendency for a crystallographic orientation relationship with (110)(cubic)//(01 (1) over bar 0)(hexagonal) and [0 (1) over bar 1](cubic)/[(2) over bar 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.