Synthesis of the cubic Z-phase in the Ti-Al-O system by a powder metallurgical method

The formation of protective or non-protective oxide scales on gamma-TiAl alloys is closely related to the composition of the subsurface depletion layer. Recently it was shown that in the depletion layer a cubic Z-phase is formed, which was until then not known in the Ti-Al-O phase diagram. In the present study a method was developed to synthesise this cubic phase by a powder metallurgical method. It was found that the Z-phase can be produced by a reaction between the alpha-TiOx solid solution and aluminium using self-propagating high temperature synthesis (SHS). The phase analysis and microstructural characterisation of the Ti-O-Al sintering products show that at 900-950 degrees C the Z-phase exists in equilibrium with gamma-TiAl(O) and alpha-Al2O3, as well as with Ti2O and alpha(2)-Ti3Al(O) ordered solid solutions. According to X-ray and electron diffraction data the Z-phase has a primitive cubic structure of one of the space groups P 2(1)3 or P 4(1)32 and has a composition close to Ti5Al3O2. The lattice parameter of the cubic phase was determined to be a = 0.691 +/- 0.001 nm. (C) 1997 Elsevier Science Ltd.