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-Ti Al alloys is closely related to the composition of the subsurface depl etion 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 phas e diagram. In the present study a method was developed to synthesise t his cubic phase by a powder metallurgical method. It was found that th e Z-phase can be produced by a reaction between the alpha-TiOx solid s olution and aluminium using self-propagating high temperature synthesi s (SHS). The phase analysis and microstructural characterisation of th e Ti-O-Al sintering products show that at 900-950 degrees C the Z-phas e 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 co mposition 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.