Effect of prior beta processing steps on microstructural refinement during thermomechanical processing of a two phase (alpha+beta) titanium alloy

Microstructural evolution in Ti-6.8Al-3.2Mo-1.8Zr-0.3Si alloy during a newly designed thermomechanical schedule has been systematically studied with the aim of obtaining a suitable microstructure for superplastic forming. The schedule involves prior processing in the beta phase field and subsequent rolling in the (alpha + beta) phase field. In all experiments the starting material was thermally or thermomechanically treated in the beta phase field and subsequently quenched in order to produce a martensitic structure. The morphology of the primary alpha in material for hot rolling could be substantially altered from that obtained with the conventional (alpha + beta) processing schedule of two phase titanium alloys. Prior beta processed microstructure, (alpha + beta) rolling temperature, and deformation were found to influence the alpha phase morphology in the alloy. The effect of subsequent annealing in the (alpha + beta) phase field on microstructural stability has been examined. The results show that the proposed thermomechanical processing schedule provides a relatively wide temperature - strain 'processing window' in beta and (alpha + beta) phase fields over which a fine grain (< 5 mum) equiaxed alpha structure can be obtained in Ti-6.8Al-3.2Mo-1.8Zr-0.3Si alloy.