Ultra-fine grain size Ti-6Al-4V alloy samples were prepared by severe plastic deformation (SPD) under non-isothermal deformation conditions by starting from beta quenched Widmanstatten needle-like structure using high hydrostatic pressure. This method produced an equiaxed, homogeneous grain structure at low process temperature, with average grain size of 0.3 - 0.4 microns of the alpha phase. The SPD Ti-6Al-4V shows enhanced superplasticity at low temperatures between 600~ 700°C (0.36~0.42 Tm). The microstructure evolution during processing and strain-rate sensitivity for superplastic deformation are studied. and possible rate controlling mechanism are presented. The strain-rate sensitivity of UFG Ti-6Al-4V was within 0.5~0.6, which suggests the grain boundary (or interface) sliding mechanism accommodated deformation; the largest elongation to failure of 1175% was achieved at 700°C and 5x10-4s-1. Flow curves of UFG Ti-6Al-4V show low yield stress at the beginning of straining, then pronounced strain hardening (sometimes called flow hardening) occurred with increasing temperature, which is related to concurrent grain growth during deformation. The apparent activation energy was determined to be 172 kJ/mol indicative of a combination of grain boundary diffusion and dislocation core diffusion. Kinetics of static grain growth shows the time exponent (n) to be 0.24, but the time exponent for dynamic grain growth is larger than that for static condition. Forming of hemispherical domes and sinewave shapes were performed on the UFG sheets at relatively low temperatures of 500-600°C, for this type of alloy.
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