Influence of phase volume fraction on the grain refining of a Ti-6Al-4V alloy by high-pressure torsion

A cold-rolled Ti-6Al-4V sheet was subjected to three different heat treatments prior to processing by high pressure torsion (HPT). Quantitative measurements revealed that the volume fractions were 70% equiaxed α phase and 30% lamellar (α + β) (Ti64-1), 47% equiaxed α phase and 53% lamellar (α + β) (Ti64-2) and 25% equiaxed α phase and 75% lamellar (α + β) (Ti64-3) and the grain sizes of the α phase were 7.0 ± 2 μm, 9.0 ± 1.5 μm and 9.5 ± 1.5 μm, respectively. The processing by HPT was performed at room temperature with a pressure of 6.0 GPa and a rotation speed of 1 rpm. Processing of the three heat treatment batches was conducted through a total number of revolutions, N, of 1/4, 1, 5, 10 and 20. The results show that the microhardness increases with increasing numbers of turns in HPT processing and for all three conditions stable microhardness values are reached after about 20 turns. The grain sizes after 20 turns of HPT were 115 ± 30 nm in Ti64-1, 85 ± 25 nm in Ti64-2 and 75 ± 15 nm in Ti64-3 so that the grain size decreases as the volume fraction of α phase decreases and the lamellar (α + β) increases.