Influence of phase volume fractions on the processing of a Ti–6Al–4V alloy by high-pressure torsion

Using appropriate heat treatments, different volume fractions of the constituent phases were produced in a Ti–6Al–4V alloy. Thus, in Ti64-1 there was ∼85% of an equiaxed α-phase and ∼15% of a lamellar (α+β) microstructure and in Ti64-2 there was ∼48% of the α-phase and ∼52% of the (α+β) microstructure. These two alloys were processed by high-pressure torsion (HPT) at room temperature under an applied pressure of 6.0 GPa. Both materials showed increases in the microhardness and decreases in the grain size after HPT with a stabilization after about 10 turns. Higher hardness values and smaller grain sizes were attained in Ti64-2 thereby demonstrating that greater grain refinement is achieved when processing an alloy with approximately equal volume fractions of the constituent phases. In these experiments, the measured equilibrium grain sizes after 20 turns of HPT were ∼130 nm in Ti64-1 and ∼70 nm in Ti64-2.