Grain refining of a Ti-6Al-4V alloy by high-pressure torsion and low temperature superplasticity

Abstract

A cold-rolled Ti-6Al-4V sheet was subjected to heat treatmentprior to processing by high-pressure torsion (HPT). Quantitative measurements revealed that the volume fractions were 25% equiaxed α phase and 75% lamellar (α+β). The grain size of the α phase was 9.5±1.5 µm. 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 material was conducted through a total number of revolutions, N, of 1/4, 1, 5, 10 and 20. After 1 turn in HPT, the traces of shear deformation appeared. When the turns increased to 5, shear deformation was more severe. After 10 turns, the fibrous structure basically disappeared and ultrafine equiaxed grains were formed. When the deformation proceeded to 20 turns, a series ofhigh-angle boundaries were formed in the sample and the average size of equiaxed grains was 77±15 nm.After HPT processing, the microhardness increased as the numbers of HPT turns increased and the distribution of hardness tended to be relatively homogeneous when the HPT continued to 20 turns.Superplastic deformation tests were performed at 873K at different strain rates in the material processed by HPT and an elongation of 790% was achieved when deforming at 873K and 10-4 s-1.