Hot deformation behavior of Ti-6Al-4V alloy: Effect of initial microstructure

Abstract

The purpose of this study was to identify plastic flow behavior and microstructural evolution during sub-transus hot deformation of a Ti-6Al-4V alloy with three initial microstructures (namely, transformed β microstructure, bimodal microstructure, and α′ martensitic microstructure) through compressive deformation at different strain rates in a Gleeble simulator and via SEM and TEM examinations. When deformed at a temperature of 800 °C and 850 °C (below the beta transus temperature of ∼975 °C), samples with a transformed β microstructure exhibited globularization of lamellar α phase at a strain rate of 10−3 s−1, while adiabatic shear banding occurred at a strain rate above 10−3 s−1 due to the presence of initial coarse β grains. Equiaxed grain microstructures were present after hot deformation in the samples containing a bimodal starting microstructure mainly at lower strain rates, and in the samples with an α′ martensitic starting microstructure at all strain rates from 10−3 s−1 to 1 s−1. Especially, a homogenous ultra-fine grained structure with an average grain size of 400 nm was achieved at both deformation temperatures at a strain rate of 1 s−1 in the case of α′ martensitic starting microstructure. The new ultra-fine grains were formed mainly through the phase transformation of α′/α+β and the subsequent continuous dynamic recrystallization during hot deformation. This opens the door to the development of homogenous ultrafine-grained structure from a fully martensitic initial microstructure via hot deformation (e.g., hot rolling, extrusion, forging).