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

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 beta microstructure, bimodal microstructure, and alpha' 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 degrees C and 850 degrees C (below the beta transus temperature of similar to 975 degrees C), samples with a transformed beta microstructure exhibited globularization of lamellar alpha 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 beta 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 alpha' 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 alpha' martensitic starting microstructure. The new ultra-fine grains were formed mainly through the phase transformation of alpha'/alpha+beta 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). (C) 2017 Elsevier B.V. All rights reserved.