Microstructural evolution and deformation mode under high-temperature-tensile-deformation of the Ti-6Al-4V alloy with the metastable α' martensite starting microstructure

Ti-6Al-4V alloy having the metastable α' martensite starting microstructure was investigated for microstructural changes during high-temperature-tensile-deformation at temperatures from 700 °C to 900 °C. As compared to the deformation of Ti-6Al-4V alloy with similar lamellar morphology consisting of an equilibrium (α+β) phase, the quite larger elongation to fracture and the higher strain-rate-sensitivity (m) are exhibited in the case of the α' martensite starting microstructure. The dynamic globularization associated with an occurrence of the discontinuous dynamic recrystallization is enhanced during tensile deformation of the α' martensite starting microstructure, resulting in the activation of grain boundary sliding at latter stage of deformation (which contributes to an increase in tensile elongation). Furthermore, dynamic β precipitation from the α' martensite during deformation also results in contribution to an additional stress-accommodation mechanism. As compared to the case in the equilibrium (α+β) lamellar microstructure, the present work indicates that the metastable microstructure of the α' martensite starting microstructure is more beneficial for an enhancement of high temperature ductility associated with enhancement of dynamic globularization and an occurrence of boundary sliding.