Hot deformation mechanisms in Ti–6Al–4V with transformed β starting microstructure: commercial v. extra low interstitial grade

Abstract

The hot deformation behaviour of commercial and extra low interstitial (ELI)grades of Ti–6Al–4V (Ti-6-4) alloy with a transformed β starting microstructure has been studied in the temperature range 750–1100°C and strain rate range 0.001–10 s-1. On the basis of the flow stress data as a function of temperature and strain rate, processing maps have been developed for these two grades and compared in order to bring out the differences, if any. While the stress–strain behaviour has not varied appreciably with the grade of Ti-6-4, significant differences have been observed in the processing maps as well as the tensile ductility variation with temperature. At lower strain rates in the α–β range (<0.01 s-1), both the grades exhibit globularisation of the lamellar structure, the optimum temperature being higher for the commercial grade than the ELI grade. The apparent activation energy for globularisation is higher in the commercial grade (455 kJ mol-1)than that of the ELI grade (370 kJ mol-1). At temperatures lower than about 900°C and strain rates less than about 0.1 s-1, a regime of strain induced porosity (SIP)at the prior β grain boundaries has been observed and the SIP regime is narrower in the ELI grade than the commercial grade. Strain induced porosity cracks are nucleated as a result of the stress concentrations produced by the sliding of prior β grain boundaries which is promoted by the lower strain rates. The mechanism of hot deformation in the β range is sensitive to the grade of Ti-6-4. In the ELI grade, the β phase deforms by large grained superplasticity, but deformation close to the transus nucleates voids within the prior β grains resulting in a drop in the tensile ductility. On the other hand, the commercial grade exhibits dynamic recrystallisation of β phase. The apparent activation energy for β deformation is lower in the commercial grade (173 kJ mol-1) than the ELI grade (287 kJ mol-1), although both the values are comparable to that for self­diffusion in β. The flow instability regime, as predicted by the continuum instability criterion, is not significantly different in the two grades of Ti-6-4 even though the domain of cracking along the adiabatic shear bands is wider in the commercial grade than the ELI grade.