Hot working of commercial Ti–6Al–4V with an equiaxed α–β microstructure: materials modeling considerations

The hot deformation behavior of Ti–6Al–4V with an equiaxed α–β preform microstructure is modeled in the temperature range 750–1100°C and strain rate range 0.0003–100 s−1, for obtaining processing windows and achieving microstructural control during hot working. For this purpose, a processing map has been developed on the basis of flow stress data as a function of temperature, strain rate and strain. The map exhibited two domains: (i) the domain in the α–β phase field is identified to represent fine-grained superplasticity and the peak efficiency of power dissipation occurred at about 825°C/0.0003 s−1. At this temperature, the hot ductility exhibited a sharp peak indicating that the superplasticity process is very sensitive to temperature. The α grain size increased exponentially with increase in temperature in this domain and the variation is similar to the increase in the β volume fraction in this alloy. At the temperature of peak ductility, the volume fraction of β is about 20%, suggesting that sliding of α–α interfaces is primarily responsible for superplasticity while the β phase present at the grain boundary triple junctions restricts grain growth. The apparent activation energy estimated in the α–β superplasticity domain is about 330 kJ mol−1, which is much higher than that for self diffusion in α-titanium. (ii) In the β phase field, the alloy exhibits dynamic recrystallization and the variation of grain size with temperature and strain rate could be correlated with the Zener–Hollomon parameter. The apparent activation energy in this domain is estimated to be 210 kJ mol−1, which is close to that for self diffusion in β. At temperatures around the transus, a ductility peak with unusually high ductility has been observed, which has been attributed to the occurrence of transient superplasticity of β in view of its fine grain size. The material exhibited flow instabilities at strain rates higher than about 1 s−1 and these are manifested as adiabatic shear bands in the α–β regime.