Flow behaviour and microstructural evolution of Ti-17 alloy with lamellar microstructure during hot deformation in α+β phase field

Abstract

The effect of process variables on flow response and microstructure evolution during hot working of Ti-17 (Ti–5Al–2Sn–2Zr–4Mo–4Cr) alloy with lamellar microstructure was established using isothermal hot compression tests at strain rates of 0·001–10 s−1, test temperature between 780 and 860°C, and height reductions of 15–75%. All of the flow curves exhibited a peak stress followed by noticeable flow softening, and tended to exhibit a noticeably lower rate of flow softening at the strain of the order of 0·7. The peak flow stress decreased with increasing temperature and decreasing strain rate. Flow softening of the Ti-17 alloy was caused by kinking, break-up and globularisation of lamellas during deformation and, to a small extent, by deformation heating. At high strain rate, adiabatic shear bands and flow localisation were observed to play a role in flow softening. In α+β phase region discontinuous yielding was discovered at higher temperature (⩾820°C) and 10 s−1. According to the deformation microstructure, lamellar α phase gradually kinked, broke up and globularised of lamellas resulting from the development of shear bands and the penetration of β phase during deformation in the α+β phase field. The degree of dynamic globularisation of α phase increased with increasing strains, temperature and decreasing strain rate.