Hot workability and microstructure evolution of highly β stabilised Ti–25V–15Cr–0·3Si alloy

Abstract

The hot workability and microstructural evolution of a highly β stabilised Ti–25V–15Cr–0·3Si alloy have been studied using constant strain rate isothermal compression tests in the temperature range 900–1100°C and strain rate range 0·01–10 s⁻¹. It was found that all the flow stress curves were characterised by a sharp initial discontinuous yielding followed by either a steady state or continuous flow softening with strain. This alloy showed dynamic recovery at temperatures less than 950°C but dynamic recrystallisation at temperatures higher than 1000°C. At higher strain rates (>1 s⁻¹), ‘necklace’ recrystallisation, in which grain boundaries were decorated by finely recrystallised grains, was operative. At lower strain rates (<0·1 s⁻¹), however, a typical continuous recrystallisation with serrated grain boundaries occurred. ‘Necklace’ recrystallisation was associated with Ti₅Si₃ particles precipitating along beta grain boundaries, whereas continuous recrystallisation was attributed to substructure formation. This alloy showed poor workability at a strain rate greater than 1 s⁻¹ due to cracking or flow localisation. At lower temperatures (<950°C), the poor workability was associated with shear cracking along 45° orientation with respect to the compression axis due to intensive slip band formation. At higher temperatures (>1000°C), it was attributed to free surface cracking due to severe oxidation of element V and the secondary tensile stresses caused by bulging of the cylindrical specimen during upsetting. The cracking behaviour of Ti–25V–15Cr–0·3Si alloy can be evaluated from the critical strain to fracture ϵ_f. This strain increased with increasing temperature and decreasing strain rate. It is demonstrated that the critical strain to fracture ϵ_f can be expressed by a single function, namely, the Zener–Hollomon parameter Z, which combines the effects of both temperature and strain rate. ϵ_f and InZ obeyed a linear relationship. Based on the experimental results, a processing window, which consisted of a temperature range 950–1050°C and strain rate range 0·01–0·1 s⁻¹, was established for optimising the process parameters and achieving microstructural control during hot working. The processing window has been validated through scaled up cylinder upsetting experiments using 140 mm diameter alloy specimens.