Deformation behaviour of beta titanium alloy Ti-10V-4.5Fe-1.5Al in hot upset forging

The characteristics of hot deformation of a beta titanium alloy Ti-10V-4.5Fe-1.5Al have been studied by upset forging in the temperature range 650-900 degreesC and strain rate range 0.001-100 s(-1). The true stress-true strain curves at 650 degreesC show continuous flow softening at strain rates above 0.1 s(-1) whereas at lower strain rates, the flow stress attains a steady-state. At temperatures higher than about 750 degreesC, there is a distinct peak in the flow stress in the early stages of deformation followed by a steady-state at higher strains. The variation of flow stress with temperature and strain rate follows the standard kinetic rate equation at strain rates lower than about 0.1 s(-1) and the apparent activation energy is estimated to be about 180 U mol(-1). The processing map exhibited a domain in the temperature range 750-900 degreesC with a peak efficiency of about 48% occurring at 850 degreesC and 0.01 s(-1). On the basis of the microstructural features, the variation of grain size with temperature and the tensile ductility variations, the domain is interpreted to represent a process of dynamic recrystallisation (DRX). The workability is optimum under peak DRX conditions and the grain size in the DRX domain is linearly dependent on the Zener-Hollomon parameter. At strain rates higher than 10 s(-1) and in a wide temperature range, the material exhibits flow instabilities, which are manifested as flow localisation. (C) 2002 Elsevier Science B.V. All rights reserved.