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

Abstract

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°C and strain rate range 0.001–100 s⁻¹. The true stress–true strain curves at 650°C 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°C, 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 kJ mol⁻¹. The processing map exhibited a domain in the temperature range 750–900°C with a peak efficiency of about 48% occurring at 850°C 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.