Unstable flow during supratransus working of Ti–6Al–4V

Abstract

The occurrence of flow instabilities during hot working of Ti–6Al–4V in the β phase field has been studied with the help of hot compression tests conducted over the ranges 1000°C–1100°C and 0.001-100 s⁻¹. The stress-strain curves exhibited steady-state flow behavior at slower strain rates while oscillations were observed above 1 s⁻¹. Instability analyses conducted using the phenomenological and continuum criteria indicated the possibility of unstable flow at faster strain rates (>1 s⁻¹). However, the microstructural manifestation of the predicted flow instability could not be retained in the compressed specimens due to phase transformation occurring while cooling to room temperature and smaller associated strains (50%) in the uniaxial compression test. A high-speed extrusion experiment conducted by imparting larger strain (270%) under unstable flow conditions revealed that the β instability manifests in the form of flow localization bands. The presence of fine-grained structure within these bands indicates that the kinetics of phase transformation are influenced by the intense localized plastic deformation.