The effect of alpha platelet thickness on plastic flow during hot working of TI–6Al–4V with a transformed microstructure Original Research Article

The effect of alpha platelet thickness on the plastic flow of Ti–6Al–4V with a transformed microstructure was established by conducting isothermal, hot compression tests at hot working temperatures on samples with identical crystallographic texture and beta grain size. Microstructures containing alpha laths/platelets ranging in thickness from approximately 0.4 to 10 μm were produced by various sequences consisting of hot rolling and heat treatment. Constant-strain-rate and strain-rate-jump compression tests were conducted at subtransus temperatures of 815, 900, and 955°C in the strain rate regime between 10−3 and 10 s−1. The rate-jump tests suggested that plastic flow is controlled by a power-law creep (dislocation glide/climb) mechanism in all cases except the low-strain rate deformation of material with the thinnest alpha laths. All of the constant-strain-rate compression tests yielded flow curves consisting of a peak stress at low strains (≤0.03), extensive flow softening, and a steady-state flow stress at large strains. The peak stress results indicated a significant Hall–Petch dependence on alpha lath/platelet thickness at the two lower test temperatures. The magnitude of this dependence was predicted by the classical Eshelby expression for grain-size strengthening. In addition, a first-order analysis demonstrated that the observed flow softening is of the same magnitude as that which would be associated with the loss of Hall–Petch strengthening (due to alpha–beta interfaces) during hot working.