The effect of alpha platelet thickness on plastic flow during hot working of Ti-6Al-4V with a transformed microstructure

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 mum 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 degreesC 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 (less than or equal to0.03), 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. Published by Elsevier Science Ltd on behalf of Acta Materialia Inc.