Transient Plastic Flow and Phase Dissolution During Hot Compression of α/β Titanium Alloys

Transients in plastic flow behavior and the kinetics of dynamic dissolution of α particles were established via isothermal, hot compression testing of Ti-6Al-4V (Ti64) and Ti-6Al-2Sn-4Zr-2Mo-0.1Si (Ti6242S). For this purpose, samples were preheated at a low subtransus temperature at which the volume fraction of α was ~ 0.90, heated at a fixed rate to one of two higher temperatures, held for a time between 0 and 900 seconds, and then upset to a 2:1 reduction using a strain rate of 0.01, 0.1, or 1 s−1. For a given alloy, test temperature, and strain rate, the flow stress decreased with increasing hold time. The observations were interpreted in terms of various models of plastic flow and microstructure evolution. The plastic-flow behavior of the two-phase microstructures was analyzed using approaches based on isostrain (upper-bound), self-consistent (SC), and isostress (lower-bound) approaches coupled with the measured (transient/non-equilibrium) phase fractions/phase compositions. The isostrain and SC methods both provided reasonable estimates of the observed flow stresses; the isostress method greatly under-predicted the measurements. Microstructure models comprised diffusion-based analyses of the dissolution of α particles into the β matrix both statically (during heating to test temperature and holding prior to deformation) and dynamically (during deformation). Static dissolution predictions showed good agreement with measurements. A comparison of static and dynamic dissolution behaviors revealed that concurrent deformation led to an enhancement of diffusion rates by a factor of approximately 8 or 4 for Ti64 and Ti6242S, respectively.