Understanding sustained coarsening driven by cyclic phase transformation in additively manufactured Ti-6Al-4V

Coarsening of the basketweave α + β microstructure in Ti-6Al-4V is a common phenomenon that occurs during thermal cycling in both additive manufacturing and cyclic heat treatment. Such a coarsening process is driven by multiple dissolution and precipitation transformations rather than by conventional Ostwald ripening. It is clear that during the first cycle, the complete dissolution of the thinnest α plates at the heating stage provides large spaces for surviving plates to regrow at the cooling stage, which results in coarsening after the first cycle. However, it is still unclear how the α plates continue to vanish and the remaining α plates sustainably coarsen in the repeated thermal cycles. In this study, cyclic heat-treatment experiments and phase-field simulations were conducted to illustrate the mechanism of sustained coarsening during thermal cycling. It was found that in addition to the complete dissolution of the thinnest α plates in the first thermal cycle, α-plate vanishing continued during repeated thermal cycling in two ways, which has not been reported before. The origin of such sustained vanishing of α plates is related to the Gibbs–Thomson effect. Moreover, the influences of cycling frequency and total duration on the transformation coarsening are detected. Both the experimental and simulation results indicate that long cycling duration results in severe coarsening, while for a given duration time, the final coarsening of the α plates is almost independent of the cycling frequency.