Superplasticity induced by the competitive DRX between BCC beta and HCP alpha in Ti-4Al-3V-2Mo-2Fe alloy

Abstract

With the purpose of identifying underlying insights into deformation mechanism (DM) of superplastic effect, a constitutive equation was developed on the basis of the experimental data. Superplastic tensile tests were conducted in the temperature range of 760–840 °C and strain rate range of 0.03–0.3 s−1. The results show that the superplastic behavior can be interpreted by grain boundary sliding (GBS) under most deformation conditions in addition to the high strain rate. To fully understand the DM of superplasticity in this exceptional condition, the corresponding microtexture and microstructure were discussed. These demonstrate that the subgrain size λ is of profound significance for accommodation mechanism responses. α grains-group migrates along the boundaries coordinated by the softer β, synchronously, inducing the accumulated stress being spreaded by dislocations traveling through β to the opposite boundaries. In the meantime, the rearranged dislocations are converted into high-angle sub-boundaries in α grains, characterizing continuous dynamic recrystallization (CDRX) mode, while the recrystallized β grains are dominantly provoked by discontinuous dynamic recrystallization (DDRX), aided by CDRX. A competitive DRX mechanism between HCP and BCC crystallographic structures is first proposed. Both grain boundaries expand towards the counterpart side and absorb surrounding dislocations until they contact each other, weakening the driving force of DRX and suppressing grain coarsening. As a result, the superplasticity of Ti-4Al-3V-2Mo-2Fe alloy is achieved by GBS associated with the refined grains caused by DRX.