The coordination behaviors of alpha and beta phases with different grain sizes in Ti-6Al-4V subjected to surface severe plastic deformation

The structure characteristics of alpha and beta phases on both sides of phase boundary in Ti-6Al-4V subjected to high energy shot peening were investigated via high-resolution transmission electron microscope. The results indicated that the incoherent phase boundary was the considerably active dislocation resource and stress concentration in phase boundary owing to plastic mismatch of two phases offered larger driving force for dislocation nucleation and motion. Correspondingly, twins were completely suppressed nearby phase boundary. Because of the diversity of grain refinement levels of alpha and beta phases, the coordination deformation behaviors were analyzed based on the ratio d_α/d_β, in which d_α and d_β were average grain sizes of alpha and beta phases respectively. As d_α/d_β was above 10 and below 30, beta phase with extremely tiny grain sizes promoted dislocation nucleation and pileup of alpha phase with larger sizes. In contrast, as d_α/d_β was above 3 and below 10, alpha phase which was severely work-hardening promoted dislocation multiplication of beta phase. As d_α/d_β was below 3, the phase with higher orientation factor primarily plastically deformed and promoted dislocation motion and pileup of the other phase, further resulting in crystal rotation of equiaxed ultrafine grains and grain refinement of nanograins.