Structure response characteristics and surface nanocrystallization mechanism of alpha phase in Ti-6Al-4V subjected to high energy shot peening

Structure features and grain refinement of alpha phase in coarse-grained Ti-6Al-4V subjected to surface severe plastic deformation by using high energy shot peening (HESP) were investigated via high-resolution transmission electron microscope. The results illustrated the novel deformation-induced structure response characteristics, including markedly reduced contribution of twinning to deformation of coarse-grained alpha phase with hexagonal close-packed (hcp) structure, dramatical rise of pyramidal <c+a>-type dislocation density accompanying an increase in grain refinement level of alpha phase, and unusual phase transformation of alpha ultrafine grains (UFGs) from hcp to face-centered cubic (fcc) structure. Based on the above-mentioned results, nanocrystallization mechanism of alpha phase was obtained as follows. The massive dislocation tangles, dislocation bands and a handful of single-system tension twins were firstly generated and subdivided the original grains into elongated UFGs, and then transversely disintegrate elongated UFGs into equiaxed UFGs, which twinning was gradually replaced by pyramidal <c+a>-type dislocation slip during the above-mentioned process. Most of equiaxed UFGs with hcp structure subsequently underwent crystal rotation to transform into randomly-oriented equiaxed nanograins. Phase transformation from hcp to fcc structure occurred in a small amount of equiaxed UFGs, in which twin-twin and dislocation-twin interactions ultimately resulted in the formation of nanograins with fcc structure.