Atomic-scale understanding of the reversible HCP⇆FCC phase transition mechanisms at {10(1)over-bar1} twin tip in pure titanium

In this paper, by using high-resolution transmission electron microscopy (HRTEM) observations and molecular dynamics (MD) simulations, the mechanisms of hexagonal close-packed (HCP) structure to face-centered cubic (FCC) structure transition and its inverse transition at the {10 (1) over bar1} twin tip in pure titanium were investigated. The HCP <-> FCC phase transition at the {10 (1) over bar1} twin tip in cold-rolled pure Ti was experimentally observed for the first time. On the other hand, under electron beam irradiation, the FCC lamella shrank gradually and eventually recovered to the HCP structure. After the HCP <-> FCC phase transformations, the interfacial structures of the twin tip changed from the (101 (1) over bar0)(T)//(101 (1) over bar3)(M) facets to a common Basal-Pyramidal (BPy) facets. MD simulations indicated that the reversible HCP <-> FCC phase transitions were caused by successive slip of Shockley partial dislocations on {0001}(HCP) and {111}(FCC) planes, respectively. The nucleation of Shockley partial dislocations stemmed from the dissociation of twinning dislocation (TD) with the Burgers vector of (b) over right arrow (4) = 4 gamma(2)-9/3+4 gamma(2) <10<(1)over bar>(2) over bar at the twin tip. The Shockley partial dislocations could be absorbed by twin tip boundaries through reacting with the residual dislocations to form new TDs (b) over right arrow (4) during the FCC <-> HCP phase transition. The periodic serrated feature of the (10 (1) over bar0)(T)// (10 (1) over bar3)(M) facets of the twin tip boundaries in pure Ti was discovered via atomistic simulations. The interfacial structure evolution of the twin tip was caused by a series of dislocation reactions and generation of phase transformation at the twin tip. The transformation from the HCP structure to the FCC structures was due to the stress concentration caused by the plugging of the periodically distributed TDs (b) over right arrow (4) at the twin tips in the case of external loading. The shrinkage of FCC lamella was caused by the relaxation of internal stress at the twin tip during the thermal relaxation by electron beam.