Proposed mechanism of HCP → FCC phase transition in titianium through first principles calculation and experiments

By means of first principles calculation and experiments, a detailed mechanism is proposed to include the stages of slip, adjustment, and expansion for the HCP -> FCC phase transformation with the prismatic relation of {10 (1) over bar0}(hcp)parallel to{1 (1) over bar0}(fcc) and [0001](hcp)parallel to[001](fcc) in titanium. It is revealed that the formation of four FCC layers is preferable after the slip of Shockley partial dislocations of 1/6 < 1 (2) over bar 10 > on {10 (1) over bar0} planes, and that the adjustment of interplanar spacing and the volume expansion are energetically favorable and could happen spontaneously without any energy barrier. It is also found that the transformed FCC lattice first follows the c/alpha ratio (1.583) of HCP and then becomes an ideal FCC structure (c/alpha = root 2). The proposed mechanism could not only provide a deep understanding to the process of HCP -> FCC prismatic transformation in titanium, but also clarify the controversy regarding volume expansion of HCP-FCC phase transition of titanium in the literature.