Abstract
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}bar{{1}}{0}{}}}_{{hcp}}{parallel }{{}{1}bar{{1}}{0}{{}}}_{{fcc}} and {{[}{0001}{]}}_{{hcp}}{parallel }{[}{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 langle {1}bar{{2}}{10}rangle on {{}{10}bar{{1}}{0}{}} 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/a ratio (1.583) of HCP and then becomes an ideal FCC structure (c/a = √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.
Highlights
Since the discovery of Burgers in 1930s, it has been generally believed that during the HCP-FCC phase transition, the two structures should follow the orientation relation of{0001}hcp {111}fcc and [112 0]hcp [110]fcc through the gliding of the Shockley partial dislocations on every two close-packed planes of HCP26
A detailed mechanism of the HCP-FCC prismatic transformation is proposed in the present study to include the stages of slip, adjustment, and expansion
It is revealed that an energy barrier of more than 600 mJ/m2 should be overcome during the slips of Shockley partial dislocations of 1/6 on {101 0} planes, while the adjustment and expansion during the HCP → FCC prismatic transformation are energetically favorable with negative relative energies and could take place spontaneously without any energy barrier
Summary
Since the discovery of Burgers in 1930s, it has been generally believed that during the HCP-FCC phase transition, the two structures should follow the orientation relation of{0001}hcp {111}fcc and [112 0]hcp [110]fcc through the gliding of the Shockley partial dislocations on every two close-packed planes of HCP26. Hong et al discovered the lattice expansion of 19.5% normal to the phase boundary in pure Ti bulk under cryogenic plain-strain compression after the HCP → FCC prismatic transition[6], and Wu et al found the volume expansion of 14.2% in polycrystalline Ti through rolling at room temperature after the HCP → FCC prismatic transition[8]. Wu et al argued that, according to the first transformation mechanism[8], the gliding of Shockley partial dislocations cannot generate the lattice expansion normal to the phase boundary[8] It is, of vital importance to theoretically clarify such a controversy regarding volume expansion during the HCP-FCC prismatic transformation of titanium. Two HCP/FCC interface models are constructed and compared with each other to express the transformed FCC lattices, and the proposed mechanism of HCP-FCC transformation of Ti would be probably generalized to other systems as well
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