Atomic scale investigation of the phase transformation path from HCP to FCC in high-purity hafnium during torsion deformation

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Although the hexagonal close-packed (HCP) to face centered cubic (FCC) martensitic phase transformation has been frequently observed and discussed in many HCP metals and alloys, the detailed phase transformation path has been disputed. Here, we studied the potential phase transformation path from HCP to FCC phase in high-purity hafnium (Hf) under torsional deformation condition. Results indicated that the HCP to FCC phase transformation was not accomplished directly by the glide of a/3[11¯00] Shockley partial dislocations on every other {0001} planes, but progressively transitions to the FCC lattice through multiple intermediate states. The complete phase transformation process can be further refined as: HCP → body-centered tetragonal (BCT)-like structure → 12R long-period stacking-ordered (LPSO) structure → FCC, and each step was accommodated by basal stacking faults. This work is instructive on the phase transformation from HCP to FCC not only in Hf but also in other HCP metals.