Characterization of Nucleation Behavior in Temperature-Induced BCC-to-HCP Phase Transformation for High Entropy Alloy

Phase transformation is one of the essential topics in the studies on high entropy alloys (HEAs). However, characterization of the nucleation behavior in the phase transformation for HEAs is still challenging through experimental methods. In the present work, HfNbTaTiZr HEA was chosen as the representative material, and molecular dynamics/Monte Carlo (MD/MC) simulations were performed to investigate the nucleation behavior in temperature-induced BCC-to-HCP transformation for this HEA system. The results indicate that Nb-Ta, Ti-Zr, Hf-Zr and Hf-Ti atom pairs are preferred in the BCC solid solution of HfNbTaTiZr HEA and Hf-Ti-Zr-rich atom cluster with chemical short range order acts as the nucleation site for HCP phase. The nucleation process follows the non-classical two-step nucleation model: BCC-like structure with severe lattice distortion forms first and then HCP structure nucleates from the BCC-like structure. Moreover, at low temperature, the BCC-to-HCP nucleation hardly occurs, and the BCC solid solution is stabilized. The present work provides more atomic details of the nucleation behavior in temperature-induced BCC-to-HCP phase transformation for HEA, and can help in deep understanding of the phase stability for HEAs.