New tetragonal phases of titanium under shock loading - Predicted by molecular dynamics and ab initio calculations

Based on molecular dynamics simulations of the long-term evolution of polycrystalline titanium structures under shock waves, a novel phase (ζ) has been identified to emerge at shock pressures of approximately 20 GPa. This phase originates from the initial HCP phase and the disordered structure induced by the shock. The energy change characteristics during this phase transition are explained based on the Helmholtz free energy under volume constraints. The ζ phase comprises framework atoms (F-atoms) with fixed positions and unfixed interstitial atoms (I-atoms), exhibiting incomplete symmetry. Validation of this phase structure using ab initio simulations and machine learning potentials supports its existence as a metastable structure. The transformation from the initial α phase to ζ phase is diffusional transformation, while the reverse transformation is multistage displacive transformation.