hcp → ω phase transition mechanisms in shocked zirconium: A machine learning based atomic simulation study

There has been much controversy over the behavior of zirconium under shock strong enough to cause the pressure-induced hcp → ω phase transformation. Due to the short time- and length scales involved, direct measurements of the microstructure are extremely challenging. We have performed molecular dynamics simulations to investigate this issue, with Zr described by a machine-learned interatomic potential. Two different orientation relationships (ORs) between the hcp and ω phases are observed under shock driven conditions. Unlike the case with Ti that is in the same group, the ORs between the hcp and ω phases show less anisotropic phase transition sensitivity and in most cases follow the Silcock relationship with(0001)_α||(12¯10)_ω. Furthermore, we find that the α→ ω transformation in shocked Zr occurs via an intermediate metastable bcc structure during the loading process, whereas no such intermediate is found during the reverse ω→α transition when the shock releases.