Nucleation mechanism for hcp → bcc phase transformation in shock-compressed Zr

We present large-scale atomic simulations of shock-induced phase transition in Zr assisted by the machine learning method. The results indicate that there exists a critical piston velocity of ${U}_{\mathrm{p}}\ensuremath{\sim}0.85\phantom{\rule{0.16em}{0ex}}\mathrm{km}/\mathrm{s}$, above which the product phase has changed from \ensuremath{\omega} to bcc. Unlike the case in Fe, the shock-induced $\mathrm{hcp}\ensuremath{\rightarrow}\mathrm{bcc}$ nucleation mechanism in hcp-Zr single-crystal shows significant dependence on crystal orientation. For shock along the $[10\overline{1}0]$ direction, the hcp phase directly transforms into bcc as expected. However, for shock compression along [0001] and $[1\overline{2}10]$ directions, the $\mathrm{hcp}\ensuremath{\rightarrow}\mathrm{bcc}$ transformation occurs in quite a different manner, i.e., the Zr single crystal transforms into a disordered intermediate that subsequently exhibits ultrafast crystallization of the bcc phase within the timescales of subnanoseconds. We associate such presence of disordered intermediate structure with the sluggishness of shear stress relaxation, which leads to an elastic unstable condition of the crystal during the first few picoseconds of uniaxial compression, and suggests that the fewer possible shear planes (related to Burgers mechanism) for [0001] and $[1\overline{2}10]$ shock loading is an underlying factor for the orientation dependence.