Identifying and assessing high-pressure phase transition in iron by unique microstructure of $\alpha \to \varepsilon \to \alpha $ transitions

Unique nanotwinned $\alpha $-Fe with threefold-symmetry characteristic was found in shock-compressed iron by using transmission electron microscopy (TEM). It was confirmed that the unique microstructure of $\alpha $-Fe was formed in two martensitic transformations during shock treatment, i.e. the $\alpha \to \varepsilon $ phase transition under shock loading and the $\varepsilon \to \alpha $ phase transition during unloading. The threefold-symmetry characteristic nanotwinned $\alpha $ phase is only correlated to the existence of $\varepsilon $ phase in high pressure during shock loading. Therefore, the volume percentage of the high pressure $\varepsilon $ phase under shock loadings could be assessed by measuring the $\alpha $ grains with the unique feature. A direct method to identify and assess high-pressure phase transition in iron has been developed.