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Phase transition of iron, as a prototype of martensite phase transition underdynamic loadings, exhibits huge diverges in its TP among experiments withdifferent pressure medium and loading rates, even in the same initial samples.Great achievements are made in understanding strain or stress dependence of theTP under dynamic loadings. However, present understandings on the strain ratedependence of the TP are far from clear, even a virgin for extreme high strainrates. In this work, large scale NEMD simulations are conducted to study theeffects of strain rates on the phase transition of iron-based single crystals.Our results show that the phase transition is preceded by lattice instabilitiesunder ramp compressions, but present theory, represented by modified Borncriteria, cannot correctly predict observed onsets of the instability. Throughconsidering both strain and strain gradient disturbances, new instabilitycriteria are proposed, which could be generally applied for studyinginstabilities under either static or dynamic loadings. For the ramp with astrain rate smaller than about 1010s-1, the observed onset of instabilities isindeed equal to the one predicted by the new instability criteria under smallgradient disturbances. The observed onsets deviates from the predicted one atlager strain rates because of finite strain gradient effect. Interestingly, thestrain rate dependence of the TP also exhibits an obvious change at the samestrain rate, i.e., 1010 s-1. When 1010 s-1, a certain power law is obeyed, butit is not applicable at larger strain rates. This strain rate effect on the TPis well interpreted with nucleation time and the finite strain gradient effect.According to these basic understandings, the roles of strain rates onnucleation and growth of the phase transition are studied.

Phase Transition of Iron-based Single Crystals at Extreme Strain Rates under Dynamic Loadings