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