Microscopic insight into the structural transition of single crystal iron under the ramp wave loading

The microscopic dynamics of the BCC to HCP/FCC structural transition (ST) in single crystal iron induced by ramp waves (RWs) has been investigated in detail with atomistic simulations. Firstly, the microstructure change during the RW propagation is demonstrated and the differences between the [0 0 1], [0 1 1] and [1 1 1] loadings are revealed. It is found that the ST products under the [0 0 1] loading tends to form lamellar twins along the (1 0 0) or (0 1 0) planes with the evolution of grain boundary, although the nucleation number will increase with the RW convergence. Nevertheless, the new phase grains under the [0 1 1] and [1 1 1] loadings are much smaller than that under the [0 0 1] loading, because more equivalent {0 1 1} planes are activated. The size of new grains is also found to reduce with the RW propagation distance, indicating a significant dependence on the loading strain rate. As a result, the maximal shear stress of the new hase can approach to zero after the ST for the [0 1 1] and [1 1 1] loadings. Besides, the change of temperature indicates the increase of entropy during the RW convergence, and the temperature and potential energy approximately keep a linear increase with the propagation distance. The modulus softening before the ST is found for all the three loading directions, which can lead to a stress climbing region and a following stress relaxation region in front of the ST wave (STW). By adjusting the piston acceleration, we further explore the strain rate effect in the range of 5×10⁹∼10¹¹ s⁻¹. Interestingly, the ST pressure (P_ST) is almost the same value for the three loading directions although there exists a significant difference in the deviator stress. What's more, P_ST approximately satisfies a linear change with the extreme strain rate, instead of the power function with an index less than 1. Finally, by tracing the stress variation and atomic motion of a certain crystal plane, we show the different ST dynamic paths when the strain rate exceeds 10¹⁰ s⁻¹ and then the strain rate effect is better interpreted.