Phase transition and dynamics of iron under ramp wave compression
The ramp wave compression experiments of iron with different thicknesses were performed on the magnetically driven ramp loading device CQ-4. Numerical simulations of this process were done with Hayes multi-phase equation of state (H-MEOS) and dynamic equations of phase transition. The calculated results of H-MEOS are in good agreement with those of shock phase transition, but are different from those under ramp wave compression. The reason for this is that the bulk modulus of the material in the Hayes model and the wave velocity are considered constant. Shock compression is a jump from the initial state to the final state, and the sound speed is related to the slope of the Rayleigh line. However, ramp compression is a continuous process, and the bulk modulus is no longer a constant but a function of pressure and temperature. Based on Murnaghan equation of state, the first-order correction of the bulk modulus on pressure in the Hayes model was carried out. The numerical results of the corrected H-MEOS agree well with those of pure iron in both ramp and shock compression phase transition experiments. The calculated results show that the relaxation time of iron is about 30 ns and the phase transition pressure is about 13 GPa. There are obvious differences between the isentropic and adiabatic process in terms of pressure–specific volume and temperature–pressure. The fluctuation of the sound speed after 13 GPa is caused by the phase transition.
KeywordsRamp wave compression Polymorphic phase transition Multiphase equation of state Sound speed
The author would like to extend his appreciation to Dr. Zhenfei Song and Dr. Hongping Zhang for their discussion on analysis of the physical process. Special thanks are expressed to Mr. Wu, Mr. Xu, Mr. Shui, and Mr. Deng for their excellent operation CQ-4 and PDV measurements. This work was supported by the National Natural Science Foundation of China (Grant 11327803), the project of Youth Innovation of Science and Technology of Sichuan Province (Grant 2016TD0022), and the National Challenging Plan (Grant JCKY2016212A501).
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