Experimental Study On The Heterogeneous State In The Near Free Surface Region Of Shock-loaded Iron

The interaction between the shock wave front and the free surface leads to the results that the particles inside the front-affected area cannot be loaded to the high stress state as the particles far from the free surface; and the phase-transitional relaxation time also makes certain contribution. These factors may lead to a mesoscopic even macroscopic heterogeneity on the material state. Micro-ejection caused by geometrical heterogeneity of the free surface and micro-spallation cause by shock-induced melting coupling the mechanism mentioned above, may lead to notable differences in the particle phase state between the near-surface and the inside areas. Based on the requirement of precise physical experiment, the characteristic physics behavior and dimensional scale of the near-surface area, with different state from the inside, needs close study.In this paper the complex mechanism of how the loading wave front affects the loading-unloading and shock-induced phase transition process in different locations inside the metal material is studied on. The dynamics behavior of the near-free-surface layer caused by the width of the wave and the phase-transitional relaxation is the focus of this study. The solid to solid polymorphic phase transition of iron is taken advantage of to simulate the solid to liquid phase transition behavior of related materials under shock-loading. The dynamic behavior of the material under shock-loading is tested, combined with the detective work of microstructural evolution inside the recovered specimens. Time-resolved velocity curves of free-surfaces of the specimens have been acquired and microstructure varieties in different locations from the impact surfaces to the free-surfaces of recovered specimens have been characterized. The near-surface layers where the original phase has been kept from the beginning to the end is specially studied on and the relations between the microstructure diversity detected in different locations of the recovered specimens and the front characteristics of the loading waves have been analyzed. Compared with the experimental results as well as the molecular dynamic simulation images, the continuum dynamic numerical simulations of phase-field method is the key point to analyze the law how shock wave front affect the stress history, how the shock-induced phase transition evolves and how the near-surface layer evolves during shock-loading process.In the loading experiments, magnetically driven technique of CQ-4equipment was used to drive high-velocity flyers to impact the pure iron specimens to achieve impact pressures from7GPa to80GPa, and laser interferometer system measurements were utilized to record the velocity profiles at the back of specimens, which is taken for the analysis of the interaction histories between the wave fronts and the free surfaces.The shock-loaded iron specimens were recovered to conduct microstructural analysis from the impact surfaces to the free surfaces. Micro-hardness tests have shown that the hardness near the impact surfaces of the specimens are greater than that near the free surfaces, and the whole trend of hardness goes down from the impact surface to the free surface in a specimen. Fine EBSD scanning and TEM analysis show that the twin crystals near the free surfaces are different from those near the impact surfaces. Testing the twin crystals in different depth of the specimens continuously from the impact surfaces to the surfaces, the α(?)ε phase transition signature twins (three sets of{112}<111> twins with a threefold symmetry) tend to decrease and eventually disappear near the free surfaces, and the quantitative description of the phase regions (especially the none phase transition region of the near-surface layer) in the depth orientation is made.Macro-continuum mechanic phase-field method and phase-transformational dynamic method were employed to simulate the dynamical behavior in the samples. The simulated wave profiles and phase regions were compared with those relevant experimental results; the simulation model was validated, and the simulation parameters were adapted by comparison with the experimental data. The availability of the classical macro-continuum mechanics method and phase field-method for the dynamic behaviors were analyzed by comparison of experimental data. With the help of numerical simulations, further quantitative analysis was made on the relationship between the wave front and the state of the near-surface layer. By analyzing the experimental and simulated results, it has been found that both the shockwave front and the phase transformational relaxation affect the heterogeneity of the near-surface layer. The simulated results of up to80GPa impact predict that the heterogeneous state of the near free surface always exists even the shock pressure is high enough to make the shock wave an over-driven one fold.