Macro-and Micro-Scale Experimental Study On Thermodynamics And Kinetic Characteristics Of Phase Transformation For Shocked Iron

Iron is the main constituent of Earth and telluric planetary cores.Therefore,the investigation of the shock-induced phase transformation in iron is of importance to fundamental science frontiers in shock wave physics,geophysics,mineralogy,and materials science.The phase transition,including structural changes,phase diagram even and complex dynamics or kinetics,has been extensively studied in the past half century since it was inferred from both dynamic and quasi-static experiment measurements.It has also enjoyed substantial theoretical attention.Nevertheless,there are several imperfectly understood aspects and inadequately tested theoretical conceptions,such as the role of temperature-,crystal orientation-and time-dependence,deviatoric stresses and strain rates in determining the associated solid-solid,solid-liquid(melting)phase boundaries and the possible metastable phase.However,measurements have not been obtained with sufficiently in-situ time-and space-resolved information to describe the detailed micro-mechanism of the phase transformation and to validate the numerical simulations.Hence,an important step for providing new insights into these processes under dynamic conditions is to develop new experimental techniques such as dynamic X-ray spectroscopy and to directly examine structural deformation and phase transitions,at the atomic length scales.In this work,we aim at providing new data on the phase transition dynamics and kinetics for shocked iron,described below,on the basis of new loading path control-techniques and in-situ multi-scale(macro-and micro-)experimental diagnostics development.Consequently,(1)the shock-induced structural changes including the ?-?-? triple point,solid-solid and solid-liquid phase boundaries were measured,in an attempt to verify the shock pressure-temperature(P-T)phase diagram of iron under 100 GPa.The temperature-and loading path-dependence of the shock response in iron were analyzed and discussed;(2)the phase transition dynamic and evolution process in Fe single crystals were studied comprehensively to reveal the orientation effect for the nucleation and growth of the new phase and to explore the micro-mechanism in the shock-induced phase transitions.The representative conclusions achieved in present work are as follows:1)For supplying the inadequacy of the experimental method for the shock-induced phase transition,new techniques and diagnostics were developed firstly,including that:(i).A preheated-target technique that consists of a Cr-Ni electrical-resistance alloy heating coil with a maximum usable temperature of 1000K is established to broaden the measurable P-T space.And a simultaneous Hugoniot and wave profile(or temperature)measurement for preheated-metal is performed successfully to solve the in-situ diagnostic issues;(ii).An in-situ dynamic X-ray diffraction(DXRD)diagnostic method is improved to probe the lattice response driven by shock waves.In order to eliminate the measurement error arising from the difference in experimental setup,the static and dynamic lattice diffraction signals are measured simultaneously in one shot by using a nanosecond burst of X-ray emitted from a laser-produced plasma.A laser pump-and-probe technique for adjusting the time-delay of DXRD diagnosis during the shock wave transit time,with a series of repeated shock loadings is then employed to generate and measure the dynamic structure evolution.In a word,our development for experimental approach provides the more opportunities for observing the phase transition kinetic and giving a micro structural change description.2)The phase transition properties associated with loading conditions(including initial temperature and strain rate)were investigated to clarify the phase boundaries and to construct the P-T phase diagram of shocked iron.We found that:(i).The phase transition is temperature-dependent,with an average rate ??tr/?T of-6.91 MPa/K below 700K and-34.7 MPa/K at higher temperatures.The shock ?-? and a-y phase boundaries intersect at 10.66±0.53 GPa and 763 K,which agrees with the ?-?-? triple point from early shock wave experiments and recent laser-heated diamond-anvil cell(DAC)resistivity and in.situ X-ray diffraction data but disagrees with the shock phase diagram reported in 2009 by Zaretsky.On that basis,we place the ?-?-? triple point of shocked iron back to its earlier accepted location;(ii).The ??? polymoephic transition is over-pressurized at high strain-rates.The corresponding transition stress is up to 17.5 GPa in laser-driven ramp-wave compression.Moreover in laser-driven shock compression,the spall stresses evaluated from the free surface velocity profiles are significantly higher after the ?-?-? cycle;(iii).Our melting data determined by the shock temperature measurement are in better agreement with the solid-liquid phase boundary using synchrotron-based fast x-ray diffraction and ab initio calculations of melting curve,but up to 1000 K higher than the melting points of fast recrystallization.The current experiments reduce the divergence between dynamic and static DAC approaches.Based on all of these observations,we generally describe the shock P-T phase diagram of iron above 100 GPa.3)The shock responses of the[100],[110]and[111]single-crystal iron were examined systematically to reveal the orientation effect of the ??? transition kinetics.Our experiments showed that in single-crystal iron:(i).The values of Hugoniot elastic limits ?HEL are greater than 6 GPa and strongly dependent on the initial crystal orientation(?HEL111>?HEL110>?HEL100).Moreover,the onset pressures ?PT for the phase transition are obtained to be 13.89±0.57,14.53±0.53 and 16.05±0.67 GPa along the[100],[110],and[111]direction,respectively.Based on these results,it is concluded that the crystal orientation dependence of ?PT agrees with the reported NEMD calculations.However,the measured values are lower;(ii).Based on the analysis of in-situ DXRD patterns,the dynamic lattice responses for different orientation are observed to be dramatically inhomogeneous,probably indicating a natural variation of the micro-structural transform path.The current data reveal that the order-disorder transformation for[100]orientation and the dynamic instability for[111]and[110]orientations are in agreement with the predictions from NEMD calculations.In conclusion,preliminary macro-and micro-scale experiments of BCC iron single crystals show that the fundamental physical behaviors involving phase transition dynamics.The findings and methods presented in this work can be used to explore much more complex metals and their response to shock loading.