Study On Kinetics Of Phase Transition Of Metals Under Ramp Wave Loading

Phase transition is a common phenomenon in nature,and the phase transition occurs under certain pressure and temperature conditions.The properties of physical,mechanical,chemical and structural are significantly changed after the phase transition.Therefore,phase transition is one of the important fields of shock wave physics and material dynamics.The mature experimental methods for the research of phase change are static and shock compression,which are approximately corresponding isothermal and adiabatic lines.It is known from the thermodynamic theory that the isentrope is between the isotherm and the Hugoniot,and the isentropic compression(ramp wave compression)is the bridge between the two experimental methods.In addition,the gravitational compression is closer to the isentropic compression.Therefore,the study of phase transition under ramp wave compression has important scientific significance.In this paper,the dynamic study of the ramp wave compression phase transition of iron,bismuth and tin is carried out on magnetically driven device CQ-4.The main conclusions and innovations are as follows:1)The magnetically driven ramp wave loading experimental technology for the study of phase transition is improved,which expands the thermodynamic loading path of CQ-4 device.The initial temperature system of presupposed sample is set up for the CQ-4 device.The temperature adjustment range is-80??180? and the precision is 0.1?.The effect of thermal effect on the phase change dynamics can be studied in a wider range of temperature.The experimental technique of shock and ramp wave compression is established.The experimental data between the isentropie and Hugoniot can be obtained.It is of great significance to establish and verify the physical model.The ramp wave loading experimental technique for liquid samples was established.2)The ramp wave compression experiments of typical metals such as iron,bismuth,tin and other typical metals were carried out.The influence of the factors such as window material,sample thickness and initial temperature on the phase transformation kinetics was studied.The velocity wave profiles of bismuth containing three phase transformation are obtained.It is clear that the bismuth in the process of dynamic compression undergo four solid phases,i.e.,I,II,III and V in order,instead of directly entering III phase or V phase from I phase.The experimental data reflects the obvious advantage of the ramp wave compression technology in the study of the dynamic of material with complex phases.The numerical results describe the elastoplastic transition,the phase transition.The expression of the Helmholtz free energy of bismuth and its parameters are corrected by the experimental results.The negative slope of velocity profile of iron with sapphire window is observed.The velocity wave profile corresponding to the phase transition is determined by the intensity of the reflected compression wave and the "sparse wave" caused by the volume collapse during the phase transition.The intensity of the reflected compression wave is determined by the acoustic impedance of the sample and the sapphire window.The intensity of the "rarefaction wave" caused by volume collapse is determined by the size of the bulk strain discontinuity and the rate of transformation.The acoustic impedance and phase relaxation time of zirconium,titanium and iron are basically the same,while the bulk strain discontinuity of zirconium(or titanium)is less than half of iron'.Therefore,there will be a strong 'sparse wave' in the inside of the iron sample.The two factors make negative slope of iron,but not for zirconium and titanium.The characteristic velocity corresponding to the onset of phase transition decreases with the increase of the initial temperature of the sample,which is due to the reduction of the difference of the Gibbs free energy of two phases,and the smaller pressure makes the material reach the critical condition of the phase transition.The experimental results of Zr under shock-ramp wave loading show that the velocity wave profile contains phase change information when the impact pressure is low.With the same loading condition,the peak of the free surface velocity of the sample increases with the increase of the impact velocity,which is due to the increase of the temperature of the sample.The thermodynamic path is between the isentrope and the Hugoniot.The characteristic velocity corresponding to phase transition gradually decreases with the increase of sample thickness,which is the result of phase transition relaxation time and stress wave evolution.The stress wave from metastable state to equilibrium state needs a certain time and distance,and the stress wave in thin samples is still in high pressure metastable state,so the corresponding characteristic velocity is higher.3)Two kinds of multiphase equation of state,the Hayes multiphase state equation based on the dynamic stress-strain and the multiphase equation of state based on Helmholtz free energy,are established.Based on the thermodynamic characteristics of the ramp wave compression process and the Murnaghan isentropic equation of state,the Hayes multiphase model is modified.The modified model is applied to both the ramp wave and the shock compression phase transition process.The uncorrected Hayes can't describe the ramp wave compression phase transition.It shows that the shock experimental results cover the effect of the pressure but the ramp wave experiment exposes the problem.The numerical results show that the multiphase state equation based on Helmholtz free energy is suitable for both of ramp wave and shock compression process.