Effect of acoustic impedance matching on kinetics of titanium phase transformation

Abstract

The phase transition experiments under ramp wave compression of titanium have been done by using a magnetic diven device CQ-4, and the influence of the acoustic impedance of the back surface of titanium on the phase transition has been studied. The experimental results show that when the acoustic impedance of the window is low, free surface or LiF, the particle velocity corresponding to the beginning of phase transition is about 408.9 m/s, while at the high impedance sapphire window, is about 373.9 m/s. The pressures corresponding to the characteristic velocities are not the phase change onset pressures. A simulation was done to well describe this complicated dynamic response including physical models of multi-phase equation of state based on Helmholtz free energy, strength model, and non-equilibrium dynamic equation of phase transition. The calculated results are in excellent agreement with the experimental data, and they exhibit the dynamic physical characteristics of Tin involving the elastoplastic transition and evolution of mass fraction of different phase. The results show that the velocity wave forms of the samples near the loading surface are the same, but the amplitude of the wave forms varies greatly due to the impedance matching of Tin and windows in the vicinity of Tin’s rear surface. The phase transition pressure of Ti is about 10.5 GPa, which is a rate dependent non-equilibrium dynamic process. In the pressure-volume thermodynamic plane, the calculated isentrope coincides with Hugoniot before phase transition, but is below the Hugoniot after phase change. The obvious decrease in sound velocity after phase transition is due to the discontinuity of the specific volume due to phase transition.