窗口声阻抗对锆相变动力学的影响

Phase transformation kinetics of zirconium under ramp wave loading with different windows

基于磁驱动加载装置CQ-4开展了锆的斜波压缩相变实验,研究了锆样品后表面窗口声阻抗对相变波形的影响.实验结果显示,锆后表面为较低声阻抗窗口(自由面和LiF窗口)时,相变起始对应的特征粒子速度约331.0 m/s,而高阻抗蓝宝石窗口时,特征粒子速度约301.9 m/s,特征速度对应的压力从约9.14 GPa下降到8.27 GPa.相变对应的速度特征拐点是与多种因素相关的实验信息,因此它对应的压力并不是材料属性参数相变压力.结合基于热力学Helmholtz自由能的多相状态方程和非平衡相变动力学方程开展了锆的相变动力学数值模拟研究,相变弛豫时间为30 ns,计算结果与三种情况的实验结果符合良好,可以较好地模拟斜波压缩下锆的弹塑性转变、相变等物理过程.在压力-比容和温度-压力热力学平面,相变前锆的准等熵线与冲击绝热线差异很小,相变后准等熵线都位于冲击绝热线下方,随着压力的增加准等熵线和冲击绝线偏差越来越大,温度-压力平面中在20 GPa时相差约100 K.相变开始后,由于相变引起比容的间断,导致锆的拉氏声速迅速下降约7%,相变完成后拉氏声速恢复到体波声速.

The effect of window acoustic impedance on the wave profile of phase transition of zirconium under ramp wave compression is investigated in experiment and simulation. In the experiments, a ramp wave driven by magnetic pressure is applied to the zirconium samples backed windows with different acoustic impedances such as LiF, sapphire and free surface based on the compact pulsed power generator CQ-4. The experimental wave profiles measured by an advanced laser interference velocimeter show that the characteristic particle velocity of the onset phase transition from （~ to w is about 331.0 m/s in the conditions of LiF widow and free surface with low acoustic impedance, and it is approximately 301.9 m/s for the sapphire window with higher acoustic impedance. The corresponding onset pressure of phase transition varies from about 9.14 GPa to 8.27 GPa. The result shows that this onset pressure of phase transition, which is affected by diverse factors, is not the inherent value of phase transition belonging to the material properties. In order to describe these dynamic responses in experiment well, the numerical simulation of phase transition dynamics of zirconium is conducted in one-dimensional hydrodynamic code, in which included are the muti-phase equation of state based on Helmholtz free energy, the equation of non-equilibrium phase transition dynamics, and Steinberg constitutive relationship. The simulated results show that they can reflect the physical processes of elasto-plastic transition and α-ω phase transition as well, which are excellently consistent with the experimental data. The relaxation times of α-ω phase transition in three different acoustic impedance experiments are nearly the same （30 ns）, and their finishing times of phase transition are all about 100 ns. The calculated quasi-isentrope of zirconium below 20 GPa in the pressure-volume and temperature-pressure thermodynamic planes shows that the isentrope and shock adiabat exhibit tiny difference before phase transition, and then separate gradually with the increase of pressure. The isentrope lies below the shock adiabat after the onset of phase transition. At about 20 GPa, the temperature of zirconium under ramp wave loading is bout 100 K lower than that under shock loading. Meanwhile, the abrupt change of volume at phase transition causes the Lagrange sound speed to reduce about 7% and then comes back to the bulk sound speed again after the phase transition has been finished.