基于原位X射线衍射技术的动态晶格响应测量方法研究

摘要

获取动态压缩条件下结构演化过程是冲击相变及其动力学机理研究最为关注的基础问题之一.对此,基于激光驱动瞬态X射线衍射技术,通过系列实验的物理状态关联和抽运-探测时序控制,实现了静态与动态晶格衍射信号的同时获取,消除了不同实验的装置结构和样品差异带来的测量误差,建立了一种基于原位X射线衍射技术的动态晶格响应测量方法.利用上述实验方法,成功实现了激光冲击加载下[111]单晶铁晶格压缩过程的原位测量,获取弹性及塑性响应的晶格压缩度与宏观雨贡纽测量结果完全符合,从晶格层面证实了超快激光加载下的高屈服强度(雨贡纽弹性极限值大于6 GPa),以及可能与晶向效应或加载率效应相关的相变迟滞现象(至终态压力23.9 GPa仍为体心立方结构),相关物理机制仍有待进一步研究.上述测量方法的建立为后续开展相变动力学机理研究提供了可行的技术途径和重要的参考价值.%Structure evolution under dynamic compression condition (high temperature, high pressure and high strain rate) is one of the most important problems in engineering and applied physics, which is vital for understanding the kinetic mechanism of shock-induced phase transition. In this work, an in-situ dynamic X-ray diffraction (DXRD) diagnostic method is established to probe the lattice response driven by shock waves. The geometry is suitable for the study of laser-shocked crystals. 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. Experimental details in our investigation are as follows. 1) The laser driven shock wave transit time?tShock and the shock pressure in sample are accurately determined from the shock-wave profile measurement by dual laser heterodyne velocimetry. 2) A laser pump-and-probe technique for adjusting the time-delay of DXRD diagnosis during?tShock, with a series of repeated shock loadings is then employed to generate and measure the dynamic structure evolution. Using this method, the dynamic lattice response of [111] single-crystal iron is studied on Shenguang-II facility. Single-shot diffraction patterns from both unshocked and shocked crystal are successfully obtained. An elastic-plastic transition process—elastic wave followed by a plastic wave—is observed in shocked [111] single-crystal iron on a lattice scale. The lattice compressibility values of the elastic wave and plastic wave are in agreement with those derived from the wave profiles. It is found that the Hugoniot elastic limit is measured to be about 6 GPa under nanosecond-pulsed laser shock compression. Such a high yield strength is consistent with recent laser ramp compression experimental results in polycrystalline Fe [Smith et al. 2011 J. Appl. Phys. 110123515], suggesting that the peak pressure of elastic wave is dependent on the loading rate and the thickness of sample. Based on the analysis of diffraction patterns, the BCC phase is determined to be stable till 23.9 GPa, the highest pressure explored in this work, which might indicate that the phase transition strongly couples with the crystal orientation and loading rate. Some possible physical mechanisms remain to be further studied: whether the transition time hysteresis occurs or the metastable FCC phase exists in shocked [111] single crystal Fe, or the phase transition onset pressure increases under high strain-rate compression. Our DXRD results provide a primary experimental reference for the follow-up study on the phase kinetics.