Femtosecond X-Ray Diffraction Studies of Crystalline Matter Deforming under Extreme Loading

X射线自由电子激光(XFELs)是地球上最耀眼的X射线源。它们发出的高度相干、近单色、亚皮秒的辐射爆发使它们成为终极的“高速相机”，能够在它们以前所未有的细节展开时捕捉到极快的原子水平现象。因此，XFELs非常适合探测经历激光动态压缩的物质，即一个或多个高功率光学激光迅速蒸发固体目标的表面，向其发射压缩波，产生比大气压力大数百万倍的内应力。在它们被分解前存活的几十亿分之一秒内，这些目标达到了通常只在行星内部遇到的那种极端压力，并经历了与流星撞击事件相媲美的变形速度。通过用极其明亮的XFEL脉冲照射这些短暂的样品，我们可以产生x射线衍射或吸收光谱，其中包含了它们在销毁前的瞬间的原子排列、结构和动力学信息。这种诊断物质在异常热力学条件下的动态响应的能力正在改变实验高压物理学，使我们不仅能够更好地了解行星天体的内部结构、形成和碰撞动力学，而且能够更好地了解如何合成和恢复物质的奇异高压相，以及工程合金和陶瓷如何响应超高速碰撞产生的巨大动态应力。在这个项目中，我们的目标是利用最近委托的欧洲XFEL(EuXFEL)的诊断能力，EuXFEL是一个由12个国家组成的财团支持的国际XFEL设备，到目前为止，英国已经承诺了大约3000万GB的资本。我们将利用EuXFEL来揭示在极端压力和应变速率下动态变形的模型金属的塑性和强度。我们的目标是利用材料科学家研究了一个多世纪的控制塑性变形的“普通”物理过程，并在通过动态压缩可以达到的“非凡”热力学条件下对它们进行研究。使用最近安装在EuXFEL的英国制造的高重复频率GB 8M DIPOLE-100激光器，我们将以前所未有的射速将一系列金属和合金激光压缩到纳秒时间尺度上的行星压力。我们将使用飞秒X射线衍射来测量我们的样品的微观结构经历的超快旋转，并使用它来确定缓解压缩过程中积累的巨大剪应力的塑性机制。从这些相同的衍射测量中，我们将提取我们的金属样品的应变状态，使我们能够测量它们在极端应变率下的动态强度。我们还将使用EuXFEL来研究这些样品在极端载荷下的X射线吸收特性，从而可以原位跟踪它们的温度动态。总之，这些XFEL实现的塑性机制、强度和温度演变的实验测量有可能改变我们对极端加载条件下材料变形物理的理解。

X-ray free-electron lasers (XFELs) are the most brilliant sources of x-rays on Earth. The highly coherent, near-monochromatic, sub-picosecond bursts of radiation they deliver make them the ultimate 'high-speed camera', capable of capturing extremely fast, atomic-level phenomena as they unfold in unprecedented detail. XFELs are therefore ideally suited to probing matter undergoing laser-based dynamic compression, whereby one or more high-power optical lasers rapidly vaporise the surface of a solid target, launching into it a compression wave that generates internal stresses many millions of times greater than atmospheric pressure. During the few billionths of a second for which they survive before being disintegrated, these targets reach extreme pressures of the kind ordinarily encountered only in planetary interiors, and experience rates of deformation rivalling those of meteoric impact events. By illuminating these short-lived samples with extremely bright XFEL pulses, we can generate x-ray diffraction or absorption spectra rich with information about their atomic arrangement, structure, and dynamics in the moments before their destruction. This ability to diagnose the dynamic response of matter under extraordinary thermodynamic conditions is transforming experimental high-pressure physics, allowing us to better understand not only the internal structure, formation, and collision dynamics of planetary bodies, but how to synthesise and recover exotic high-pressure phases of matter, and how engineering alloys and ceramics respond to the huge dynamic stresses created by hypervelocity impacts.In this project, we aim to leverage the diagnostic power of the recently commissioned European XFEL (EuXFEL), an international XFEL facility backed by a consortium of twelve countries to which the UK has committed approximately £30M in capital to date. We will exploit the EuXFEL to shed new light on the plasticity and strength of model metals dynamically deforming at extreme pressures and strain rates. Our aim is to take the 'ordinary' physical processes controlling plastic deformation that materials scientists have studied for over a century, and to examine them under the 'extraordinary' thermodynamic conditions accessible via dynamic compression. Using the UK-built, high-repetition-rate, £8M DiPOLE-100 laser recently installed at EuXFEL, we will laser-compress a range of metals and alloys to planetary pressures over nanosecond timescales at an unprecedented shot rate. We will use femtosecond x-ray diffraction to measure the ultrafast rotation experienced by our samples' microstructure, and use it to identify the plasticity mechanisms that relieve the colossal shear stresses accumulated during compression. From these same diffraction measurements, we will extract the strain state of our metallic samples, allowing us to measure their dynamic strength at extreme strain rates. We will also use EuXFEL to study these samples' x-ray absorption properties under extreme loading, with which we can track their temperature dynamics in situ. Together, these XFEL-enabled experimental measurements of plasticity mechanisms, strength, and temperature evolution have the potential to transform our understanding of material deformation physics under extreme loading conditions.