Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, California 94550, USA.
Department of Physics, Clarendon Laboratory, University of Oxford, Parks Road, Oxford OX1 3PU, UK.
Nature. 2017 Oct 25;550(7677):496-499. doi: 10.1038/nature24061.
Pressure-driven shock waves in solid materials can cause extreme damage and deformation. Understanding this deformation and the associated defects that are created in the material is crucial in the study of a wide range of phenomena, including planetary formation and asteroid impact sites, the formation of interstellar dust clouds, ballistic penetrators, spacecraft shielding and ductility in high-performance ceramics. At the lattice level, the basic mechanisms of plastic deformation are twinning (whereby crystallites with a mirror-image lattice form) and slip (whereby lattice dislocations are generated and move), but determining which of these mechanisms is active during deformation is challenging. Experiments that characterized lattice defects have typically examined the microstructure of samples after deformation, and so are complicated by post-shock annealing and reverberations. In addition, measurements have been limited to relatively modest pressures (less than 100 gigapascals). In situ X-ray diffraction experiments can provide insights into the dynamic behaviour of materials, but have only recently been applied to plasticity during shock compression and have yet to provide detailed insight into competing deformation mechanisms. Here we present X-ray diffraction experiments with femtosecond resolution that capture in situ, lattice-level information on the microstructural processes that drive shock-wave-driven deformation. To demonstrate this method we shock-compress the body-centred-cubic material tantalum-an important material for high-energy-density physics owing to its high shock impedance and high X-ray opacity. Tantalum is also a material for which previous shock compression simulations and experiments have provided conflicting information about the dominant deformation mechanism. Our experiments reveal twinning and related lattice rotation occurring on the timescale of tens of picoseconds. In addition, despite the common association between twinning and strong shocks, we find a transition from twinning to dislocation-slip-dominated plasticity at high pressure (more than 150 gigapascals), a regime that recovery experiments cannot accurately access. The techniques demonstrated here will be useful for studying shock waves and other high-strain-rate phenomena, as well as a broad range of processes induced by plasticity.
在固体材料中，压力驱动的冲击波会导致极端的破坏和变形。了解这种变形以及在材料中产生的相关缺陷，对于研究广泛的现象至关重要，包括行星形成和小行星撞击点、星际尘埃云的形成、弹道穿透体、航天器屏蔽以及高性能陶瓷的延展性。在晶格水平上，塑性变形的基本机制是孪晶（即形成具有镜像晶格的晶粒）和滑移（即产生并移动晶格位错），但确定在变形过程中哪种机制起作用具有挑战性。表征晶格缺陷的实验通常研究变形后样品的微观结构，因此受到冲击后退火和回波的影响。此外，测量的压力相对较小（小于 100 千兆帕斯卡）。原位 X 射线衍射实验可以提供对材料动态行为的深入了解，但最近才应用于冲击压缩过程中的塑性，并且尚未提供有关竞争变形机制的详细信息。在这里，我们展示了具有飞秒分辨率的原位 X 射线衍射实验，该实验捕获了驱动冲击波驱动变形的微观结构过程的晶格级信息。为了展示这种方法，我们对体心立方材料钽进行了冲击压缩，钽因其高冲击波阻抗和高 X 射线不透明度而成为高能密度物理学中的重要材料。钽也是一种先前的冲击压缩模拟和实验提供了关于主导变形机制的相互矛盾信息的材料。我们的实验揭示了在数十皮秒的时间尺度上发生的孪晶和相关的晶格旋转。此外，尽管孪晶与强冲击波之间存在普遍关联，但我们发现从高压（超过 150 千兆帕斯卡）开始，孪晶向位错滑移主导的塑性转变，这一区域是恢复实验无法准确到达的区域。这里展示的技术将有助于研究冲击波和其他高应变速率现象，以及由塑性引起的广泛过程。
