A study of elastic precursor decay in FCC, BCC and HCP metals under shock loading

弹性前兆随应变率和距离的演化包含了关于强动态载荷下屈服起源的丰富信息。尽管这是一个研究了40多年的话题，但这种行为的晶格和微结构起源仍然模糊不清，并且强烈依赖于材料。基于基本位错理论对前驱衰变进行分析建模的努力通常会导致对初始可移动位错密度的高估，至少高达2个数量级。这种现象被归因于许多可能的来源，包括高阶滑移系统的高速激活，应力波有限上升过程中位错的快速形核，以及跨音速位错速度。尽管对Al和Fe进行了大量的实验研究，但扩展到其他面心立方或体心立方材料，到更高对称性的hcp系统，甚至考虑微结构变化的材料还很少，限制了对诸如Peiriel应力、热激活和层错能等因素的依赖的仔细研究。仔细研究前体衰变的手段在于高度仪表化的ISP100毫米口径单级气枪的独特能力。对单晶和多晶钽、工业纯铝和MA2镁合金进行的初步实验表明，可以同时加载多个靶材，允许在不同厚度、成分或缺陷密度的样品之间进行直接比较。本博士试图在这些初步研究的基础上，通过将这项研究扩展到特定的FCC、BCC和HCP感兴趣的金属的研究。本PHD将通过对弹性前驱衰变现象的系统研究来研究位错的产生和迁移的动力学。这个基于实验的PHD项目将使用一套在线编程实验平台，从大口径气炮、中尺度气体发射器到长脉冲激光冲击驱动器，对特定的面心立方(Al，Cu，Ni)，体心立方(Ta，Mo，W)和六方(钛，锆，镁)金属系统进行全面的研究，跨越数十年的应变率(10^4到10^10 S^-1)和靶材厚度(厘米到Mu)。100毫米口径的大口径isp气枪将能够同时装载多个目标，便于直接检查应变率和目标厚度、晶体系统、织构的影响，如果是单晶靶，则有利于定向。这种方法适用于几毫米到亚毫米长度的目标，并将采用移频HETV。由于与峰值弹性状态相关的相对较低的速度，以及加载表面附近极快的衰减率，向更薄的样品移动是一个挑战。这些都处于传统的位移模式(即HETV)和速度模式(VISAR)干涉测量技术的灵敏度和精度的边界。在中尺度气体发射器和长脉冲激光上的实验将允许询问这一具有挑战性的制度，并试验一种新型的成像位移干涉仪。由于这个项目试图将晶格和微观结构与塑性松弛联系起来，彻底了解靶子的初始条件是至关重要的。为此，在使用帝国理工学院广泛的X射线和金相学套件进行测试之前，将对样品进行全面的表征，以揭示初始位错密度、晶粒度、织构(多晶)和晶体取向(单晶)。这些实验的结果将指导帝国理工学院最近开发的动态离散位错塑性(D3P)程序，该程序将使人们能够探索特定的位错行为。这将反过来阐明在极端条件下缺陷产生和不稳定形成的初始阶段。

The evolution of the elastic precursor with strain rate and distance contains rich information regarding the origins of yielding under intense dynamic loading. Despite being a topic of study for over 40 years, the lattice and microstructural origins of this behaviour remain obscured and strongly material dependent. Efforts to analytically model the decay of the precursor based upon elementary dislocation theory typically result in an overprediction of the initial mobile dislocation density of at least 2 orders of magnitude. This phenomenon has been attributed to a number of possible sources, including the high-rate activation of high order slip systems, the rapid nucleation of dislocations during the finite rise of the stress wave, and transonic dislocation velocities. Despite a wealth of experimental studies of Al and Fe; extension to other FCC or BCC materials, to higher symmetry HCP systems, or indeed materials considering microstructural variation have been few, limiting careful study of the dependence of factors such as Peiriel's stress, thermal activation, and stacking-fault energy. Means to carefully study precursor decay lies in the unique capabilities of the highly instrumented ISP 100 mm bore single stage gas gun. Preliminary experiments performed on single crystal and polycrystalline tantalum, commerically pure aluminium, and Ma2 magnesium alloy have revealed the potential for performing simultaneous loading of multiple targets, allowing direct comparison between samples of differing thickness, composition, or defect density. This PhD seeks to build upon these pilot studies, by extending this research to the study of specific FCC, BCC, and HCP metals of interest. This PhD will investigate the kinetics of dislocation generation and mobility through a systematic study of the elastic precursor decay phenomenon. Using the suite of ISP experimental platforms, from the large-bore gas gun, mesoscale gas launcher, to the long-pulse laser shock driver, this experimentally based PhD project will perform a comprehensive study of specific FCC (Al, Cu, Ni), BCC (Ta, Mo, W), and HCP (Ti, Zr, Mg) metal systems, across decades of strain-rate (10^4 to 10^10 s^-1) and target thickness (cm to mu). The large format of the 100 mm bore ISP gas gun will enable simultaneous loading of multiple targets, facilitating direct scrutiny of the effects of strain-rate and target thick- ness, crystal system, texture, and in the case of single crystal targets, orientation. This approach is suitable for targets on the several mm to sub-mm length scale, and will employ frequency-shifted HetV. Moving to thinner samples presents a challenge due to the relatively low velocities associated with the peak elastic states, and the extremely rapid decay rates near the loading surface. These lie at the boundary of both sensitivity and accuracy for both conventional displacement-mode (i.e. HetV) and velocity-mode (VISAR) interferometry techniques. Experiments on the mesoscale gas launcher and long-pulse laser will allow interrogation of this challenging regime, and trialling of a novel imaging displacement interferometer. As this project seeks to correlate lattice and microstructure to plastic relaxation, a thorough understanding of the initial condition of the targets is of critical importance. To this end, samples will be fully characterised prior to testing using the extensive X-ray and metallography suite at Imperial College, to reveal initial dislocation densities, grain size, texture (polycrystals), and crystal orientation (single crystals). The results of these experiments will guide the Dynamic Discrete Dislocation Plasticity (D3P) code recently developed at Imperial, which will enable specific dislocation behaviours to be explored. This will in turn shed light on the incipient stages of defect generation and instability formations at extreme conditions.