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

弹性前体随应变速率和距离的演化包含了关于在强动态载荷下屈服起源的丰富信息。尽管是一个研究课题超过40年，这种行为的晶格和微观结构的起源仍然模糊和强烈的材料依赖性。基于基本位错理论对前体的衰减进行分析建模的努力通常导致对初始移动的位错密度的过度预测至少2个数量级。这种现象已被归因于一些可能的来源，包括高速率激活高阶滑移系统，快速成核的位错在有限的应力波的上升，和跨音速位错速度。尽管丰富的实验研究的Al和Fe;扩展到其他FCC或BCC材料，更高的对称性HCP系统，或实际上考虑微观结构变化的材料已经很少，限制仔细研究的依赖因素，如Peiriel的应力，热活化，和堆垛层错能。仔细研究前体衰变的方法在于高度仪表化的ISP 100 mm口径单级气炮的独特能力。对单晶和多晶钽、高纯铝和Ma 2镁合金进行的初步实验揭示了同时加载多个靶的潜力，从而可以直接比较不同厚度、成分或缺陷密度的样品。该博士旨在建立在这些试点研究的基础上，通过将这项研究扩展到特定FCC，BCC和HCP金属的研究。这个博士将通过对弹性前体衰减现象的系统研究来研究位错产生和迁移的动力学。利用ISP的一套实验平台，从大口径气体炮，中尺度气体发射器，到长脉冲激光冲击驱动器，这个基于实验的博士项目将对特定的FCC进行全面的研究（Al、Cu、Ni）、BCC（Ta、Mo、W）和HCP（Ti、Zr、Mg）金属系统，在几十年的应变速率（10^4 ~ 10^10 s^-1）和靶厚度（cm ~ mu）范围内。100 mm口径ISP气枪的大尺寸将能够同时加载多个目标，便于直接检查应变率和目标厚度、晶体系统、纹理以及单晶目标的取向的影响。这种方法适用于几毫米到亚毫米长度尺度的目标，并将采用频移的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的极快衰减速率，移动到较薄的样品提出了挑战。这些谎言的边界的灵敏度和准确度的传统位移模式（即HetV）和速度模式（VISAR）干涉测量技术。中尺度气体发射器和长脉冲激光器的实验将允许询问这种具有挑战性的制度，并试用一种新的成像位移干涉仪。由于该项目旨在将晶格和微观结构与塑性弛豫相关联，因此对目标初始条件的透彻理解至关重要。为此，在使用帝国理工学院的广泛X射线和金相套件进行测试之前，将对样品进行全面表征，以揭示初始位错密度、晶粒尺寸、织构（多晶体）和晶体取向（单晶）。这些实验的结果将指导最近在帝国开发的动态离散位错塑性（D3 P）代码，这将使特定的位错行为进行探索。这反过来又将阐明在极端条件下缺陷产生和不稳定性形成的初始阶段。

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.