Quasi-static and Impact Nanoindentation to Study Deformation Mechanisms in Metals

纳米压痕是一种很有吸引力的技术，它可以以一种通用和经济的方式研究局部机械特性。戈登实验室中的两个MML压痕代表了当前的技术水平。已经开发了热和冷阶段，允许在高达~800℃和低至-170℃的温度下工作。可以进行冲击压痕，以产生高达约10^4 S^-1的(瞬时)局部应变率。这两个系统中的一个可以在真空或可控气氛下运行，大大减少了高温下样品氧化和金刚石尖端侵蚀的发生率，也消除了低温下的冷凝。此外，还开发了一套有限元模拟程序，允许从纳米压痕数据推断准静态材料本构关系和特定试件的残余应力水平，具有良好的可靠性和准确性。拟议的工作将涉及开发这些能力，特别是与冲击模式压痕有关的能力。这将需要改进当前的模型套件，并全面描述压头运动的动力学特性。这应该允许在传统测试存在严重困难的应变率范围内提取应变率敏感性信息。这些技术将被应用于“标准”材料，如纯铜，也将被应用于特别感兴趣的合金。其中包括将由AWE供应的(贫化)铀。这类合金在相对较低的应变下可以表现出明显的超弹性变形，将开发出提取表征这种行为的参数的方法。还将研究时效对这些合金的影响，这将有助于在压痕阶段(在真空室内)进行现场热处理。AWE将为博士生提供资金，该项目将特别针对这些合金以及超弹性变形在其整体机械行为中的作用。

Nanoindentation is an attractive technique, allowing study of local mechanical characteristics in a versatile and cost-effective manner. The two MML indenters in the Gordon Laboratory represent the current state of the art. Hot and cold stages have been developed, allowing operation at up to ~800 C and down to -170 C. Impact indentation can be carried out so as to generate (transient) local strain rates ranging up to about 10^4 s^-1. One of the two systems can be operated in vacuum or under controlled atmosphere, greatly reducing the incidence of specimen oxidation and diamond tip erosion at high temperature, and also eliminating condensation at low temperature. Furthermore, a suite of FEM modelling routines has been developed, allowing quasi-static material constitutive relations, and residual stress levels in particular specimens, to be inferred from nanoindentation data, with good degrees of reliability and accuracy.The proposed work will involve development of these capabilities, particularly relating to impact mode indentation. This will require enhancement of the current modelling suite, and comprehensive characterisation of the dynamics of indenter motion. This should allow the extraction of strain rate sensitivity information, over ranges of strain rate for which conventional testing presents severe difficulties. These techniques will be applied to "reference" materials, such as pure copper, and also to alloys of particular interest. These include (depleted) uranium, which will be supplied by AWE. Such alloys can exhibit pronounced superelastic deformation at relatively low strains and methodology will be developed for the extraction of parameters characterising such behaviour. The effect of ageing on these alloys will also be investigated, which will be facilitated by the capacity for heat treatments to be carried out in situ on the indenter stage (inside the vacuum chamber). AWE will fund a PhD studentship, which will be oriented particularly towards these alloys and the role of superelastic deformation in their overall mechanical behaviour.