Scale-Dependent Crystal Plasticity and Nanoindentation-Induced Dislocation Microstructure

CMMI 0800168：依赖尺度的晶体塑性和纳米压痕诱导的位错微结构高燕飞(PI)，田纳西大学诺克斯维尔分校Pharr(Co-Pi)本研究的目的是通过对测量的和模拟的纳米压痕诱导的近表面位错微结构的定量比较，从机理上理解单晶的尺度相关塑性行为。现有的理论/实验比较通常仅限于宏观信息，如硬度。在实验上，将使用纳米压痕测试来通过改变材料类型、压头形状和晶体取向来实现受控的缺陷分布。将检查连续硬度测量和可靠硬度提取中的伪影。将使用微焦点X射线技术进行亚微米分辨率的空间分辨晶格取向偏差测量。从建模的观点来看，测量的位错微结构将被用作输入，以检验晶格旋转、不相容晶格变形和几何上必要的位错分布之间关系的结晶学性质。从而可以对各种梯度塑性理论进行验证和标定。通过在压痕前施加不同程度的预应变，并利用离散位错模型来定性地研究源限制行为。这项研究将为力学理论家在对尺度相关本构模型的批判性评估中带来好处和影响，并为材料科学家提高小规模材料结构的设计能力带来好处和影响。多学科性质将成为本科生和研究生教育的传播范例。将开发新的教学材料和动手实验室演示，以纳入外展和多样性计划，这些计划旨在激发高中生的兴趣，并帮助招收工程科学中代表性不足的群体的学生。

CMMI 0800168: Scale-Dependent Crystal Plasticity and Nanoindentation-Induced Dislocation MicrostructureYanfei Gao (PI), George M. Pharr (Co-PI)University of Tennessee, KnoxvilleThe objective of this research is directed to a mechanistic understanding of the scale-dependent plastic behavior of single crystals by a quantitative comparison of measured and simulated near-surface dislocation microstructure induced by nanoindentation. Existing theoretical/experimental comparison has generally been limited to macroscopic information such as hardness. Experimentally, nanoindentation tests will be employed to achieve controlled defect distributions by varying the material type, indenter shape, and crystallographic orientations. Artifacts in continuous stiffness measurement and reliable hardness extraction will be examined. The spatially resolved lattice-misorientation measurements with sub-micron resolution will be performed using microfocus x-ray technique. From modeling standpoint, the measured dislocation microstructure will be used as input to examine the crystallographic nature of the relationship between lattice rotation, incompatible lattice deformation, and geometrically-necessary-dislocation distributions. Thus various gradient plasticity theories can be validated and calibrated. The source limited behavior will be investigated by applying various degrees of pre-strain prior to indentation and qualitatively by utilizing a discrete dislocation model. The research will lead to benefits and impacts for mechanics theorists in the critical evaluation of scale-dependent constitutive models and for materials scientists in advancing design capabilities of small scale material structures. The multidisciplinary nature will be a disseminative example for undergraduate and graduate education. New instructional materials and hands-on laboratory demonstrations will be developed for incorporation into outreach and diversity programs, which are designed to inspire interests of high school students and to help recruit students from underrepresented groups in engineering sciences.