Scale-Dependent Crystal Plasticity and Nanoindentation-Induced Dislocation Microstructure

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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.