A Quantized Crystal Plasticity Model for Nanocrystalline Metals: Connecting Atomistic Simulations and Physical Experiments

by Lin Li

Institution: The Ohio State University
Department: Materials Science and Engineering
Degree: PhD
Year: 2011
Keywords: Materials Science; quantized crystal plasticity; nanocrystalline metals; mechanical behaviors; deformation mechanisms
Record ID: 1919709
Full text PDF: http://rave.ohiolink.edu/etdc/view?acc_num=osu1299605340


Nanocrystalline (NC) metals, which consist of grains or crystallites with sizes less than 100 nm, have exhibited unique mechanical and physical properties, in comparison to coarse-grained (CG) counterparts. The appealing mechanical properties, for instance, include extremely high strengths, very extended elastic-plastic transitions, and unprecedented magnitudes of recoverable plastic strain. Further, footprints of inter-granular stresses measured from diffraction experiments are distinct for NC metals vs. CG metals. In particular, recent in-situ synchrotron measurements reveal that residual lattice strains change rather modestly even after imposing macro plastic strains to ~1%. Remarkably, over the same regime, the corresponding residual peak widths decrease. These phenomena are in sharp contrast to CG metals, for which residual lattice strain and peak widths both increase with deformation. In this dissertation, a quantized crystal plasticity (QCP) model is developed to explore the aforementioned unique NC features. The QCP model employs a crystallographic description of dislocation slip plasticity; in particular, single slip events across nano scale grains impart large (~1%) increments in grain-averaged plastic shear. Therefore, plasticity does not proceed in a smooth, continuous fashion but rather via strain jumps, imparting violent grain-to-grain redistribution in stress. This discrete feature is consistent with recent Molecular Dynamics (MD) simulations, which illustrate a dramatic jump in grain-averaged shear strain when a dislocation spontaneously transverses a nano grain interior after depinning from grain boundary (GB) ledges. Finite element simulations implementing this quantized plasticity approach predict the experimental properties of enhanced strength, extended elastic-plastic strain, and recoverable plastic strain, as well as the trends in residual lattice strain and peak width mentioned, but only under certain conditions. First, the grain-to-grain distribution of critical stress for slip activation is very different from that for CG materials. In particular, no events occur below a rather large threshold stress ~ 1/grain size; and above this threshold, a very asymmetric distribution predominates, signifying that a relatively large number of easier-to-slip grains are balanced by a minority of harder-to-slip grains. Second, there exists a large residual stress state, which can be removed via post deformation. The quantized crystal plasticity provides an alternate view of NC deformation, compared to hypotheses in the literature that are centered on GB sliding or deformation of a GB phase separated from grain interior. The QCP model is capable of bridging the disparity in length and time scales between MD simulations and physical experiments, and as well establishes an insightful connection between them.