Microstructural Behavior and Failure of FCC Crystalline Aggregates
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0976-EE02-02
Microstructural Behavior and Failure of FCC Crystalline Aggregates O. Rezvanian, M. A. Zikry, and A. M. Rajendran Department of Mechanical and Aerospace Engineering, North Carolina State University, Raleigh, NC, 27695 ABSTRACT A unified dislocation density-based microstructural representation of f.c.c. crystalline materials, has been developed such that the microstructural behavior can be accurately predicted at different physical scales. This microstructural framework is based on coupling a multiple-slip crystal plasticity formulation to three distinct dislocation densities, which pertain to statistically stored dislocations (SSDs), geometrically necessary dislocations (GNDs), and grain boundary dislocations (GBDs). This interrelated dislocation-density formulation is then used with specialized finite-element modeling techniques to predict the evolving heterogeneous microstructure and the localized phenomena that can contribute to failure initiation as a function of inelastic deformation. INTRODUCTION During inelastic deformations of crystalline aggregates, geometrically necessary boundaries (GNBs) form due to gradients of plastic deformation, which are caused by material texture or loading conditions. Dislocations stored in these boundaries are the geometrically necessary dislocations (GNDs) needed to preserve the lattice continuity through accommodating lattice misorientations across the GNBs. Statistically stored dislocations (SSDs) accumulate by the statistical trapping of dislocations during plastic slip. The SSDs are generally heterogeneously distributed, and comprise a cell-type microstructure with low-density cell interiors and high-density cell walls. Lattice miorientations across the low-angle grain boundaries (GBs) are accommodated by misfit dislocations. Experimental analyses of the correlation between the slip pattern and the microstructure [1, 2] have indicated that some grain orientations develop GNBs that contain Burgers vectors, which belong to one slip plane. These GNBs are commonly denoted as crystallographic boundaries (CBs) [3], and form when two coplanar active slip systems account for a large fraction of the plastic slip. Some other grain orientations result in the formation of GNBs with Burgers vectors belonging to two active slip planes. These GNBs are denoted as non-crystallographic boundaries (NCBs) [3]. It has been further observed [2] that CBs have a mixed tilt and twist characteristic, while the NCBs have predominantly tilt characteristic. Hence, CBs can then consist of both types of GNDs, namely edge and screw types, while NCBs are mostly comprised of GNDs of edge type. This study provides a physically-based unified formulation for microstructural evolution through a representation of dislocation densities associated with SSDs, GNDs, and GBDs, coupled to a multiple-slip crystalline plasticity formulation for f.c.c. materials, which is then coupled to specialized finite-element methods to track intergranular and intragranular microstructural evolution. This interrelated m
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