ABSTRACT
Uneven stress distribution and stress localization during deformation are key factors for fracture and failure in poly crystalline and nanocrystalline metals. The inhomogeneous polycrystalline microstructure that consists of grains of different sizes and shapes joined together at different angles and forming various types of grain boundaries (GBs) creates inhomogeneous deformation fields under homogeneous loading. There are a number of factors responsible for the appearance of inhomogeneous deformation inside the polycrystal. The coexistence of grains of different sizes and orientations with anisotropic elastic properties is one factor. The difference in the structures and properties of the GBs between grains of various misorientations is a second factor for uneven stress distribution. A third factor is grain-boundary sliding (GBS), i.e., the rigid translation of one grain relative to another at the GB interface. When grains deform, GBS is an inevitable process as a result of the relative movements and rearrangements of the grains.1 GBS is a strongly inhomogeneous mode of deformation localized at a very narrow interface layer, thus creating very strong shear forces. This, together with the fact that the shear strength of a general GB between grains of high misorientation angle (highangle GB) is much lower than the shear strength of the perfect crystal,2,3 causes GBS to be a prominent deformation mode. Particularly in nanocrystalline metals, because of their extremely small grain size (less than 100 nm) resulting in a dense network of GBs, GBS assisted through stress-driven GB diffusion for grains smaller than 20 nm4-6 and through dislocation slip in larger grains7-9 was found to be a major mode of deformation. During GBS, the load transfer between the sliding surfaces is significantly reduced, and the load is redirected to other places more resistant to sliding such as the triple junctions where three GBs meet and GBS cannot be accommodated. In this way, sliding creates redistribution of the load and the appearance of stress localization in the microstructure that, in the absence of an efficient accommodation mechanism, can lead to void formation and microcracking starting at the triple junctions.10,11
This chapter describes a multiscale modeling strategy that is used to study the effect of GBS for stress localization in a polycrystalline microstructure. Although they reveal system behavior at atomic-level resolution, molecular dynamics (MD) calculations of large systems of grains are computationally prohibitive to perform, and a more efficient analysis technique is sought. Finite element method (FEM) analysis is an obvious choice, but the FEM models cannot simulate the deformation mechanisms found within the systems of grains a priori. Thus, the FEM models must be tuned in order to reproduce the
stress localization observed in the atomistic simulations. For this purpose, a nanocrystalline model of bimodal grain-size distribution was constructed and is convenient for study by both MD and FEM simulations. The MD simulations revealed the behavior of this model with atomic-scale details. Additional MD simulations on a bicrystal model were performed to extract the elastic and yield properties of the GBs presented in the model. These GB properties were then used to tune the FEM model to reproduce closely the behavior of the nanocrystalline model to match with the MD simulation results. The FEM model then served as part of a multiscale modeling strategy to extrapolate the MD-derived information to larger scales to study the failure properties of nanocrystalline metals.
