The Effect of Microstructural Inhomogeneity on Grain Boundary Diffusion Creep
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The Effect of Microstructural Inhomogeneity on Grain Boundary Diffusion Creep Kanishk Rastogi and Dorel Moldovan Department of Mechanical Engineering, Louisiana State University, Baton Rouge, LA 70803 ABSTRACT Stress concentration at grain boundaries (GB), a phenomena arising from microstructural inhomogeneity, is an important factor in determining the mechanical properties of polycrystalline materials. In this study we use mesoscopic simulations to investigate characteristics of the deformation mechanism of grain-boundary diffusion creep (Coble creep) in a polycrystalline material. The stress distribution along the grain boundaries in a polycrystalline solid under externally applied stress is determined and the mechanism of how topological inhomogeneities introduce stress concentrations is investigated. Microstructures with inhomogeneities of various sizes and distributions are considered and their effect on the stress distribution and creep rate is quantified. INTRODUCTION In high temperature Coble creep deformation of fine-grained materials atoms are transported by grain boundary diffusion from boundaries in compression to those in tension [1]. The macroscopic creep behavior of a polycrystal undergoing diffusional creep is complex and generally governed by overall characteristics of the microstructure in which inhomogeneities play a significant role [2-7]. Stress concentration at grain-boundaries (GBs), a phenomenon arising from microstructural inhomogeneity, is an important factor in determining the mechanical properties of polycrystalline materials, such as crack nucleation or creep fracture in metals at elevated temperatures [8,9]. Although such stress concentrations may be of secondary importance for propagating the failure mechanism, their role is critical, for example, when a preferred site is selected for dislocation emission from a GB or for the nucleation of cavities or cracks along the GBs. At elevated temperatures, the very prominent GB sliding process prevents stress concentration at the triple junctions (the preferred site at low temperature). Instead, as illustrated below, there is a shift of the regions with higher stress away from the triple junctions into the GBs. In the present study we use a mesoscale simulation approach pioneered by Cocks [6] to simulate the stress distribution and the evolution of two-dimensional model microstructures subject to uniaxial tensile stress. Our focus is on elucidating the effect of microstructural inhomogeneity on stress distribution and on the creep rate. SIMULATION METHODOLOGY Following the work of Pan and Cocks [6] and Cocks and Searle [9], we briefly summarize the equations and the concepts used in our mesoscopic simulations. The diffusion of atoms along GBs is driven by the gradient of the chemical potential, which is induced by the gradient of the stress, σ, acting along each boundary. The chemical potential in the GB plane is related to the stress, σ, by the relation: µ = µ0 − σΩ , where µo is the chemical potential of an atom in a stressfree syst
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