Phenomenology of Abnormal Grain Growth in Systems with Nonuniform Grain Boundary Mobility
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I.
INTRODUCTION
ABNORMAL grain growth (AGG) occurs when a subset of grains grow to extreme sizes at the expense of the uniformly growing normal grains and is associated with the development of a bimodal grain size distribution and a shift in crystallographic texture.[1] Understanding and controlling AGG is relevant to a wide range of technologically important applications, including nanocrystalline metals, which can undergo significant microstructural evolution via AGG at room temperature,[2–4] and silicon transformer steels, in which abnormal growth of Goss-oriented grains improves magnetic properties.[5] In nanocrystalline materials, AGG is sometimes attributed to the high density of interfacial area, which provides a large driving force for grain growth.[6,7] However, this hypothesis offers no compelling explanation as to why only a small fraction of grains grows abnormally. A more detailed analysis is necessary to identify the crystallographic conditions necessary for sustained AGG to occur. This has posed a significant challenge to the grain growth community, because there are many different physical mechanisms responsible for AGG in different material systems. For a grain to grow abnormally, it requires a growth advantage over neighboring grains and a persistence mechanism to retain that growth advantage as the abnormal growth front sweeps through new microstructural neighborhoods. A simple size advantage is insufficient for AGG to persist due to curvature-driven grain boundary motion: unusually large grains grow slowly
BRIAN L. DECOST, Graduate Student, and ELIZABETH A. HOLM, Professor, are with the Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213. Contact e-mail: [email protected] Manuscript submitted January 8, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
relative to smaller surrounding grains and return to the normal grain size distribution.[8,9] Some of the persistent growth advantages associated with AGG include selective grain boundary depinning in microstructures containing second-phase particles,[10] gradients in crystallographic texture,[11] crystallographic outlier grains in tightly textured polycrystals,[12,13] preferred growth of grains with low surface energy,[14] preferential growth of certain faceted grain boundaries,[15,16] solid-state wetting transitions at grain boundaries and triple junctions,[17,18] and transitions in grain boundary structure resulting in high mobility.[19,20] Recent atomistic simulations of grain boundaries in Ni indicate that grain boundary mobility can vary by orders of magnitude at temperatures as low as 600 K (327 C).[21] Of particular interest, about 20 pct of the boundaries surveyed had mobility that remained constant or even increased with decreasing temperature.[21] A significant fraction of these antithermal boundary types are incoherent R3 boundaries. The tendency for FCC metals to have large populations of R3 boundaries (40 to 60 pct),[22,23] therefore, poses a possible mechanism for low-temperature AGG in
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