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How Grain Growth Stops: A Mechanism for Grain-Growth Stagnation in Pure Materials

Taking the Rough with the Smooth Even with extensive annealing at high temperatures, most polycrystalline materials will not become a perfect single crystal, which would represent the thermodynamically preferred state. The stability of the polycrystalline state has been attributed to the presence of impurities that accumulate at the grain boundaries, but even very pure materials show grain growth stagnation. Using simulations, Holm and Foiles (p. 1138) show that grain boundaries can be classified as “rough” and “smooth.” Rough boundaries move continuously with well-defined activation energies, while the smooth boundaries have low mobility and move in a jerky, stepwise manner. With heating, a boundary can change from smooth to rough, but the transition temperature can vary by hundreds of degrees from one grain boundary to the next. These smooth, low-mobility boundaries thus pin the polycrystalline structure, even in the absence of impurities. Smooth, low-mobility grain boundaries limit crystallite growth when a pure metal is heated. The thermodynamic equilibrium state of crystalline materials is a single crystal; however, polycrystalline grain growth almost always stops before this state is reached. Although typically attributed to solute drag, grain-growth stagnation occurs, even in high-purity materials. Recent studies indicate that grain boundaries undergo thermal roughening associated with an abrupt mobility change, so that at typical annealing temperatures, polycrystals will contain both smooth (slow) and rough (fast) boundaries. Mesoscale grain-growth models, validated by large-scale polycrystalline molecular dynamics simulations, show that even small fractions of smooth, slow boundaries can stop grain growth. We conclude that grain-boundary roughening provides an alternate stagnation mechanism that applies even to high-purity materials.