Grain Rotation as a Mechanism of Grain Growth in Nanocrystalline Materials

Abstract

Grain-boundary (GB) properties in a polycrystalline system are generally anisotropic; in particular, both the GB energy and mobility depend on the GB misorientation. Moreover, in nanocrystalline materials, in which the grain size is less than 100 nm, grain rotations leading to the coalescence of neighboring grains via elimination of the common GB between them may provide a new mechanism for grain growth. Here we investigate the combined effect of curvature-driven GB migration and grain-rotation grain-coalescence on the kinetics, topology and morphology of grain growth. A stochastic velocity-Monte-Carlo algorithm based on a variational formulation for the dissipated power is implemented. The presence of both growth mechanisms introduces a physical length scale RC into the system, enabling the growth process to be characterized by two regimes. If the average grain size is smaller than RC, grain growth is dominated by the grain-rotation-coalescence mechanism. By contrast, if the average grain size is greater than RC, growth is dominated by curvature-driven GB migration. The values of the growth exponents, different for the two growth regimes and different from a system with isotropic GB properties, are rationalized in terms of the detailed growth mechanism and the continuous change of the fraction of low-angle GBs in the system. An extended von Neumann-Mullins relation based on averaged GB properties is proposed and verified.