Energy dissipation by grain boundary replacement during grain growth

The changes in both the grain boundary area and grain boundary energy that occur during grain growth have been measured in polycrystalline Ni using high energy diffraction microscopy. In addition to the reduction of grain boundary area, the average grain boundary energy decreases as higher energy grain boundaries are replaced by lower energy grain boundaries. This energy dissipation mechanism influences grain boundary migration and might explain the absence of a correlation between grain boundary curvature and migration velocity. Classical studies of isotropic grain growth in polycrystals have been based on the idea that grain boundary (GB) migration is driven by the product of the GB energy and curvature. [1,2] While support for this foundational concept is certainly found in studies of bicrystals [3] and simulations [4], recent experimental evidence contradicts this idea. When measured curvatures and velocities were compared in Ni [5] and SrTiO3, [6] they were found to be uncorrelated. Similar observations were reported for alpha-Fe, [7] where the measured constant of proportionality between curvature and velocity did not behave in expected ways. The purpose of this letter is to present evidence that, during grain growth, the replacement of higher energy GBs by lower energy boundaries dissipates energy by a mechanism that is not related to curvature. This reduction of the average GB energy represents an additional driving force for grain growth. For a GB network comprised of N triangular mesh elements of area ai, the total free energy (F) is: