Challenging Some Free - Energy Reduction Criteria for Grain Growth
S G. Srinivasan, John W. Cahn
Laboratory experiments on wedge shaped bicrystalline specimens ahve provided much insight into grain-boundary migration [for example: Rath and Hu, Trans. AIME, VOl. 245, pp. 1577 (1969)]. We revisit this geometry, but employ a wedge angle of 2 pi, at the nanometer scale using atomistic molecular dynamics simulations in three dimensions. That is, we use bicrystals of circular cylindrical grains embedded in another, with a variety of initial misorientations. The rotation of some included grains increases the grain boundary specific free-energy, gamma, spontaneously. We show that this result does not violate thermodynamics, which requires only that the total free energy not increase. We infer that grain rotation and the grain-boundary motion are processes coupled to one another. The increase in gamma results from grain boundary motion that decreases both the grain boundary area and its total free-energy. For small misorientations this result can be understood in term of grain boundary dislocations. Since this behavior continues smoothly to high angles, we suggest that there is a single coupling law which spans the entire misorientation range we studied.