A novel approach to the mechanism for the grain refining effect of melt superheating of Mg–Al alloys

The possible grain refining candidates for Mg-Al alloys that may occur in the superheating process have been systematically studied from a crystallographic perspective. The edge-to-edge matching model was used to investigate and compare the grain refining efficiency of various (Al, Mn) containing phases existing in the melt. The results show that the metastable tau-AlMn phase possesses significantly better crystallographic matching to the Mg matrix than the other Al-Mn intermetallic phases and could therefore serve as an effective heterogenous nucleation site. However, since the metastable tau-AlMn phase can only be generated from the high temperature epsilon-Alkin phase through a massive transformation, overheating the melt is necessary to produce the high temperature epsilon-AlMn phase. During the subsequent cooling to pouring temperatures, the epsilon-AlMn phase will partially or fully transform into the tau-AlMn phase, which in turn can act as a nucleation site. The. rapid cooling and the short isothermal holding time prevent it from further transforming into the equilibrium Al8Mn5 and beta-Mn phases. (c) 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.