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

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 τ-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 τ-AlMn phase can only be generated from the high temperature ε-AlMn phase through a massive transformation, overheating the melt is necessary to produce the high temperature ε-AlMn phase. During the subsequent cooling to pouring temperatures, the ε-AlMn phase will partially or fully transform into the τ-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 β-Mn phases.