Ichihito Narita

Phosphorus (P) addition has been previously reported to refine the primary Mg₂Si phase in Mg-based alloys. However, the refinement mechanism has not been fully clarified. This study aims to elucidate the mechanism of the refinement based on the crystallization behavior during the solidification process. Mg-9Al-1Zn alloy adding 0.5 mass% P and 3 mass% Si was prepared to investigate the effect of P on the growth behavior of Mg₂Si during solidification. The alloys were melted at 1023 K, held for 30 minutes, and then slowly cooled to 903 K at a rate of 0.05 K/s to facilitate the growth of Mg₂Si particles. Microstructural analysis revealed that the morphology of the primary Mg₂Si phase transformed from dendritic or equiaxed forms to polygonal shapes due to the addition of P. EPMA analysis confirmed the formation of Mg₃P₂ as a heterogeneous nucleus near the center of the Mg₂Si particles, along with the distribution of P within the particles. These findings suggest that P promotes the refinement of Mg₂Si by the facilitating heterogeneous nucleation and further contributes to refinement by suppressing crystal growth via a poisoning effect.

It is well known that the addition of phosphorus (P) or calcium (Ca) to Mg-Si alloy promotes the refinement of the primary Mg₂Si phase. However, the influence of simultaneous addition of P and Ca on the refinement behavior of Mg₂Si has not yet been clarified. The present study aims to elucidate the mechanism by which P and Ca affect the growth behavior of Mg₂Si phase in magnesium alloy during the solidification. Mg-9Al-1Zn-1Ca alloys adding 0.5 mass% P and 3 mass% Si were melted at 1023 K, held for 30 minutes, and slowly- (furnace-) cooled to 903 K at a rate of 0.05 K/s to promote the growth of Mg₂Si. Microstructural analysis revealed that the morphology of the primary Mg₂Si changed from dendritic or equiaxed shapes to polygonal granular shapes with the addition of P and Ca. The coexistence of P and Ca promoted the refinement.