The heterogeneous nucleation behavior of Al–Hf–P master alloy and its influence on the refinement of Mg2Si phase in Mg2Si/Al composites

A novel Al–7Hf–1.2P master alloy with HfP particles was successfully synthesized by in-situ melt reaction, in which HfP particles exhibited hexagonal plates in three–dimensional spaces. Under the optimized conditions, i.e. addition level of 2.5% Al–7Hf–1.2P master alloy, melting temperature of 780 °C and holding time of 60 min, the primary Mg2Si in Mg2Si/Al composites can be significantly refined from more than 121.70 μm to 19.20 μm, with morphologies improved from coarse dendrite to octahedron or truncated octahedron obviously. Furthermore, it was found that Hf also has a significant modification effect on eutectic Mg2Si simultaneously. However, the microstructure of Mg2Si/Al composite becomes slightly coarser with further increase of Hf addition exceeding 1.5%. Meanwhile, it was found that Hf precipitates in the form of HfSi2 with near spherical shape. Based on FESEM photo-micrographs, it was first observed that HfP could act as the heterogeneous nucleation sites for primary Mg2Si particles, resulting in the excellent refinement of Mg2Si/Al composites. Keywords: Mg2Si/Al composite, Al–Hf–P master alloy, Mg2Si, Refinement, Modification, Heterogeneous nucleation