The heterogeneous nucleation behavior of Al–Hf–P master alloy and its influence on the refinement of Mg 2Si phase in Mg 2Si/Al composites

摘要

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 60min, the primary Mg 2 Si in Mg 2 Si/Al composites can be significantly refined from more than 121.70μm to 19.20μm, 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 Mg 2 Si simultaneously. However, the microstructure of Mg 2 Si/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 HfSi 2 with near spherical shape. Based on FESEM photo-micrographs, it was first observed that HfP could act as the heterogeneous nucleation sites for primary Mg 2 Si particles, resulting in the excellent refinement of Mg 2 Si/Al composites.