Reactive synthesis of hexagonal Ti5P3.16 crystals and their heterogenous nucleating mechanism on primary Si

Reactive synthesis of Ti₅P_3.16 crystals was firstly investigated in the Al–Ti–P–C alloy and their effective refinement on primary Si in Al–Si alloys was also revealed. It was found that the Ti₅P_3.16 phase was formed by the direct reaction of P atoms with TiC particles in Al melt. During the reaction process, P atoms absorbed on TiC agglomerates and initiated the gradual phase decomposition, and then the releasing Ti atoms combined surrounding P ones together to form Ti-P phase. Based on the TEM and SEM characterization, it is identified that the Ti₅P_3.16 phase has an hexagonal close packed crystal lattice structure and mainly displays a regular hexagonal prism morphology enclosed by six {10-10} facets and two parallel {0001} ones. Furthermore, the Al–Ti–P–C alloy containing the Ti₅P_3.16 particles showed an efficient refinement performance on primary Si particles for hypereutectic Al–Si alloy, whose average size was reduced from 85 μm to 12.5 μm for A390 alloy. In order to reveal the refinement mechanism, the evolution process of the Ti₅P_3.16 phase induced by Si atoms in the Al–Si melt was carried out, indicating that the Ti₅P_3.16 transformed into amounts of nano AlP particles with sizes of about 100–200 nm after added into the melt. Then, these in-situ AlP particles with fresh interfaces distributed uniformly in the melt and act as the effective heterogenous nuclei for the primary Si during the following solidification process of Al–Si melt with the decrease of the melt temperature, which leads to significantly refinement of the microstructure of A390 alloys.