Evolution of a novel Si-18Mn-16Ti-11P alloy in Al-Si melt and its influence on microstructure and properties of high-Si Al-Si alloy

[Display omitted] •The Si-P alloy shows better refining effect than Cu-14P in this experiment.•The core-shell model reveals a mechanism of the transformation of TiP to AlP.•P refinement is beneficial to reduce the coefficient of thermal expansion value. A novel Si-18Mn-16Ti-11P master alloy has been developed to refine primary Si to 14.7±1.3μm, distributed uniformly in Al-27Si alloy. Comparing with traditional Cu-14P and Al-3P, Si-18Mn-16Ti-11P provided a much better refining effect, with in-situ highly active AlP. The refined Al-27Si alloy exhibited a CTE of 16.25×10−6/K which is slightly higher than that of Sip/Al composites fabricated by spray deposition. The UTS and elongation of refined Al-27Si alloy were increased by 106% and 235% comparing with those of unrefined alloy. It indicates that the novel Si-18Mn-16Ti-11P alloy is more suitable for high-Si Al-Si alloys and may be a candidate for refining hypereutectic Al-Si alloy for electronic packaging applications. Moreover, studies showed that TiP is the only P-containing phase in Si-18Mn-16Ti-11P master alloy. A core-shell reaction model was established to reveal mechanism of the transformation of TiP to AlP in Al-Si melts. The transformation is a liquid-solid diffusion reaction driven by chemical potential difference and the reaction rate is controlled by diffusion. It means sufficient holding time is necessary for Si-18Mn-16Ti-11P master alloy to achieve better refining effect.