Microstructural evolution of grain refined in situ Sip/ZA27 composite during partial remelting

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The microstructural evolution of Si particle reinforced ZA27 (Sip/ZA27) composite refined by Zr and rare earth was investigated during partial remelting. The phase transformations and the effect of the Si particles were also discussed. The results indicated that a semisolid microstructure with small and spheroidal primary particles could be obtained after being partially remelted. The microstructural evolution was divided into four steps, the initial rapid coarsening, structure separation, spheroidisation and final coarsening, which resulted from the phase transformations of α+η+ϵ→β, η+β+ϵ+Si→L and β→α′+L, Si+α′→L, and Si+α′→L and L→Si+α′ respectively. The variation of the primary particle size with heating time obeyed the Lifshitz, Slyozov and Wagner law during the final coarsening stage. The Si particles had no effect on the general microstructural evolution steps of the ZA27 matrix. But due to the effect of Si element on the as cast microstructure, the final coarsening step could be further divided into two stages. Furthermore, the evolution progress of the four steps was slowered due to the existence of the Si particles. The size of the Si particles first decreased due to their partial remelting and then coarsened because of Ostwald ripening, and their morphologies first became blunt and then spheroidal.