Roles of the semisolid heating on the microstructure of a hypereutectic Al-Fe-Cu alloy

A hypereutectic Al-Fe-Cu alloy with a high-volume fraction ferro-aluminum second phase (AlFe phases for short) was reheated in the solid-liquid region and the microstructure evolution was investigated. During semisolid heating, the high-melting AlFe phases in the Al-Fe-Cu alloy were demonstrated to stunt the grain growth and to block the liquid coalescing and the solid moving. Consequently, the grain sizes in the alloy increased rapidly and then slowly with increasing holding time, and the grains increased gradually with increasing temperature. Smaller grain grew into the large grain but it did not continually grow into the larger grain with increasing temperature or holding time. The shape factor (SF) of the alloy increased gradually and then decreased quickly with increasing temperature or holding time. The major alloying elements in addition to magnesium in the hypereutectic Al-Fe-Cu alloy were finally enriched at the grain boundaries or around the AlFe phases. Besides dissolving in the grains or AlFe phases, copper also diffused between the grains or around AlFe phases, resulting in the formation of diverse Cu-enriched zones. Cu constituents in the inter-grains are outnumbered in the intra-grains. The coarsening kinetics of the alloy was controlled by grain boundary diffusion. The coarsening rate constants K in the initial stage of heating (5-20 min) were several times larger than that in the later stage of heating (20-60 min), indicating the blocking effect of AlFe phases on coarsened grain being obvious.