On the crystal shapes of primary silicon in hypereutectic Al-Si alloys

Abstract

Shapes and habit planes of primary silicon in sand and metal molded Al-15 to 40%Si alloys were studied with a metallurgical and a gonio-microscope by using polished alloys or extracted silicon crystals. The results obtained are as follows:(1) The crystal shapes strongly depended on the Si content, cooling rate and refining treatment.(2) The silicon particles in high silicon alloys were coarse and irregular while those in low silicon alloys with less than 15%Si were of fine polyhedral form.(3) The irregular particles were composed of thin hexagonal flakes, the flat surfaces of which seemed to be {111} or {100}.(4) The silicon particles in rapidly cooled alloys were smaller than those in slowly cooled ones. Their shapes suggested that they consisted of polyhedral Si crystals.(5) With addition of phosphorus, most small Si crystallised independently. They were bounded by {111} faces and had forms of polyhedrons growing along the <110> direction or of hexagonal pyramids.(6) It was suggested that the silicon particles, of octahedral or other shapes, in refined alloys were single crystals.