Microstructure evolution and growth behaviors of faceted phase in directionally solidified Al-yalloys I. Microstructure evolution of directionally solidified Al-15%Y hypereutectic alloy

The intermetallic compound has been widely introduced in alloys as a reinforced phase due to its high strength, high hardness and enhanced heat stability. The size, morphology, distribution and volume fraction of these intermetallic compounds affect the mechanical properties of materials significantly. In this work, the microstructures evolution and growth behoviors of primary intermetallic Al₃Y phase have been investigated in directionally solidified Al-15%Y (mass fraction) hypereutectic alloy at a wide range of pulling rates (1~100 μm/s). The ascast Al-15%Y alloy is composed of primary intermetallic Al₃Y phase and Al₃Y/Al eutectic structure. At relatively low pulling rates (1~5 μm/s), primary Al₃Y phase exhibits irregular and having a branching structure on the top of the specimens. Primary Al₃Y phase also precipitates in a faceted growth with sharp edges and corners. As the pulling rate increases, the morphologies of Al₃Y phase transit to elongated prism. Al₃Y phase distributes dispersively in the eutectic structure at a higher pulling rate, presenting a crossing shape with two prisms crossed vertically. Further increasing the growth rate to 100 μm/s, the cross morphology such as two six prismatic vertical cross structure of primary Al₃Y appear, similar to the growth in the form of dendrites. During the increase of pulling rates, the leading-phase at solid-liquid interface appear gradually, and the growth distance of primary phase increases with the pulling rates increase.