Microstructure and dendrite morphology of Si p /ZA27 composite

Abstract

The effects of pouring temperature, Si content and P modification on the microstructures and dendrite morphologies of Si_p/ZA27 in situ composites have been investigated. Especially, the dendrite morphologies have been visually shown through observation of shrinkage porosity microstructure. The results indicate that the primary α-Al grains change from equiaxed dendrites to parallel distributed columnar-like dendrites as the pouring temperature rises from 575 to 800°C. The change tendency in dendrite morphology with increasing Si content is opposite to that with rising pouring temperature. However, these parallel distributed dendrites with highly anisotropic morphology are not feathery grains. The P modification promotes the formation of feathery grains, and this kind of grains can form when the P modified 4·76 wt- Si_p/ZA27 composite is poured at 750°C. There are four lines of secondary dendrite arms around one primary trunk of a feathery grain. Two of them are located in the twin plane (111), and they are easier to grow up than the other two. Therefore, the feathery grains are always in a laminar structure that is overlapped by two-dimensional structured laminar dendrites. The formation process and three-dimensional structure of the feathery grains have also been discussed.