Microstructural defects and their origins in Cd–CuCd 3 eutectic

In this investigation, macrostructural and microstructural imperfections, formed in the fibrous Cd–CuCd₃ eutectic during unidirectional solidification, were characterized and the factors important in their formation were studied. The effect of growth rate R, imposed thermal gradient G, and natural convection on the defect morphology were investigated, and the important physical factors that determine the manner in which microstructural defects form were considered. Convection was found to play a minimal role in defect formation. The most important variable was found to be the ratio G/R, which determined the solid/liquid interface shape during solidification. When low values of G/R were used, eutectic cells were formed. For intermediate values of G/ R, the microstructure was found to contain features analogous to nodes in single–phase materials. Only when high values of G/R were used were all microstructural defects eliminated, producing a nearly perfect structure of parallel CuCd₃ rods in a matrix of cadmium. The morphology of branching and curving rods, which were found to be the primary microstructural defects, indicates that both the solid/solid interfacial free energy and its anisotropy are important factors in rod branching, and a branching mechanism consistent with the observations made was proposed. Anisotropy in the solid/solid interfacial free energy was considered to account for the formation of blades; however, kinetic considerations were required to account for the growth dependence of the rod-to-blade transition observed in this eutectic.

MST/130