Modification of fibrous morphology in the Cd--CuCd3 and Mg--Mg2Si eutectics by ternary additions

A study of the microstructural changes caused by ternary additions to two directionally solidified eutectics, Cd-CuCd3 and Mg-Mg2Si, was carried out using the technique of selective sublimation to reveal the eutectic morphology. The mechanism by which the observed modification of the eutectic structure occurs is thought to be the same in both systems studied. By electron microprobe analysis, it was found that the third component, Zn or Pb, was present with much larger concentrations in the intermetallic phase than in the matrix phase. Thus, the third component is preferentially rejected by the matrix phase during freezing and the liquid adjacent to the matrix phase may become undercooled as a result. Branching of fibers and formation of plates are believed to be responses to this undercooling. The presence of some branched rods and plates in Mg-Mg2Si specimens without ternary additions is thought to be due to residual amounts (0.02 weight percent maximum) of lead present in the starting materials. The occurrence of a fiber to plate modification with the addition of a suitable ternary component in the eutectic systems studied indicates that some degree of control over the microstructure of such eutectics is possible with suitable alloying