Microsegregation in unidirectionally cast copper alloys

Solute concentrations in dilute Cu-Ni, Cu-Ag, and Cu-Sb alloys have been measured at the centers of primary and secondary dendrites and in the interdendritic regions, using electron probe microanalysis. It was found that in the Cu-Ni alloys, the segregation ratio was independent of distance from the chill and alloy composition, with the effective distribution coefficient differing significantly from the equilibrium value. In the Cu-Ag and Cu-Sb alloys the segregation ratio was markedly dependent on distance from the chill and alloy composition, being greatest farthest from the chill. The microsegregation associated with secondary dendrite branches differs from that of the primary branches for the same spacing, the amount depending on the system. One ternary alloy, Cu-Ag-Ni, was examined and it was demonstrated that the presence of a second solute markedly influenced the microsegregation of the first solute. Measurements were made of primary and secondary dendrite arm spacing in the castings for increasing distances from the chill. The spacings are considered in terms of pouring temperature, freezing rate, and cooling rate. It was found that, in general, no simple relationship existed between dendrite spacing and the above variables within the range of conditions investigated in this study. Solute distributions across individual primary dendrite stalks were determined and related to curves calculated on the basis of complete mixing and diffusion solute transport in the liquid. For the Cu-Ag and Cu-Sb alloys(k < 1), good agreement was obtained between the experimental and complete mixing curves, using an effective value fork in the calculations determined from the dendrite center concentrations. For the Cu-Ni alloys(k > l), only a part of each experimental curve could be fitted by either the complete mixing or diffusion transport calculated curves.