Undercooling in Co-Cu alloys and its effect on solidification structure

Undercooling behaviour and solidification morphology change of various Co-Cu alloys were examined. Each alloy was melted in an alumina crucible under an argon atmosphere by high-frequency induction, and the cyclic heating and cooling was repeated several times in the temperature range between 1300 and 1850 K. The temperature change during the experiment was analysed under the Newtonian cooling assumption. The temperature curve showed that the undercooling in a first few cycles was negligibly small but it increased remarkably. The alloy was undercooled below the metastable liquid miscibility gap after the next several cycles. In these samples, liquid separation was observed. The homogeneously mixed spherical grains of copper-enriched phase were observed in cobalt-enriched matrix for the samples solidified immediately after the liquid separation. The two melts became coarser after the separation by mutual coalescence. In the case of the slow start of the solidification after the separation, they formed a clear interface between the upper cobalt-enriched layer and the lower copper-enriched layer located in the lower part according to the density difference between the two melts. It depended on the cooling rate after liquid separation. The very fine duplex structure can be obtained by the rapid cooling of the melt at the initial stage of the separation.