Solidification of Cu-Cu₂S alloys in stable and metastable systems

Cu-Cu₂S-X alloys specimens solidified at a cooling rate of about 20 K/sec have been examined by SEM, and the key factors in controlling the morphology of sulfide inclusions have been studied from a thermodynamic viewpoint. In the specimens containing some crystalline inoculants, such as Al₂O₃ and ZnO produced by the deoxidation in the molten state, the solidification proceeds according to the stable phase diagram, forming dendritic or polyhedral Cu₂S by primary crystallization, and rod-like or stringy Cu₂S by the eutectic reaction. Without crystalline inoculants, droplets of Cu₂S are formed by the monotectic reaction, because the eutectic temperature in the stable Cu-Cu₂S system is estimated to be only 1 K higher than the monotectic temperature in the metastable system. These findings are in good accordance with those on MnS in Fe-MnS-X alloys.