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.
