Cooling curve and graphite morphology in Ni–C alloys

Ni–C alloys were used to study the solidification sequence of the graphite morphology based on a thermal analysis. Ninety grams of Ni–2˙2C alloys, with or without the addition of Mg, Ce and Ca, were melted at 1773 K and then cooled at 20 or 40 K min−1. The graphite morphology is chunky in the Ce added specimen cooled at 20 K min−1. Meanwhile, chunky graphite (CHG) and spheroidal graphite (SG) are observed in the pure Ni–C and Ca added Ni–C alloys. Spheroidal graphite forms in the Ni–C–Ce alloy cooled at 40 K −1. This cooling curve shows a continuous temperature decline during the eutectic reaction identical with that of the SG iron. Only flake-like graphite is formed in the Mg added specimen. From these experimental results, it was concluded that the formation of CHG occurs earlier than that of the SG in the Ni–C alloys. Thus, the solidification mechanism of the Ni–C system differs from that of the Fe–C one.