Phase relations in the Co–CoAl–C system involving E21 type Co3AlCx phase

Solid–liquid phase equilibria have been studied in a part of the Co-rich corner of the Co–CoAl–C alloy system where the ternary Co3AlCx carbide is a significant component in the microstructure of alloys on solidification. fcc-Co can coexist with Co3AlCx, graphite, and β-CoAl. They form inter alia two ternary eutectics and an invariant fcc-Co–Co3AlCx eutectic reaction between them. The eutectic reaction was utilized to grow directionally solidified (DS) ingots that exhibit aligned regular lamellar Co–Co3AlCx microstructures. Melt equilibria were established via differential thermal analysis (DTA) and light microscopy. signal observed in the solid-state with the DS microstructure and alloy composition was associated to X-ray diffraction, dilatometry and transmission electron microscopy work and attributed to the action of a second order reaction.