Effects of cobalt content on the microstructures of Fe-9Al-30Mn-1C-xCo alloys

The microstructure of the as-cast Fe-Al-Mn-C-Co alloy was a mixture of the (α + DO₃ + γ + κ) phases. However, the microstructure of the alloy with 9.0 wt.% Co content was nearly a single γ phase. As the alloys underwent heat treatment between 1000 °C and 1100 °C, their microstructure changed to the (γ + κ) phase. The κ phase carbides ((Fe,Mn)₃AlC_x) had an ordered L′1₂-type structure with lattice parameter a = 0.385 nm and were formed by a spinodal decomposition during quenching. The alloy with Co content ≥5.0 wt.% was aged at 650-950 °C, a (γ + κ) → (γ + κ + Co₂₃C₆) → (γ + Co₂₃C₆) phase transition was observed in the matrix and on the grain boundaries. It is believed that the addition of Co not only stabilizes the austenitic structure at high temperatures but also inhibits and/or prolongs the precipitation of the κ phase during the aging process.