Experimental observation and crystallographic description of M7C3 carbide transformation in Fe–Cr–Ni–C HP type alloy

Abstract The microstructure and phase composition of the heat resistant Fe–Cr–Ni–C alloy of the HP series (0.45С–25Cr–35Ni) in the as cast condition and after holding at 1150 °C during 2–100 h have been studied with the help of light and electron microscopy, electron microprobe and X-ray diffraction. The fragmentation of the initial single crystal М7С3 carbide particles into subdomains of ∼500 nm size and a subsequent gradual transition of certain subdomains into the М23С6 carbide have been detected after 2 h holding at 1150 °C. After 100 h holding at 1150 °C, complete transition of the hexagonal eutectic carbide М7С3 into the М23С6 carbide with face-centered cubic lattice occurred as ‘in situ’ transformation process. The mechanism of the in situ transformation from М7С3 to М23С6 carbides by increasing of the metal atoms number in the carbon coordination polyhedron from six to eight has been explained within the framework of the recent combinatory model for polymorph transformations in metals (Talis and Kraposhin, 2014).