Multiple morphologies and twin structure generated by a definite growth behavior of grain boundary M23C6in tempered Fe–15Mn–3Al-0.7C steel

M23C6carbide can precipitate simultaneously on both sides of austenite grain boundary in Fe–15Mn–3Al-0.7C steel tempered at 600 °C. In this paper, the morphology and growth behavior of M23C6are studied by means of OM, SEM and TEM. The results show that the morphology of M23C6located on both sides of grain boundary is different. On one side of the grain boundary, the carbides appear to be dispersed and disconnected, each with an irregular particle-like morphology, while on the opposite side, they are dispersed along the grain boundary in a short-rod morphology, and the adjacent ones tend to join together to form a network structure. In fact, regardless of which side of the grain boundary the carbides are on, they exhibit a definite growth behavior, growing along the close-packed plane and grain boundary. Also, the morphology of M23C6depends on the angle between the close-packed plane and the grain boundary. In addition, twin structures with the same orientation can be found in both austenite and M23C6. A notable distinction between them is that there is a flat twin plane in the austenite twin structure but not in the carbide twin structure. Therefore, this paper highlights that the formation of these two twin structures is different. The austenite twin structure is caused by quenching treatment, while the carbide twin structure is formed by a new method called “growing-contacting”.