The phase transformation of primary MC carbides during degradation is a critical issue in nickel-based superalloys. This study reports a previously long-overlooked composite structure within MC-type carbides. This carbide consists of two distinct regions: a slightly cubic core and an encapsulating outer layer. Atomic-resolution imaging and elemental analysis reveal subtle lattice differences between these regions, attributed to minor compositional variations. Furthermore, different elemental diffusion kinetics in the composite structure during long-term high-temperature aging leads to increased Ti content and reduced lattice parameters in the outer MC region, which may further facilitate the continuous precipitation and growth of the η phase. These findings provide new insights into the structure of MC carbides and their effect on η nucleation in Ni-based superalloys and offer potential strategies for controlling η phase formation through MC carbide regulation.
