Evolution of grain boundary carbides in thermal exposed Ti-modified alloy N

The evolution of grain boundary MC carbides during the thermal exposure has been investigated in the Ti-modified Hastelloy N alloy. MC carbides precipitate at the grain boundaries after exposed at 650 °C and 750 °C for 500 hours (h). The morphology of MC carbides at the grain boundaries was characterized as separated particles with a cube-on-cube orientation relationship with the matrix. The MC/Matrix interface exhibits a semi-coherent character, on which misfit dislocations are repeated in every sixth layer of (111)MC. With the further thermal exposure, the size of MC carbides remain stable at 650 °C. By contrast, M2C carbides undergo the initial coarsening and subsequent re-dissolution at 750 °C. At the same time, the deviation angles from the cube-cube orientation relationship (OR) between MC carbides and the matrix become larger at both 650 °C and 750 °C, which indicate the semi-coherent interfaces are transforming to the incoherent ones.