Crystallography and morphology evolution of MC carbides in Ni-12Mo-7Cr-2 Nb based superalloy

Abstract

The crystallography and morphology evolution of MC carbides in Ni-12Mo-7Cr-2 Nb based superalloy have been systematically investigated in this study. After the thermal exposure at 750 °C, the secondary MC carbides can be observed in the matrix in the forms of stacking fault precipitates (SFP) and flake-shaped ones. At the first stage of exposure, the SFP preferentially nucleates around the primary MC carbides, and then spread throughout the matrix. The flake-shaped MC carbides were also formed at the dislocations and pile-ups. After the long-term exposure, the SFP starts to degenerate, while flake-shaped ones still remain and become coarsen. All secondary MC carbides possess the cube-on-cube orientation relationship with the matrix but the different interface structures. For the SFP, the semi-coherent interface is along the {111} planes. In the flake-shaped MC carbides, it is along {001} planes. All secondary MC carbides possess the same chemical constitution, that is approximately (Mo0.66Nb0.24Cr0.1) C. Based on the first-principle calculations, the Mo doping in NbC was prove to decrease the chemical interface energy and favor the nucleation process. The evolution mechanism of the secondary MC carbides was also briefly discussed by exploring the role of crystal defects (stacking faults and dislocations). Our results can help to deepen the understanding of the helium trapping mechanism of MC carbides.