Microstructural characteristics of the microphase Y-Ti₂SC in nickel-based superalloys

The microstructural characteristics of microphase Y-Ti₂SC in Ni-based superalloys have been studied by means of analytical transmission electron microscopy (TEM). In the standard heat treated samples, the Y phase always exhibits slab morphology forming either the sandwiched plate in MC or the side-by-side structure with MC. Moreover, the Y phase also maintains an definite crystallographic orientation relationship of [_12̄10]Y// [_11̄0]MC and (0 0 0 1)_Y//(1 1 1)_MC with the MC-type carbide. Stacking fault which results from the insertion of two Ti-C layers has been identified in the Y-Ti₂SC phase. Combined with high resolution TEM (HRTEM) and first-principles calculations, we also determine the most stable interfacial structure between Y phase and MC carbide. Additionally, we find that Y-Ti₂SC phase can solutionize amounts of beneficial elements Hf and Zr from the matrix during long-term annealing process, which leads to its lattice expansion. This solution phenomenon is detrimental for superalloys due to the consumption of the beneficial elements.