Origin of the morphological change from rafted structure to irregular shape of the γ′ phase in single crystal nickel-based superalloys

Microstructural evolution in single crystal Ni-based superalloys is investigated by the phase field simulation. During creep of the alloys, the morphology of the γ′ phase changes from the cuboidal shape to the rafted one and the rafted structure is collapsed in the later stage of creep. It is understood that rafting is caused by the strain energy resulting from γ/γ′ coherent interface. In this study, we focused on the decrease of interfacial coherency in creep condition and investigated the dependence of morphological evolution on it. Creep strain, misfit strain and gradient energy coefficient are changed with time in a two dimensional (2D) phase field simulation to express the γ/γ′ interfacial transition from coherent to semi-coherent in creep condition. The semi-coherent interface model recreates the two types of microstructural evolution of the rafted structure to collapse observed in CMSX-4 and NKH71 superalloys. Furthermore, creep strain rate, instantaneous strain and time of starting the interfacial transition from coherent state to semi-coherent state decide substantially the morphology in the later stage of creep. These morphological evolutions can be explained by the relationship between the amount of creep strain and degree of coherency.