The effect of pre-aging on the creep behavior of Ni-12Mo-7Cr-2Nb superalloy

Abstract

The effects of pre-aging treatment on the creep fracture behavior of Ni-12Mo-7Cr-2Nb alloy were systematically investigated. Pre-aging treatment can introduce stacking fault precipitates (SFPs) in the matrix, which contain nanoscale MC carbides on the extrinsic stacking faults. At the same time, the dense MC carbides appear at grain boundaries. The creep test results showed that pre-aging treatment can improve the rupture elongation by 2–3 times as compared with the solution-treated specimens and change the fracture mode from intergranular to transgranular. Microstructural observations indicated that such variation of creep performance is closely linked to the distribution and morphology of secondary MC carbides. During creep tests of solution-treated specimens, needle-like MC carbides and a few SFPs form in the matrix, and discrete MC carbides form at grain boundaries. Such microstructures tend to enhance strain concentration and crack development at grain boundaries. In contrast, in pre-aged specimens, dense intergranular MC carbides promote local dynamic recrystallization, which can effectively alleviate the strain concentration at grain boundaries and shift the fracture position from grain boundaries to SFPs-matrix interfaces. The merging of SFPs-related cracks across the matrix is more difficult than the propagation of intergranular ones, which contributes to the better rupture elongation of pre-aged specimens. This study provides an effective path to enhance the ductility of Ni-12Mo-7Cr-2Nb alloy at high temperature, which is expected to overcome irradiation embrittlement problem in molten salt reactor applications.