Microstructure and mechanical properties of novel Ni-20Co-12Cr superalloys, with and without Nb addition, were systematically studied during long-term thermal exposure. With increased exposure time, the average diameter of the γ' precipitates increased in both alloys in succession; this is more obviously observed in alloy containing 1 wt% Nb (1Nb). It is suggested that Nb increased the γ' coarsening rate by accelerating the diffusion of Al and Nb in γ matrix. In addition, the γ' phase fraction is increased by about 4% in 1Nb compared to the alloy without Nb (0Nb). The morphology of the γ' phase changed from near-spherical to cuboidal shape during exposure in both alloys. Due to the increased γ/γ' lattice misfit by Nb addition, 1Nb alloy showed an earlier tendency of shape change. Vickers hardness results revealed that the overall hardness decreased with the exposure time because the size increment of the γ' precipitate weakened the precipitates strengthening and Orowan strengthening.
Keywords: Nb; Vickers hardness; strengthening mechanisms; superalloy; thermal exposure; γ′ coarsening.