Effects of Cr content on the microstructure and stress rupture property of a directionally solidified Ni-based superalloy during long-term thermal exposure

The effects of Cr addition on the microstructure and stress rupture property during long-term thermal exposure have been investigated in a directionally solidified Ni-based superalloy. The increment of Cr content resulting in the excessive precipitation of σ phase is the principal reason for the catastrophic reduction in the stress rupture properties. The amounts of rounded cuboidal γ′ and its coalescence increase, but the tendency of γ′ alignment decreases with the increment of Cr content, which leads to the reduction of the stress rupture property to some degree. The change of lattice misfit shows a good agreement with the change of γ′ morphology and the tendency of γ′ alignment. The change of γ′ shape is accompanied by the increase of the misfit dislocation density, which causes the rise of γ′ coalescence. The increase of Cr content can promote the formation of M23C6 or M6C in the vicinity of degenerating primary MC. Thus, the increased Cr content facilitates the decomposition of primary MC, which deteriorates the stress rupture properties. In addition, the increase of Cr content can promote the formation M23C6 and M6C along gain boundaries (GBs), facilitating the formation of a semicontinuous carbide chain to some degree and broadens GBs and thus promoting the crack nucleation at GBs, which plays a certain role in the degradation of the stress rupture properties.