Effect of μ precipitation and γ′ evolution on intra-/intergranular strength of Ni-Co-Cr-based superalloys after 800 ℃ thermal exposure

Abstract

The initiation of microscopic defects and the microstructural stability are crucial factors in determining the tensile strength and failure mechanism of Ni-Co-Cr-based superalloys. This study involved the long-term thermal exposure of Ni-Co-Cr-based superalloys at 800 ℃ and their elevated-temperature tensile testing at 750 ℃ to investigate the microstructural and properties evolution. A 100 h-exposure led to the formation of a significant number of μ precipitates rich in refractory elements at the grain boundaries, particularly on the large-sized γ′ precipitate surfaces. The intragranular γ′ precipitates considerably coarsened with an increasing thermal exposure duration while its volume fraction remained relatively constant. Cracks tended to nucleate preferentially at the μ phase interface during high-temperature tensile testing, leading to the breakage of the γ/γ′ precipitates and formation of pores owing to crack propagation. The dislocation motion as a deformation response of the intragranular γ/γ′ precipitates to tensile testing was dominated by a shearing mechanism, with a portion of the intragranular γ′ precipitates deforming through dislocation shearing. The primary failure mechanism during high-temperature tensile testing changed from intergranular to intragranular with an increasing exposure time. Investigating the relationship between the phase evolution and failure mechanism will enhance our understanding of the fracture behavior of Ni-Co-Cr-based superalloy hot-end structural parts.