On the mechanisms controlling grain boundary cracking in Ni-based superalloy René 41 with boron and carbon additions

Cast & wrought Ni-based superalloys with the highest alloying contents are designed to endure the harshest environments in gas turbine engines. Their high-temperature properties are unlocked by complex microstructures of a γ-matrix, γ’ precipitates, and various grain boundary (GB) (co–)precipitates, typically GB-γ’, carbides, and/or borides. However, their applications are often limited by GB cracking. Micro-alloying additions of B and C are added to improve GB cohesion but may further promote M₆C, M₂₃C₆, and M₂B precipitation in addition to GB-γ’. Three mechanisms known to control GB cracking are mesoscale strains and stresses, the nanoscale structure, and the nanoscale chemical environment of such GB microstructures, but systematic studies are unavailable.

We aim to advance the limited understanding of the onset of GB cracking in the Ni-based superalloy René 41 by investigating these three mechanisms for GB cracking. In-situ tensile testing reveals a sequence of slip band formation, interface decohesion and the onset of GB cracking. Crystal plasticity finite element modeling shows no direct correlation between equivalent strains and stresses and GB cracking susceptibility. No local nanoscale phase transformations and/or formation of defect structures are detected across interfaces between the γ-matrix and GB-M₂B, M₆C, or M₂₃C₆. Atom probe microscopy reveals a correlation between low interfacial excess of B and Mo and severe decohesion at γ-matrix / GB-M₂B interfaces. In contrast, GB microstructures with GB-γ’ encapsulating GB-M₂B preserve a high interfacial excess of B and, thus, GB cohesion. A microstructural model summarizes the GB microstructure – property relationship applicable to various similar Ni-based superalloys.