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

• Cracking in René 41 controlled by composition of interfaces around grain boundaries. • In-situ tensile tests: Slip band formation, interface decohesion & cracking onset. • Crystal plasticity modelling: No correlation of equivalent strains & stresses and cracking. • No phase transformation or crystal defect structures at particle–matrix interfaces. • Grain boundary-γ’ encapsulating M 2 B preserve high interfacial B excess & cohesion. 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 M 6 C, M 23 C 6 , and M 2 B 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-M 2 B, M 6 C, or M 23 C 6 . Atom probe microscopy reveals a correlation between low interfacial excess of B and Mo and severe decohesion at γ-matrix / GB-M 2 B interfaces. In contrast, GB microstructures with GB-γ’ encapsulating GB-M 2 B 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.