Enhanced crack propagation resistance after healing treatment by addition of boron in FeNi-base superalloy

The objective of the present study was to investigate whether the crack propagation resistance of boron-containing FeNi-based superalloy could be enhanced by healing treatment. The healing treatment was performed by pre-straining and subsequent heat treatment, resulting in a pronounced boron segregation zone at the interface. Formation of boron segregation zone reinforced the crack propagation path, thereby effectively enhancing the mechanical properties of FeNi-based superalloy. The tested alloys were prepared by hot-rolling followed by aging treatment, resulting in precipitation of γ'-Ni3Ti and γ''-Ni3Nb embedded in the γ matrix. After pre-straining and healing heat treatment, η-Ni3Ti phase formed at the grain boundary, leading to the formation of a boron segregation zone at the interface between the η-Ni3Ti phase and γ matrix. Both strength and ductility were improved with increasing boron content from 0 at% to 0.2 at% after healing treatment. The spontaneous segregation of boron at the precipitate-matrix interface played a role in enhancing both strength and ductility by interface strengthening. Moreover, it was revealed that a critical amount of boron segregation is required for interface strengthening. Dislocation density analysis revealed that the recovery effect was weaker in alloys with boron than in those without it, leading to a more pronounced work-hardening effect.