Grain-boundary precipitation in Allvac 718Plus

Allvac 718Plus is a newly developed nickel-based superalloy designed to replace Inconel 718 in static and rotating applications in gas turbine engines. Fine γ′ precipitates act as its principal strengthening phase and a second plate-like phase is generally present at grain boundaries. This has been reported to be the δ phase (Ni3Nb, D0a, orthorhombic), and is used to pin grain boundaries and improve resistance to intergranular fracture. In Allvac 718Plus non-uniform precipitation of a δ-like phase was observed along the grain boundaries; however, no relation was found between grain-boundary misorientation and the occurrence of precipitation. The crystal structure and chemistry of this phase was found to be different from the orthorhombic Ni3Nb δ phase reported previously in Inconel 718 and Allvac 718Plus. The true structure was found to be consistent with the hexagonal η-Ni3Ti D024 structure, but its chemistry was close to Ni6AlNb with partial ordering of Al and Nb over the prototype Ti sites. It was found that the serrated boundaries observed in the alloy were a result of discontinuous precipitation of η-Ni6AlNb, which was a predominant precipitation mechanism throughout the microstructure.