Influence of interrupted ageing on the temporal evolution of the γ′ size distribution and the co-precipitation of γ″ in alloy 718Plus

The influence of interrupted ageing on the temporal evolution of γ′ distribution in alloy 718Plus has been examined. Various bimodal γ′ distributions were generated as a function of ageing duration, and the peak-aged distribution was identified through hardness tests. The volume fractions of the large and fine γ′ precipitates evolved into a near 50:50% uniform distribution. The γ″ precipitation succeeded the γ′ precipitation during low-temperature ageing. The nucleation of the γ″ on γ′ is explained on the basis of the Al + Ti to Nb ratio approaching the critical value in the matrix during the ageing treatment. The preferential nucleation site for the co-precipitation of γ″ is explained on the basis of lattice misfits of the γ, the large and the fine γ′, and the γ″ phases.