Unexpected formation of delta (δ) phase in as-cast niobium-bearing superalloy at solution annealing temperatures

The annealing of as-cast Inconel 718 superalloy at high solution annealing temperatures for short holding durations was studied. It was found that the needle-shaped delta phase formed from the eutectic constituent, which was followed by the dissolution of this phase by continued annealing. In fact, the formation of delta phase was characterized as an intermediate stage for the dissolution of Laves phase and back-diffusion of the segregated elements from the interdendritic regions. Thermodynamics analysis revealed that the upper temperature limit for the formation of delta phase is dependent on the Nb content. During annealing, the Laves phase started to dissolve, which resulted in the rejection of Nb to the interlamellar austenite, which was characterized as the primary Nb source for the observed local increase in the formation temperature of delta phase (C) 2019 Elsevier B.V. All rights reserved.