Effect of long-time exposure on the evolution of minor phases in Alloy 718

The effects of exposure to 25,000 h on the microstructural stability of Alloy 718 have been examined at 538, 621, 677 and 732 °C, with emphasis on the evolution of minor phases. Significant structural changes occurred as the material was aged beyond 621 °C. In addition to coarsening of the principal phases γ′, γ″ and δ, nucleation and growth of the equilibrium δ-Ni3Nb phase results in precipitation of minor constituents, α-Cr, a high Cr–Mo Laves phase with Si, and a Cr–Ni–Fe σ phase, out of the retained solute. Though all the minor constituents are present at all the temperatures, each of the higher temperatures relate to one predominant minor phase as its signature.