Discontinuous Precipitation of γ′ Phase in Ni-Co-Al Alloys

Abstract

Decomposition of nickel-base alloys by precipitation of the γ′ phase occurs either continuously (homogeneously) or discontinuously. Under certain conditions of solute content and temperature, discontinuous precipitation is observed. Ni-Al-Co alloys have been characterized by coupling atom probe tomography with transmission electron microscopy studies. The primary focus was to investigate the discontinuous precipitation of γ and γ′ phases. When subjected to fast quenching after solution treatment, the γ′ precipitates exhibit a near-spherical shape and monomodal size distribution with an average size of less than 5 nm. After early stage annealing at 600°C for 10 min, discontinuous precipitation nucleated near the grain boundaries while some regions of homogeneous γ′ precipitates were observed. Discontinuous γ + γ′ product was completely transformed throughout the grain after 600°C/1 h. On long-term annealing (600°C/256 h), coarser γ + γ′ lamellae products replaced the fine discontinuous products that exhibited after 1 h annealing at 600°C. Equilibrium compositions of the γ and γ′ phases were achieved in this coarsening stage. The γ′ phase has an Al content of 25 at.%, which is consistent with the as-quenched condition where the (Ni + Co)3Al stoichiometry is maintained after 600°C/256 h annealing.