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

Decomposition of nickel-base alloys by precipitation of the gamma' 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 gamma and gamma' phases. When subjected to fast quenching after solution treatment, the gamma' precipitates exhibit a near-spherical shape and monomodal size distribution with an average size of less than 5 nm. After early stage annealing at 600A degrees C for 10 min, discontinuous precipitation nucleated near the grain boundaries while some regions of homogeneous gamma' precipitates were observed. Discontinuous gamma + gamma' product was completely transformed throughout the grain after 600A degrees C/1 h. On long-term annealing (600A degrees C/256 h), coarser gamma + gamma' lamellae products replaced the fine discontinuous products that exhibited after 1 h annealing at 600A degrees C. Equilibrium compositions of the gamma and gamma' phases were achieved in this coarsening stage. The gamma' phase has an Al content of 25 at.%, which is consistent with the as-quenched condition where the (Ni + Co)(3)Al stoichiometry is maintained after 600A degrees C/256 h annealing.