Abstract. ATI 718Plus R is a polycrystalline Ni-base superalloy for aero-engine disc applications, which relies on the precipitation of η-phase (Ni 6 Nb[Al,Ti], D0 24 ) at grain boundaries for resistance to intergranular failure and grain size control. The nucleation and evolution of the η particles have been examined throughout the manufacturing process. During hot working below the η solvus, existing precipitates lose their orientation relationship with the matrix and become aligned with the forging flow. During heat treatment, a dual morphology develops by the uniform growth of those incoherent precipitates into coarse plates, together with the new nucleation of thin lamellae from selected grain boundaries. The balance between the two morphologies in the final component can be achieved by controlling precipitation and dissolution processes during forging.

Alloy ATI 718Plus® relies upon the co-precipitation of secondary phases n and 5 at the grain boundaries to control grain structure during processing. Abundant precipitation can occur in the billet material during heating time before sub-solvus forging, which persists to a certain extent in the microstructure of the final component and modifies the compression flow stress. As these phases are sensitive to the thermo-mechanical history of the material, any such inhomogeneity in the billet stock will be reflected in the precipitate population. A TTT diagram of second-phase precipitation in 718Plus billet material is presented. The effects of initial grain size and residual strain on the morphology and precipitation kinetics of the precipitates have been investigated by scanning and transmission electron microscopy and EBSD. Plastic deformation was found to determine precipitate morphology, largely through its effect on the degree of coherency at the precipitate/matrix interface. Approaches to modify the precipitate growth mechanism through a deformation process are suggested.

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