Development of microstructure in isothermally forged Nimonic alloy AP1

The development of microstructure in the nickel alloy Nimonic AP1 owing to isothermal forging and subsequent heat treatment has been investigated. The microstructure initially consisted of coarse grains, which corresponded to the prior powder particles; these were surrounded by a coarse dispersion of γ′. On deformation, the volume fraction of material with a coarse γ′ dispersion increased, and this was associated with localised grain refinement following heat treatment. High volume fractions of this structural type gave some increase in tensile strength but reduced stress rupture life and increased fatigue crack growth rates. High temperature compression tests were used to develop a constitutive equation for the material which was then applied, via a finite element model, to simulate the isothermal forging process. The relationships between deformation and structural development, the modelling, and the mechanical properties are discussed. The approach is potentially useful for the design of forging operations for turbine disc production.