Morphology Evolution of γ′ Precipitates for Wrought Ni-Based Superalloys

The morphology evolution of gamma' precipitates during isothermal aging at 1173K is investigated for wrought Ni-based superalloys with various combinations of lattice misfit (vertical bar delta vertical bar) and gamma' volume fraction (f(v)) (i.e., Inconel X-750, Alloy 80A, Udimet 520, and Udimet 720Li). The resulting morphology evolution strongly depends on vertical bar delta vertical bar, which was classified into three types. The spherical morphology for vertical bar delta vertical bar < 0.10% remains unchanged with the increasing diameter of the gamma' precipitates (d) (Type A). In the case of 0.10 <= vertical bar delta vertical bar < 0.30%, the gamma' morphology evolves from spherical to cuboidal with a maintained coherency at the gamma/gamma' interface with an increasing d, accompanied by the alignment of the cuboidal gamma' precipitates (Type B). The morphology evolution of the gamma' precipitates when vertical bar delta vertical bar >= 0.30% is from spherical to cuboidal and finally to globular. The coherent gamma/gamma' interface turns to a semi-coherent one with the coalescence of the gamma' precipitates (Type C). The alignment of the cuboidal gamma' precipitates is promoted for superalloys with a larger f(v) value, whereas the effect of f(v) on the morphology evolution of the gamma' precipitates during aging treatment is quite limited.