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

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