The lattice parameters of γ and γ″ phases and the γ″/γ lattice mismatches in a nickel-base superalloy, a modified NCF 3 type alloy, were investigated by X-ray diffraction. The measurment on the γ″ phase was carried out using γ″ precipitates extracted from the alloy. The relationship between the morphology of the γ″ phase and the lattice parameters of the γ and γ″ phases and the γ″/γ lattice mismatches was discussed in detail.
With increasing aging time, the lattice parameter of the γ phase decreased and those of the γ″ phase increased. The former after a certain aging time reached at a constant value at each aging temperature. The changes in the lattice parameters in various aging conditions suggest a change in the composition of the γ and γ″ phases. The γ″/γ lattice mismatch in the direction normal to the plane of the γ″ plate, i.e. parallel to the c axis was nearly twice greater than that in the plane. The dependence of the morphology of the γ″ phase on the lattice parameters, the γ″/γ lattice mismatches and the axis ratio c₀ /a₀ for the γ″ phase was not clear. The γ″ phase precipitated at 1073K in a square plate-like morphology was incoherent with the matrix in the direction of the c axis, while that precipitated at up to 1033K in a near disc-shaped morphology was maintained to be coherent.