Lattice constants of γ and γ″ phases and γ″/γ lattice mismatches in a Ni-15Cr-8Fe-6Nb alloy

Abstract

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, are investigated by X-ray diffraction. The measurement on the γ″ phase is 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 γ″/γ lattice mismatches is discussed in detail. With increasing aging time, the lattice parameter of the γ phase decreases and those of the γ″ phase increase. The former after a certain aging time reaches 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 habit plane of the γ″ plate, i.e. parallel to the c axis is nearly twice greater than that in the plane. The dependence of the morphology of the y″ phase on the lattice parameters, the γ″/γ lattice mismatches and the axis ratio co/ao for the γ″ phase is not clear. The γ″ phase precipitated at 1 073 K in a square plate-like morphology is incoherent with the matrix in the direction of the c axis, while that precipitated at up to 1 033 K in a near disc-shaped morphology is coherent.