LATTICE-CONSTANTS OF GAMMA-PHASE AND GAMMA''-PHASE AND GAMMA'' GAMMA-LATTICE MISMATCHES IN A NI-15-CR-8-FE-6-NB ALLOY/

The lattice parameters of gamma and gamma '' phases and the gamma ''/g amma lattice mismatches in a nickel-base superalloy, a modified NCF 3 type alloy, are investigated by X-ray diffraction. The measurement on the gamma '' phase is carried out using gamma '' precipitates extracte d from the alloy. The relationship between the morphology of the gamma '' phase and the lattice parameters of the gamma and gamma '' phases and gamma ''/gamma lattice mismatches is discussed in detail. With inc reasing aging time, the lattice parameter of the gamma phase decreases and those of the gamma '' 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 gamma and gamma '' phases. The gamm a ''/gamma lattice mismatch in the direction normal to the habit plane of the gamma '' plate, i.e. parallel to the c axis is nearly twice gr eater than that in the plane. The dependence of the morphology of the gamma '' phase on the lattice parameters, the gamma ''/gamma lattice m ismatches and the axis ratio c(0)/a(0) for the gamma '' phase is not c lear. The gamma '' phase precipitated at 1073 K in a square plate-like morphology is incoherent with the matrix in the direction of the c ax is, while that precipitated at up to 1033 K in a near disc-shaped morp hology is coherent.