LATTICE-CONSTANTS AND COMPOSITIONS OF THE METASTABLE NI3NB PHASE PRECIPITATED IN A NI-15CR-8FE-6NB ALLOY

The lattice constants of gamma and metastable gamma '' (Ni3Nb) phases and the composition of the gamma '' phase in a nickel-based superalloy (a modified JIS NCF 3-type alloy) are investigated using an X-ray dif fractometer and an analytical electron microscope with an energy-dispe rsive X-ray (EDX) system. The lattice constant of the gamma phase decr eases and those of the gamma '' phase increase during the aging time. The gamma '' phase is composed of nickel, niobium, chromium, and iron. While the content of nickel in the gamma '' phase increases and reach es a constant composition during the aging time, the chromium and iron content decrease and reach a constant value during aging. The niobium content maintains a constant value regardless of the time that the al loy has aged. The relationship between the lattice constants and the c omposition of the gamma '' phase is discussed in detail. In the early stages of aging, there is the possibility that nickel (including chrom ium and iron) and niobium atoms mutually substitute for one another, r esulting in a somewhat lower degree of order than would normally be ex pected.