Abstract
The lattice constants of γ and γ"phases and the composition of γ"phase in a nickel-base superalloy, a modified NCF 3 type alloy (X-750M), are investigated by using an X-ray diffractometer and an analytical electron microscope with an energy dispersive X-ray (EDX) system. The measurement on the γ"phase is carried out using γ"precipitates extracted from the aged alloy. The relationship between the lattice constants and the composition of the γ"precipitates is discussed in detail. The lattice constant of the γ phase decreases and those of the γ" phase increase with aging time. The change in the lattice constants in various aging conditions suggests a change in the composition of the γ and γ" phases. The γ"phase is composed of nickel, niobium, chromium and iron. While the content of nickel in the γ"phase increases with increasing aging time and reaches at a constant composition, 72mol%, after 72ks aging at 993K, the contents of chromium and iron decrease with increasing aging time and reach at constant values, 6.5mol% and 3mol%, respectively, after 72ks aging. The content of niobium maintains constant value of 18mol% regardless of aging time. A non-stoichiometric composition of γ"phase suggests that the normal sites of nickel and niobium atoms are occupied by chromium and/or iron atoms. If nickel and niobium atoms occupy normal sites regardless of the sites of chromium and iron atoms, it is difficult to explain the change in lattice constants of the γ" phase with aging time. If a part of nickel (including chromium and iron) and niobium atoms can be replaced each other site, it can be explained well.
