Phase relation and microstructure in Ni3Al–Ni3Ti–Ni3Nb pseudo-ternary alloy system

The phase relation and microstructure of alloys based on the Ni3Al–Ni3Ti–Ni3Nb pseudo-ternary alloy system at 1273 K were investigated by an optical microscope, an X-ray diffraction and a scanning electron microscope (attached with a wavelength dispersive spectroscope). As the constituent intermetallic phases, L12 (Ni3Al), D024 (Ni3Ti), D0a (Ni3Nb) and D019 (Ni3Ti0.7Nb0.3) were identified and then their phase fields were shown to depend upon the electron concentration (e/a) and the atomic size factor (Rx/RNi) of constituent atoms. Among four intermetallic phases identified, five kinds of two-phase relations (i.e. L12–D024, D024–D019, D019–D0a, D0a–L12 and D024–D0a) and two kinds of three-phase relations (i.e. L12–D024–D0a and D024–D019–D0a) were found to exist. Also, D024 (Ni3Ti) phase extended up to concentration field in which a majority of constituent Ti elements were replaced by Al and Nb elements, keeping their concentration ratios identical. The prepared alloys exhibited widely different microstructures, depending on whether their alloy compositions exist in a single phase, a two-phase or a three-phase region, and also on what kind of intermetallic phases their alloy compositions are composed of.