Further investigation on phase relation and microstructures in Ni3Si–Ni3Ti–Ni3Nb pseudo-ternary alloy system

The isothermal phase diagram at 1323 K in the Ni3Si–Ni3Ti–Ni3Nb pseudo-ternary alloy system was re-investigated by scanning electron microscopy (attached with a wavelength dispersive spectroscope), X-ray diffraction and transmission electron microscopy (TEM), focusing on the phase relation among possible geometrically-close-packed (GCP) Ni3X phases. The prepared alloys exhibited widely different microstructures, depending on alloy compositions whether they exist in a single phase, a two- or a three-phase region, and also on the constituent GCP Ni3X phases. The L12(Ni3Si), D024(Ni3Ti) and D0a(Ni3Nb) phases were directly equilibrated one another or each other when keeping Ni content 79.5 at.%. On the other hand, the D019(Ni3Ti0.7Nb0.3) phase was identified to exist when keeping Ni content 75 at.%. The phase stability and existing region of each GCP Ni3X phase identified in the Ni3Si–Ni3Ti–Ni3Nb pseudo-ternary alloy system were discussed, based on the electron concentration (e/a) and the atomic size factor (Rx/RNi) of the constituent atoms.