Further investigation on phase relation and microstructures in Ni 3Si–Ni 3Ti–Ni 3Nb pseudo-ternary alloy system

Abstract

The isothermal phase diagram at 1323 K in the Ni 3Si–Ni 3Ti–Ni 3Nb 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) Ni 3 X 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 Ni 3 X phases. The L1 2(Ni 3Si), D0 24(Ni 3Ti) and D0 a(Ni 3Nb) phases were directly equilibrated one another or each other when keeping Ni content 79.5 at.%. On the other hand, the D0 19(Ni 3Ti 0.7Nb 0.3) phase was identified to exist when keeping Ni content 75 at.%. The phase stability and existing region of each GCP Ni 3 X phase identified in the Ni 3Si–Ni 3Ti–Ni 3Nb pseudo-ternary alloy system were discussed, based on the electron concentration ( e/a) and the atomic size factor ( R x / R Ni) of the constituent atoms.