Phase relation and microstructure in multi-phase intermetallic alloys based on Ni3Al–Ni3Ti–Ni3V pseudo-ternary alloy system

Abstract

The phase relation and microstructure of alloys based on the Ni3Al–Ni3Ti–Ni3V pseudo-ternary alloy system at 1273 and 1373 K were investigated by optical microscopy, X-ray diffraction, differential scanning calorimetry and scanning electron microscopy (attached with a wavelength dispersive spectroscope). As the constituent intermetallic phases at 1273 K, L12(Ni3Al), D024(Ni3Ti), D022(Ni3V) and rhombohedral (Ni3Ti0.7V0.3) 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 the constituent atoms. Among four intermetallic phases identified at 1273 K, five kinds of two-phase relations (i.e. L12–D024, D024–rhombohedral, rhombohedral–D022 D022–D024 and D022–L12) and two kinds of three-phase relations (i.e. L12–D024–D022 and D024–rhombohedral–D022) 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 V elements, keeping their concentrations identical. The prepared alloys exhibited widely different microstructures, depending on alloy composition and temperature. In the low Ti concentration region, dual multi-phase microstructures composed of L12+D024+A1(→L12+D022) were formed by a eutectic reaction.