Phase relation and microstructure in multi-phase intermetallic alloys based on Ni3Si–Ni3Ti–Ni3Nb pseudo-ternary alloy system

The phase relation and microstructure of alloys based on Ni3Si–Ni3Ti–Ni3Nb pseudo-ternary alloy system at 1323 K were investigated by optical microscopy, X-ray diffraction and scanning electron microscopy (attached with a wavelength dispersive spectroscope). In high Ti concentration region, L12(Ni3Si), 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, four kinds of two-phase relations (i.e. L12–D024, D024–D019, D019–D0a and D019–L12) and two kinds of three-phase relations (i.e. L12–D024–D019 and L12–D019–D0a) were found to exist. In low Ti concentration region, a four-phase microstructure consisting of L12, D8a(Ni16Si7(Ti,Nb)6), D0a and Al(Ni solid solution) was observed to exist. The L12 phase cannot be directly equilibrated with the D0a phase. The prepared alloys exhibited widely different microstructures, depending on alloy compositions whether they exist in a single phase, a two-phase or a three-phase region, and also on constituent intermetallic phases.