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 Ni ₃Si–Ni ₃Ti–Ni ₃ Nb 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, L1 ₂(Ni ₃Si), D0 ₂₄(Ni ₃Ti), D0a(Ni ₃Nb) and D0 ₁₉(Ni ₃Ti _0.7Nb _0.3 ) were identified and then their phase fields were shown to depend upon the electron concentration ( e/a) and the atomic size factor ( R _x /R _Ni ) of constituent atoms. Among four intermetallic phases identified, four kinds of two-phase relations (i.e. L1 ₂–D0 ₂₄, D0 ₂₄–D0 ₁₉, D0 ₁₉–D0a and D0 ₁₉–L1 ₂) and two kinds of three-phase relations (i.e. L1 ₂–D0 ₂₄–D0 ₁₉ and L1 ₂–D0 ₁₉–D0a) were found to exist. In low Ti concentration region, a four-phase microstructure consisting of L1 ₂, D8a(Ni ₁₆Si ₇(Ti,Nb) ₆), D0a and Al(Ni solid solution) was observed to exist. The L1 ₂ 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.