The phase relation and microstructure of alloys based on Ni 3 Si-Ni 3 Ti-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 3 Si), D0 24 (Ni 3 Ti), D0a(Ni 3 Nb) and D0 19 (Ni 3 Ti 0.7 Nb 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 16 Si 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. #
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