Phase equilibria and γ′-L12 phase stability in the Ni-rich portion of Ni-Fe-Si and Ni-Fe-Al systems

Stable and metastable phase equiliblia among γ-FCC, γ′-L12 Ni3Si and δ-Ni31Si12 phases in the Ni-Fe-Si system and among γ-FCC, γ′-L12 Ni3Al and β-B2 NiAl phases in the Ni-Fe-Al system were examined. The stable phase diagrams in the Ni-rich portion were determined by energy dispersion X-ray spectroscopy (EDX) and electron probe microanalysis (EPMA) using specimens fully annealed at temperatures in the interval 700-1300 °C. On the other hand, the γ/γ′ metastable phase equilibria were investigated by the electrical resistivity (ER) measurement technique, transmission electron microscopic (TEM) observation and TEM-EDX analysis. It was confirmed that the metastable γ′ phase coherently precipitates in the γ matrix due to aging even in the specimens with a high Fe composition over 60 at.%, and that both the γ′-Ni3Si or γ′-Ni3Al phase fields extend not only toward the Ni3Fe portion but also toward the Fe3Si or Fe3Al portion, forming metastable (Ni,Fe)3Si or (Ni,Fe)3Al intermetallic compound with the L12 structure.