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.