Phase equilibria and γ′-Ll2 phase stability in the Ni-rich portion of Ni-Fe-Si and Ni-Fe-Al systems

Stable and metastable phase equiliblia among gamma-FCC, gamma'-Ll(2) Ni3Si and delta-Ni31Si12 phases in the Ni-Fe-Si system and among gamma-FCC, gamma'-Ll(2) Ni3Al and beta-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 degrees C. On the other hand, the gamma/gamma' 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 gamma' phase coherently precipitates in the gamma matrix due to aging even in the specimens with a high Fe composition over 60 at.%, and that both the gamma'-Ni3Si or gamma'-Ni3Al phase fields extend not only toward the Ni3Fe portion but also toward the Fe3Si or Fe3Al portion, forming metastable (Ni,Fe)(3)Si or (Ni,Fe)(3)Al intermetallic compound with the Ll(2) structure. (c) 2004 Elsevier Ltd. All rights reserved.