Miscibility gap of B2 phase in NiAl to Cu3Al section of the Cu-Al-Ni system

The phase separation in the bee phase of the Cu-Al-Ni system at 600-700 degrees C was investigated mainly by energy dispersion X-ray spectrometry (EDS) and differential scanning, calorimetry (DSC). The compositions of the beta(1) (A2 or B2: Cu-rich), beta(2) (B2: NiAl-rich) and gamma (gamma-brass type) phases in equilibrium were determined. It was found that there is a beta(1) + beta(2) miscibility gap in the beta phase region as previously reported by Alexander. It was confirmed by means of high temperature in situ TEM observation that this miscibility gap consists of the B2+B2 phases but not the A2+B2 phases which is sometimes observed in many other Ni-Al and Co-Al base ternary bee alloys. Thermodynamic calculation was performed which indicates that this characteristic feature suggests that the beta(1) (B2) + beta(2) (B2) miscibility gap is a part of a Cu-rich B2 + NiAl-rich B2 miscibility gap island formed around the center of the composition triangle of the isothermal section. The phase separation in the beta phase region and the stability of the ordered bcc alummide are presented and discussed. (c) 2004 Elsevier Ltd. All rights reserved.