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

The phase separation in the bcc phase of the Cu-Al-Ni system at 600-700 °C was investigated mainly by energy dispersion X-ray spectrometry (EDS) and differential scanning calorimetry (DSC). The compositions of the β1 (A2 or B2: Cu-rich), β2 (B2: NiAl-rich) and γ (γ-brass type) phases in equilibrium were determined. It was found that there is a β1+β2 miscibility gap in the β 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 bcc alloys. Thermodynamic calculation was performed which indicates that this characteristic feature suggests that the β1 (B2)+β2 (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 β phase region and the stability of the ordered bcc aluminide are presented and discussed.