Phase equilibria on Ni–Al interface under low oxygen pressure

Seventeen phases of the Ni–Al–O system at high temperatures were analyzed using thermodynamic calculations. An Ni–Al–O isothermal stability diagram was obtained from the thermochemical data. The diagram describes the interface equations for Ni/Al intermetallic compounds, Al/Al2O3, and Al2O3/AlXNiY compounds, and their corresponding regions. Four univariant equilibria points and ten bivariant equilibria lines below 1126 K were obtained. The equations for the coexistence points and interface lines were also obtained. A three-domain diagram of Ni–Al–O phase arrangement at temperatures between 900 and 1191 K is shown. Thermodynamic calculations confirmed that the formation of nickel aluminate spinel (NiAl2O4) requires a threshold NiO activity (log aNiO = −205.3/T − 0.347) and the partial pressure of oxygen (log PO2=−24622/T+8 atm).(log PO2=−24622/T+8 atm). In the Ni–Al–O system, aNiO < 0.266 at 900 K, the compounds in the Ni/Al interface are formed in the order Al3Ni(s) → Al3Ni2(s) → AlNi(s) → AlNi3(s) → Al2O3(α). When aNiO < 0.351 at 1911 K, the compounds in the Ni/Al interface are formed in the order AlNi(s) → Al2O3(α).