Oxidation of NiAl-Ag Alloys at 1000℃ in 0.1 MPa O2

Small amounts of silver have been added to the intermetallic compound NiAl with the purpose of improving its mechanical properties. Four ternary NiAl-Ag alloys containing 0.5, 1, 5 and 10 at. pct Ag, denoted as NiAl-0.5Ag,NiAl-1Ag, NiAl-5Ag and NiAl-10Ag, and the Ag-free NiAl have been oxidized at 1000℃ for 24 h in 0.1 MPa O2 to study the effect of the presence of silver on the oxidation behavior of NiAl. All the NiAl-Ag alloys are composed of a matrix ofβ-NiAl containing a dispersion of isolated particles of a second silver-rich phase. A continuous external layer of Al2O3 formed on all the alloys. In addition, the scales formed on NiAl-5Ag contained a thin and discontinuous layer of pure silver located at the alloy/Al2O3 interface, while those formed on NiAl-10Ag contained isolated particles as well as discontinuous layers of silver at various locations in the scale extending up to the gas/scale interface. The kinetic curves of all the alloys were generally composed of two main parabolic stages with smaller parabolic rate constants for the final stage. The addition of silver does not significantly affect the oxidation behavior of the NiAl intermetallic compound in all cases, as expected because silver is essentially present only as a second phase due to its very small solubility inβ-NiAl.