Oxidation of NiAl-Ag Alloys at 1000 deg C in 0.1 MPa O_2

摘要

Small amounts of silver have been added to the intermetallic compound NiAl with the purpose of improving its mechanical properties. Four ternary NiAI-Ag alloys containing 0.5, 1, 5 and 10 at. pet Ag, denoted as NiAI-0.5Ag, IMiAI-lAg, NiAI-5Ag and NiAi-10Ag, and the Ag-free NiAl have been oxidized at 1000 deg C for 24 h in 0.1 MPa O_2 to study the effect of the presence of silver on the oxidation behavior of NiAl. All the NiAI-Ag alloys are composed of amatrix of beta-NiAI containing a dispersion of isolated particles of a second silver-rich phase. A continuous external layer of Al_2O_3 formed on all the alloys. In addition, the scales formed on NiAI-5Ag contained a thin and discontinuous layer of pure silver located at the alloy/Al_2O_3 interface, while those formed on NiAI-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 osly as a second phase due to its very small solubility in beta-NiAl.