CYCLIC OXIDATION BEHAVIOR OF 2-PHASE NI-CR-AL ALLOYS AT 1100-DEGREES-C

The cyclic oxidation behaviour in static air of four Ni-Cr-Al alloys of different chromium contents was studied at 1100-degrees-C. All of the alloys were two-phase, consisting of nickel-rich beta-NiAl and alpha-Cr. The volume fractions of the two phases were different in each of the alloys, although their compositions were kept relatively constant. All of the alloys exhibited relatively large weight gains and formed an external scale containing Al2O3. The large weight gains were attributed to extensive internal Al2O3 formation which was in turn associated with scale spallation. The extent of internal attack was greatest for alloys containing up to 30 at.% Cr. In these alloys, a destabilization of the original alpha-Cr + beta-NiAl equilibrium to gamma-Ni + beta-NiAl occurred as a result of the selective oxidation of aluminium. For an alloy containing 40 at. % Cr, the selective oxidation of aluminium resulted in a destabilization of the alpha-Cr + beta-NiAl equilibrium to alpha-Cr + gamma-Ni. Such a destabilized structure was more resistant to internal Al2O3 formation.