Simultaneous internal oxidation and nitridation of Ni-Cr-Al alloys

A series of Ni-Cr-Al alloys was subjected to thermal cycling to 1100 degreesC in air for up to 260 1-hr cycles. All alloys exhibited poor corrosion resistance. Repeated scale spallation led to subsurface alloy depletion in aluminum and, to a lesser extent, chromium. This caused transformation of the prior alloy three-phase structures (alpha -Cr + beta -NiAl + gamma -Ni) to single-phase gamma -nickel solution. Destruction of the external scale allowed gas access To this metal, which was able to dissolve both oxygen and nitrogen. Inward diffusion of the two oxidants led to development of a complex interval-precipitation zone: Al(2)O(3) and Cr(2)O(3) beneath the surface, followed by Al(2)O(3), then AIN, then AlN + Cr(2)N, and finally, AlN atone in the deepest region. This distribution is shown to reflect the relative stabilities of the precipitates and the higher permeability, of nitrogen. Diffusion-controlled kinetics were in effect initially, but mechanical damage to the interval-precipitation zone led to more rapid gas access and approximately linear kinetics in the long term.