The emergence and disappearance of a high density of microcracks in nitrided Fe-4.65 at. % Al alloy

Abstract

Nitriding of recrystallised and cold-rolled Fe-4.65 at. % Al alloy at 550°C in an ammonia-hydrogen gas mixture of nitriding potential 0.104 atm-1/2 indicated that nucleation of the thermodynamically (most) stable hexagonal (wurzite) AlN in the ferrite matrix of recrystallised specimen is difficult. The competition between the precipitation of nitrogen gas (dissociation of nitrogen supersaturated ferrite into pure ferrite and nitrogen gas) at the grain boundaries and the precipitation of hexagonal (wurzite) AlN led, via the coalescence of pores filled with nitrogen gas, to the emergence of microcracks along the grain boundaries and the formation of nitride-precipitate free zones near the grain boundaries of the recrystallised specimens. Upon continued nitriding the initially partially nitrided grains became fully nitrided and the microcracks disappeared. © Carl Hanser Verlag GmbH & Co. KG.