Interrelationships Of Defects, Nitride Modification And Excess Nitrogen In Nitrided Fe-4.75 At.% Al Alloy

In order to investigate the influence of crystal-lattice defects (dislocations) and of a relatively large Al content on the development of different modifications of aluminium nitride in ferrite and on the associated uptake of excess nitrogen, recrystallised and cold rolled specimens of Fe-4.75 at.% Al alloy were subjected to gaseous nitriding treatments at 550 degrees C employing a nitriding potential of 0.104 atm(-1/2). In contrast with earlier results for an alloy of relatively low Al content, in both the nitrided cold rolled and the nitrided recrystallised specimens nanosized AlN precipitates developed, both of the metastable, cubic, NaCl type modification and of the stable, hexagonal, wurtzite type modification, which exhibit with the ferrite matrix a Baker-Nutting orientation relationship and a Pitsch-Schrader orientation relationship, respectively. A high-temperature hydrogen reduction treatment confirmed the stoichiometry of the precipitated nitrides as AlN. The fraction of cubic, NaCl type, AlN is significantly higher in the nitrided cold-rolled specimens because of its preferential development along dislocations. As compared to the nitrided recrystallised specimens, the higher dislocation density and the higher amount of cubic AlN in the nitrided cold-rolled specimens leads to a larger amount of excess nitrogen in the nitrided cold-rolled specimen.