Phase transformations and phase equilibria in the Fe-N system at temperatures below 573 K

The phase transformations of homogeneous Fe-N alloys of nitrogen contents f rom 10 to 26 at. pet were investigated by means of X-ray diffraction analys is upon aging in the temperature range from 373 to 473 K, It was found that precipitation of alpha " -Fe16N2 below 443 K does not only occur upon agin g of supersaturated alpha (ferrite) and alpha' (martensite), but also upon transformation of gamma'-Fe4N1-z and epsilon -Fe2N1-x (<20 at. pct N). No < alpha>" was observed to develop upon aging of gamma (austenite). Therefore, it is proposed that gamma' is a stable phase at temperatures down to (at l east) 373 K, Phase formation upon annealing at low temperatures is apparent ly governed by the (difficult) nucleation and (slow) growth of new Fe-N pha ses: alpha " forms as a precursor for a because of slow nitrogen diffusion, and nitrogen-enriched epsilon develops as a precursor for gamma' because o f a nucleation barrier.