It was found that, upon annealing non fully-nitrided powder in an Ar atmosphere, some of the N atoms (which had weak interactions with interstitial sites) could be excited from the interstitial sites of Fe17Sm2 molecules by thermal energy, and migrate again. However, some of the N atoms were still located in the interstitial sites of the Fe17Sm2 molecules, due to strong interactions between the gas atoms and the interstitial sites. During the N diffusion process, the diffusion force for N atoms decreased with increasing occupation of the interstitial sites by N atoms, due to changes in the chemical potential of the system. This resulted in differing diffusive activities of the N atoms for various N absorption contents in Fe17Sm2Nx. Surface and grain boundary diffusion played important roles in the migration of N atoms into small Fe17Sm2 particles.

J.Hu, T.Zhao, H.Guo, B.Shen, Z.Wang, G.Liu, K.Wang, Y.Wang, B.Hu: Journal of Alloys and Compounds, 1995, 222, 113-6