During nitriding of nanocrystalline iron and reduction of iron nitrides with NH/H mixtures, at the stationary states a hysteresis phenomenon is observed where two phases coexist, according to the extended phase rule of Gibbs due to the presence of an additional degree of freedom associated with the size of the nanocrystallites. Nanocrystallites transform from the biggest ones to the smallest due to differences in energy demand.

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On the basis of the results of experiments concerning nanocrystalline iron nitriding with ammonia−hydrogen mixtures, it was found that α-iron crystallites transform to iron nitride γ′-Fe 4 N in their whole volume in the order of their size, from the largest to the smallest ones. During the phase transition, the saturated solid solution of nitrogen in α-iron, being in the chemical equilibrium state, changes into an unsaturated solid solution of nitrogen in γ-iron, being in the transition state, and then to γ′-nitride, being again in the chemical equilibrium state. The phase transition is an adiabatic and isothermal process, and thus the change in Gibbs energy of the system is zero. On that basis the energy balance of the system was performed in this paper. The Gibbs energy required for phase transition was supplied to the iron nanocrystallite by the nitriding potential of the ammonia− hydrogen mixture. This energy depends on the ratio of the nanocrystallite active surface to its volume ratio and is the lowest for the largest nanocrystallites. Therefore, large crystallites have to accumulate the least energy, in comparison with the smaller ones. This explains the phenomenon that nanocrystallites undergo the phase transition in a specific order.

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