Size-Dependent Transformation of α-Fe into γ′-Fe4N in Nanocrystalline the Fe–NH3–H2 System

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.