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The thermodynamics of allotropic transformation in nanocrystalline Fe is studied. For this purpose, the grain boundary structure is initially examined using a novel method based on molecular dynamics (MD) simulation, the results of which are exploited to determine grain boundary thickness and excess volume for different grain sizes. The dilated crystal model and the quasi-harmonic Debye approximation method are subsequently employed to calculate the values of Gibbs free energy for grain boundaries of different allotropic phases. Using the quantitative parameters obtained from the simulations, the relevant Gibbs free energy curves are plotted in terms of grain size and analyzed for different temperatures. This is followed by deriving stability curves for the Î³-Fe and Î±-Fe phases in terms of grain size for different temperatures. Ultimately, the parameters obtained from MD simulation studies and the stability diagrams thus obtained are used to explore the changes in the Î³âÎ± allotropic transformation temperature of iron nanocrystal for different grain sizes. It will be that allotropic transformation temperature declines from 900 to 300 K with rising grain size from 4 to 33 nm.
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