Size effect on the Fe nanocrystalline phase transformation Original Research Article

A thermodynamics for the phase transformation from γ(fcc) to α(bcc) in nanocrystalline (NC) Fe is considered. Gibbs free energies of the interfaces in NC γ- and α-Fe particles were calculated, respectively, by means of a quasiharmonic Debye approximation, yielding a larger increase in the total Gibbs free energy of α-Fe than that of γ-Fe. This is attributed to the difference in their interfacial energies. As a result, the fcc NC Fe can be thermodynamically stable at room temperature when the grain size is sufficiently small. Taking into account the thermodynamic equilibrium condition, the critical grain size for the γ-Fe phase to exist in stable form at 300 K was quantitatively calculated for different excess volumes ΔV, a parameter describing the state of interface based on a dilated crystal model. The assumptions made in the present model and the factors influencing the critical grain size are discussed.