Thermodynamic study on phase stability in nanocrystalline Sm-Co alloy system

To study the phase stability and the phase transformation behavior in the nanocrystalline (NC) stoichiometric alloys, a thermodynamic model has been developed in the present paper. Using the NC Sm-Co alloy as an example, the thermodynamic properties of various phases in the alloy system were evaluated systematically. In particular, the grain-size-dependence of the Gibbs free energy of each alloy phase at different temperatures was provided. Based on the model calculations, the stabilities of different phases in the NC Sm-Co system were analyzed. As distinctly different from the phase stability in the conventional polycrystalline alloys, the Gibbs free energies of some NC phases become positive at the room temperature when the nanograin size is reduced to below a certain critical value, which implies that these phases cannot stably exist at the room temperature. In order to verify the thermodynamic model, the stoichiometric Sm-Co alloy was prepared, and the grain structure and the phase constitution of the alloy were characterized by combining x-ray diffraction and transmission electron microscopy analyses. The experimental findings have confirmed the thermodynamic model predictions for the NC alloy system.