MULTIPHASE EQUILIBRIUM, PHASE STABILITY AND PHASE TRANSFORMATION IN NANOCRYSTALLINE ALLOY SYSTEMS

摘要

A nanoscale thermodynamic approach is developed to quantify the multiphase equilibrium in nanocrystalline alloy systems. The nanoscale multiphase equilibrium calculations indicate that the activities and mole numbers of phases in alloy systems change diversely as a function of the nanograin size at constant temperature, which results in distinctly different characteristics of phase stability and phase transformation behavior in nanocrystalline alloys with respect to coarse-grained polycrystalline counterparts. Take the nanocrystalline Sm—Co alloy system as an example, the diverse phase stabilities and phase transformations are demonstrated. To verify the thermodynamic predictions, a series of experiments on preparation and characterization of nanocrystalline SmCo_3, Sm_2Co_7 and SmCo_2 alloys have been performed. The experimental results are in agreement with the model calculations, which validate the present thermodynamic approach for nanoscale multiphase equilibrium.