Nanoscale thermodynamic study on phase transformation in the nanocrystalline Sm2Co17 alloy

The characteristics of phase transformation in nanocrystalline alloys were studied both theoretically and experimentally from the viewpoint of thermodynamics. With a developed thermodynamic model, the dependence of phase stability and phase transformation tendency oil the temperature and the nanogram size were calculated for the nanocrystalline Sm2Co17 alloy. It is thermodynamically predicted that the critical grain size for the phase transformation between hexagonal and rhombohedral nanocrystalline Sm2Co17 phases increases with increasing temperature. When the grain size is reduced to below 30 nm, the hexagonal Sm2Co17 phase can stay stable at room temperature, which is a stable phase only at temperatures above 1520 K in the conventional polycrystalline alloys. A series of experiments were performed to investigate the correlation between the phase constitution and the grain structure in the nanocrystalline Sm2Co17 alloy with different grain size levels. The experimental results agree well with the thermodynamic predictions of the grain-size dependence of the room-temperature phase stability. It is proposed that at a given temperature the thermodynamic properties, as well as the phase stability and phase transformation behavior of the nanocrystalline alloys, are modulated by the variation of nanogram size, i.e. the grain size effects oil the structure and energy state of the nanogram boundaries.