Thermal stability of nanocrystalline materials

The thermal stability, i.e. grain growth, of nanocrystalline (nc) materials is addressed in this paper. Methods for experimentally measuring nc grain size and their limitations are presented. The grain growth exponents n from D-1/n - D-0(1/n) = kt for nc and conventional polycrystalline (D > 1 mu m) materials are compared. Grain growth data in nc Fe are analyzed by the above equation and one which takes pinning forces into account. The n vs T/T-M data for nc Fe appear to be an extension of data for large grain size (> 30 mu m) zone-refined Fe to lower T/T-M regime. An activation energy for grain growth in nc Fe was determined to be 178 kJ/mol compared to 174 kJ/mol for grain boundary self diffusion in Fe.