Microstructure and phase evolution in nanocrystalline materials during ball milling

Microstructure and phase evolution in nanocrystalline materials synthesized by ball milling have been investigated. Elemental Fe powder was milled in a low-energy ball mill at different temperatures and intensities. Grain size and strain were determined by the Warren-Averbach analysis of X-ray Bragg-peak broadening, and compared with the results of simplified analysis methods over a wide range of grain sizes. Systematic errors introduced by simplified methods were investigated. It was found that the grain size decreases and strain increases with time until reaching a steady state. The steady state grain size increases weakly with decreasing milling intensity or increasing temperature. A kinetic model was proposed, assuming simultaneous grain refinement and growth and incorporating the effect of deformation-enhanced diffusion. The model fits all of the data sets well. The weak temperature dependence of the grain-growth term is consistent with nonequilibrium vacancy production. The fit results also indicate that the efficiencies for grain refinement and vacancy production decrease with increasing vibration amplitude. A universal relationship for all vibration amplitudes is obtained after proper normalization of grain size and time. This model was also applied in our study of more complicated systems, in which phase transformations are coupled with microstructure evolution.; The decomposition kinetics of supersaturated Ag50Cu50 solid solutions have been studied. The solid solutions were formed by high-energy ball milling and subsequently milled at elevated temperatures ranging from 333 to 433 K in a low-energy ball mill. X-ray diffraction patterns suggest a wide distribution of compositions at steady state. At low temperatures, the stored enthalpy first decreases to a minimum value and subsequently increases toward a steady state, while at high temperatures it decreases monotonically. A model is proposed using the effective-temperature concept and a rate equation approach. A dependence of the effective temperature on grain size is introduced through the deformation-enhanced diffusion coefficient. The time dependence of the effective temperature, caused by the evolving grain size described by the model mentioned above, explains the non-monotonic behavior and the distribution of compositions. The model is also used to predict mechanical alloying kinetics.