Optimal structural superplasticity in metals and ceramics of microcrystalline- and nanocrystalline-grain sizes

Optimal structural superplasticity (defined as the range from the lowest strain rate to the point of inflection on the isostructural, isothermal sigmoidal ln(strain rate)–ln(stress) plot) in both metals and ceramics of grain sizes ranging from a few nanometers to a few micrometers is explained using a model in which rate controlling grain/interphase boundary sliding at the level of atomistics develops by boundary migration (rate controlled by boundary diffusion) to a mesoscopic scale (defined to be of the order of a grain diameter or more). Expressions for the threshold stress that should be exceeded for the onset of mesoscopic boundary sliding as a function of grain size and temperature and the steady state strain rate as a function of stress, grain size and temperature have been derived. A conclusion is reached that when the grain size is in the lower nanometer range, grain boundary migration will take place entirely by diffusion. At coarser grain sizes this process will involve a combination of (non-rate controlling) dislocation emission and (rate controlling) boundary diffusion. Experimental support for the above conclusions is demonstrated.