Transition of deformation mechanisms and its connection to grain size distribution in nanocrystalline metals

A polycrystalline constitutive theory is developed based on the recently proposed model of Asaro, Krysl, and Kad (AKK) for deformation mechanisms in nanocrystalline metals and the extended aggregate Taylor model of Asaro and Needleman (AN). The AKK model is particularly concerned with mechanisms including the emission of perfect and partial dislocations, along with deformation twins from grain boundaries, and grain boundary sliding, and with the grain size, strain rate, and temperature dependent transition between these mechanisms. Thus of specific concern is the effect of grain size distribution as well as the mean grain size on overall nanocrystalline aggregate response. Grain size distributions are simulated with log-normal distributions and incorporated into the AN aggregate model in the form of distributions of the volume fraction of grains with various sizes. Numerical results for the case of electrodeposited Ni are shown to be in good agreement with available experimental data.