Search

The model of Zhu et al. [Zhu B, Asaro RJ, Krysl P, Bailey R. Transition of deformation mechanisms and its connection to grain size distribution in nanocrystalline metals. Acta Mater 2005;53(18):4825–38] is further developed and used to explore the effect of grain size and grain size distribution, along with the influence of material parameters, on the mechanical response of nanocrystalline face-centered cubic aggregates. This model accounts for the simultaneous contributions of deformation mechanisms including grain boundary emission of dislocations and/or stacking faults, as well as for mechanisms such as grain boundary sliding and for natural transitions between the relative dominance of each. The effect of grain growth during deformation is also quantitatively assessed via simulation of recently obtained data on indentation tests in which dynamic grain growth was documented through the measurement of changes in grain size distribution and concomitant changes in hardness. The simulations provide a plausible description of the observed phenomenology and further underscore the unstable nature of nanocrystalline grain size distributions. The possibility of incorporating additional potential deformation mechanisms such as Coble creep, as has been proposed in other models, is discussed and shown to be straightforward addition to the model. Recently obtained data on texture development is analyzed via texture predictions for aggregates subject to finite deformations via high pressure torsion (HPT). The phenomenology is assessed specifically with regard to the potential use of texture measurements for confirming the importance of crystallographic mechanisms vs. those such as grain boundary sliding.

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.