Extrinsic size-effects in metals and alloys have been a rich topic of research over the past decade, and continue to grow with numerous new challenges and questions emerging at the micro- and nano-scales. We utilize atomistic simulations, discrete dislocation dynamics simulations and microscale experiments were performed to identify the mechanisms governing size-scale effects. While previous experimental studies, limited to a very small subset of starting initial dislocation densities and microcrystal sizes (mostly submicron), suggest that the Taylor hardening law fails at micron and submicron length scale, DDD simulations show that a strength-dislocation density relationship exists for bulk as well at the micron scales and Taylor hardening is recovered above a size-dependent critical dislocation density. We discuss the role of this critical dislocation density in controlling the size-affected flow response, and rationalize the results based on the stochastic of the existing dislocation network in the microcrystals. Finally, we examine the effect of crystal size on the twinning deformation in submicron hexagonal packed single crystals.
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