Emergence of external size effects in the bulk-scale polycrystal to small-scale single-crystal transition: A maximum in the strength and strain-rate sensitivity of multicrystalline Cu micropillars

Using microcompression methodology, various plastic deformation characteristics, namely the strength, strain-rate sensitivity (SRS) and activation volume, of 〈1 0 0〉-single-crystalline (SC) Cu micropillars and nanostructured multicrystalline (MC) Cu micropillars with several grains were systematically investigated at different strain rates for a wide range of external sample sizes ranging from 300 to 2000 nm. The results revealed that the paradigm of “smaller is stronger” for small-scaled SC pillars holds true in nanostructured MC Cu micropillars above the strongest external size; however, below this size, a decrease in the external size triggers the reverse size effect, i.e. “smaller is weaker”. The appropriate introduction of grain boundaries into SC pillars can markedly improve the smoothness of their plastic flow and significantly enhance their SRS. Additionally, we demonstrated that the SRS of MC Cu pillars increases with decreasing size up to a certain limit and then decreases with a further decrease in the external sample size; thus, a maximum SRS emerges. A phenomenological model was proposed to rationalize the scaling behavior of the activation volume with the external sample size and to highlight the effect of the internal feature size on the rate-limiting behavior of conventional bulk nanostructured metals (Cu).