Transition from homogeneous-like to shear-band deformation in nanolayered crystalline Cu/amorphous Cu-Zr micropillars: Intrinsic vs. extrinsic size effect

The microcompression method was used to investigate the compressive plastic flow behavior of nanolayered crystalline/amorphous (C/A) Cu/Cu-Zr micropillars within wide ranges of intrinsic layer thicknesses (h similar to 5-150 nm) and extrinsic sample sizes (350-1425 nm) with the goal of revealing the intrinsic size effect, extrinsic size effect and their interplay on the plastic deformation behavior. The nanolayered C/A micropillars exhibited deformation behaviors of strain-hardening followed by strain-softening that were dependent on the thickness of the layers. At h <= 10 nm, the strain-softening is related to shear deformation that is caused by fractures in the amorphous layers. At h > 10 nm, however, the strain-softening is related to the reduction in dislocation density caused by dislocation absorption. Correspondingly, the deformation mode of the C/A micropillars transitioned from homogeneous-like to shear band type as h decreased to the critical value of similar to 10 nm, which is indicative of a significant intrinsic size effect. The extrinsic size effect on the plastic deformation also became remarkable when h was less than similar to 10 nm, and the interplay between the intrinsic and extrinsic size effects leads to an ultrahigh strength of similar to 4.8 GPa in the C/A micropillars, which is close to the ideal strength of Cu and considerably greater than the ideal strength of the amorphous phase. The underlying strengthening mechanism was discussed, and the transition in deformation mode was quantitatively described by considering the strength discrepancy between the two constituent crystalline and amorphous layers at different length scales. (C) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.