Intrinsic and extrinsic size effects in the deformation of metallic glass nanopillars

Nanosized pillars with diameters ranging from 90 to 600 nm of four amorphous alloys, Cu47Ti33Zr11Ni6Sn2Si1, Zr50Ti16.5Cu15Ni18.5, Zr61.8Cu18Ni10.2Al10 and Al86Ni9Y5, were fabricated and tested in situ in a transmission electron microscope. The major consideration when varying the composition was the change in bulk modulus and Poisson's ratio, which may affect the deformation mode and ductility of metallic glasses (MGs) at the nanoscale. Differences between the deformation behavior of tapered (1.5-3°) and taper-free systems were also investigated. The yield stress of all the MGs measured through the in situ experiments is found to be essentially size independent, irrespective of tapering. With increasing size, all the MGs examined show a ductile-to-brittle transition under compression; the transition point, however, depends on the chemical composition of the specific MG investigated. The lower the μ/B ratio, the larger the pillar diameter above which more brittle behavior occurs. Al86Ni9Y5 taper-free MG showed a transition threshold to brittle behavior at the largest pillar diameter of 300 nm. A micromechanical model is presented to explain the various dependencies.