Sample size and preparation effects on the tensile ductility of Pd-based metallic glass nanowires

Abstract

Glass materials, including metallic glasses (MGs), typically fracture in tension at room temperature in a globally elastic manner. Although homogeneous tensile plasticity and necking of nanoscale MGs have been reported, controversy exists regarding possible contributions from specimen preparation and testing techniques. Here, we show the separate effects of sample size reduction and extrinsic effects on the homogeneous tensile plasticity and necking of Pd40Cu30Ni10P20 glassy wires tested at room temperature. An intrinsic transition from catastrophic shear fracture to plasticity and necking was obtained in this glass when its diameter approached the estimated length scale of the shear-band nucleus size (i.e. 500nm). A further reduction in the wire diameter to 267nm produced homogeneous flow and complete ductile necking, with a true fracture strain in excess of 2.0. Our theoretical analysis shows that the plasticity of nanoscale MG wires with diameters smaller than a critical length scale is mediated by shear transformations catalyzed by local shear dilatation, and the predicted critical length scale for the brittle-to-ductile transition of the glassy wires is consistent with our experimental results. Extrinsic effects introduced during sample preparation and/or testing produce entirely different results and are reviewed in the light of previous work.