Transition from a strong-yet-brittle to a stronger-and-ductile state by size reduction of metallic glasses

Journal name:
Nature Materials
Volume:
9,
Pages:
215–219
Year published:
DOI:
doi:10.1038/nmat2622
Received
Accepted
Published online

Amorphous metallic alloys, or metallic glasses, are lucrative engineering materials owing to their superior mechanical properties such as high strength and large elastic strain. However, their main drawback is their propensity for highly catastrophic failure through rapid shear banding, significantly undercutting their structural applications. Here, we show that when reduced to 100nm, Zr-based metallic glass nanopillars attain ceramic-like strengths (2.25GPa) and metal-like ductility (25%) simultaneously. We report separate and distinct critical sizes for maximum strength and for the brittle-to-ductile transition, thereby demonstrating that strength and ability to carry plasticity are decoupled at the nanoscale. A phenomenological model for size dependence and brittle-to-homogeneous deformation is provided.

At a glance

Figures

  1. Nanotension results for the specimen larger than 200[thinsp]nm in diameter.
    Figure 1: Nanotension results for the specimen larger than 200nm in diameter.

    a,b, SEM images of typical as-fabricated (a) and deformed (b) metallic glass nanopillars for tension tests. The inset in b is a top-down zoomed-in image of the fracture surface within the square. c, Typical tensile stress versus strain curves for each diameter. The value of strain in c has no units.

  2. Monotonic nanotension results for the 100-nm-diameter specimen.
    Figure 2: Monotonic nanotension results for the 100-nm-diameter specimen.

    a, SEM image of a typical as-fabricated 100-nm-diameter tensile sample. bf, Images captured from a movie recorded during an in situ tension test at εE of 0 (b), 0.04 (c), 0.06 (d), 0.07 (e) and the final fracture (f). The square in e indicates the region where a neck is formed. g,h, The engineering (g) and true (h) stress–strain curves of the nanotension test. True stresses and strains after necking were obtained by directly measuring the diameter in the necked region. The error bars in h reflect the uncertainty in measuring pillar dimensions on the captured images of the movie. The value of strain in g and h has no units.

  3. Multiple-loading nanotension results for the 100-nm-diameter specimen.
    Figure 3: Multiple-loading nanotension results for the 100-nm-diameter specimen.

    a, Engineering stress–strain curve of the 100nm tension sample loaded and unloaded three times before attaining its final plastic strain. b,c, SEM images before (b) and after (c) the multiple-loading test, showing that the sample became taller and that a very small neck formed near the head (the yellow vertical lines were inserted to help identify the necked region). d, Dark-field TEM image and corresponding electron diffraction pattern. e, Zoomed-in bright-field TEM image of the region within the square in d. The halo-like coating around the pillar in ce is probably due to the amorphous carbon contamination deposited after the sample was tested. The value of strain in a has no units.

  4. Tensile yield strength as a function of sample diameter.
    Figure 4: Tensile yield strength as a function of sample diameter.

    At 100nm, the filled square is the 0.2% offset yield strength, and the open square is the fracture strength of the same sample. The dashed line represents the compressive yield strength in the bulk measured by W. L. Johnson’s group at the California Institute of Technology.

  5. Schematic representation of the applied stresses required to initiate shear-band propagation versus homogeneous deformation as a function of sample diameter, d.
    Figure 5: Schematic representation of the applied stresses required to initiate shear-band propagation versus homogeneous deformation as a function of sample diameter, d.

    When the sample diameter is larger than the critical size, d*, defined as the intersection of the two curves, the material fails by shear-band propagation without notable plasticity. When d<d*, homogeneous plastic deformation precedes shear-band propagation, showing significant plasticity quantified by the height difference between the two curves.

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Affiliations

  1. Division of Engineering and Applied Science, California Institute of Technology, 1200 E. California Blvd, MC 309-81, Pasadena, California 91125-8100, USA

    • Dongchan Jang &
    • Julia R. Greer

Contributions

D.J. and J.R.G. designed the research and J.R.G. supervised the project. D.J. carried out the experiments. D.J. and J.R.G. contributed to the interpretation of the results and to the writing of the paper.

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The authors declare no competing financial interests.

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