Plastic zone at crack tip: A nanolab for formation and study of metallic glassy nanostructures
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A. Lindsay Greer Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB2 3QZ, United Kingdom (Received 11 November 2008; accepted 20 March 2009)
We report that various metallic glassy nanostructures including nanoridges, nanocones, nanowires, nanospheres, and nanoscale-striped patterns are spontaneously formed on the fracture surface of bulk metallic glasses at room temperature. A clear correlation between the dimensions of these nanostructures and the size of the plastic zone at the crack tip has been found, providing a way to control nanostructure sizes by controlling the plastic zone size intrinsically or extrinsically. This approach to forming metallic glassy nanostructures also has implications for understanding the deformation and fracture mechanisms of metallic glasses. I. INTRODUCTION
Novel system performance through nanostructuring has been investigated in many branches of science in recent years.1 One of the most active trends in this field is the development of synthetic methods to obtain shapeand size-controlled nanostructures, because various behaviors of these nanostructures such as physical and chemical properties2,3 are strikingly sensitive to both morphology and size. The categories of metallic alloys and metal oxide nanostructures are important and have been extensively studied due to their scientific interest and practical applications. However, most of these metallic nanostructures are crystalline,4,5 despite the fact that the superiority of the amorphous over the nanocrystalline state has been demonstrated in some cases such as chemical catalysis.6 More recently, it has been predicted theoretically7–9 and confirmed experimentally6,10 that the amorphous state is naturally favorable and feasible for stable nanostructures. Bulk metallic glasses (BMGs), exhibiting unique physical, chemical (such as high resistance to corrosion and oxidation), and mechanical (such as extraordinarily high strength, hardness, and elastic limit) properties, have attracted extensive attention because of their potential for engineering application and their scientific interest.11 In contrast to dislocation-mediated plastic flow in crystalline metallic alloys, BMGs normally display little global plasticity before fracture because their plastic deformation at room temperature is highly localized in nanoscale shear a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0362
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http://journals.cambridge.org
J. Mater. Res., Vol. 24, No. 9, Sep 2009 Downloaded: 13 Mar 2015
bands, where a large plastic strain is accumulated in a very thin region (10–20-nm thick) exhibiting strain softening or thermal softening.12 The softening leads to the formation of a viscous fluid-like layer that manifests itself in remarkable patterns when the shear band comes apart in final fracture (for a review see Ref. 12). Using the unique nature of the localized plasticity of BMGs, in this letter, we report a consequence of the local plastic behavior, namely spontaneous fo
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