Direct observations on the evolution of shear bands into cracks in metallic glass
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Fu-Fa Wu Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, People’s Republic of China; and School of Materials and Chemical Engineering, Liaoning University of Technology, Jinzhou 121001, People’s Republic China
Zhe-Feng Zhanga) Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, People’s Republic of China
Ju¨rgen Eckertb) IFW Dresden, Institute for Complex Materials, D-01171 Dresden, Germany (Received 20 April 2009; accepted 19 July 2009)
The evolution of shear bands (SBs) into cracks was observed by using a high-resolution scanning electron microscope in Zr59Cu20Al10Ni8Ti3 metallic glassy samples after a small punch test with different strain rates. As shear strain increased along a radial SB, three distinctive regions of morphologies were found (I) bonded SB, (II) microcrack plus bonded SB, and (III) full crack. In region II with moderate shear strain, some glassy “extrusions” were also observed. Once shear offset increases to a critical value, the SB becomes a full crack. For two different SBs in one specimen, the critical shear offsets maintain approximately the same value, which sheds light on the critical shear failure condition of metallic glass. The critical shear offset was also found to be sensitive to the strain rate and a higher strain rate led to less critical shear offset. It is suggested that the structure evolution and heat evolution within a shearing SB should be responsible for the present results. I. INTRODUCTION
Metallic glass (MG) has been a hot topic during the last decades because of its unique physical and chemical properties. Due to its dislocation-free feature, the strength of MGs approaches the theoretical limit, which gives potential to apply them as structural materials.1–5 However, the fact that no dislocation exists and thus no slip happens in MG makes it deform and fail mainly by localized shear banding at room temperature.4,5 Therefore, the evolution of the shear bands (SBs) into cracks should be the key factor controlling the plastic deformation ability of the MGs. While owing to the limited plasticity and the rapid shear failure process,5 the evolution of SB into crack is hardly observed directly by using the conventional uniaxial tests. a)
Address all correspondence to this author. e-mail: [email protected] b) This author was an editor of this journal during the review and decision stage. For the JMR policy on review and publication of manuscripts authored by editors, please refer to http://www.mrs.org/jmr_policy DOI: 10.1557/JMR.2009.0374 3130
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J. Mater. Res., Vol. 24, No. 10, Oct 2009 Downloaded: 13 Mar 2015
The critical condition for the failure of an SB is also an important issue for the plastic deformation behavior of MGs. In the unconstrained uniaxial tension tests, specimens fail often accompanying with the activity of few SBs and exhibit nearly zero tensile plasticity.2–5 More recently,
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