In situ thermographic observations on the compression behavior of a relaxed Zr-based bulk-metallic glass
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H. Choo Department of Materials Science and Engineering, The University of Tennessee, Knoxville, Tennessee 37996; and Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831
P.K. Liaw Department of Materials Science and Engineering, The University of Tennessee, Knoxville, Tennessee 37996 (Received 15 June 2006; accepted 4 October 2006)
Using an infrared camera, the plastic deformation of a relaxed Zr52.5Cu17.9Ni14.6Al10.0Ti5.0 bulk-metallic glass in a moderately high strain rate compression was observed in situ. The specimen exhibits an inhomogeneous deformation, which is manifested by serrated plastic flow, shear banding, and obvious work softening. Shear-banding operations were observed throughout the plastic deformation. Shear-banding operations started before the nominal yielding; shear bands could not block each other, but their interaction seems to accelerate the plastic deformation. A significant increase in the specimen’s temperature was observed due to shear banding.
I. INTRODUCTION
Metallic glasses are receiving more extensive attention than ever before, because a number of bulk glass-forming alloys have been discovered, which makes metallic glasses close to practical applications as structural materials. The bulk-metallic glasses (BMGs) have outstanding mechanical properties, such as a high strength of up to 5 GPa,1 a large elastic deformation limit of around 2%,2 as well as good corrosion and fatigue resistances.3–5 However, the poor ductility and subsequent catastrophic fracture severely limit their processing and application. At low homologous temperatures, typically including room temperature, the plastic deformation of the BMGs is manifested by inhomogeneity, and is restricted to highly localized shear bands.6–8 The excessive propagation of individual shear bands substantially decreases the plastic-deformation resistance of the BMGs, and may cause premature fracture. The inhomogeneous deformation is characterized by the serrated plastic flow and the formation of the shear bands. It was widely observed in various loading modes that with increasing strain rates, the serrations in a plastic
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0041 368 J. Mater. Res., Vol. 22, No. 2, Feb 2007 http://journals.cambridge.org Downloaded: 26 Mar 2015
flow tend to diminish.9–19 Recent research attributed this trend to the transition from the inhomogeneous to the homogenous plastic deformations at high strain rates by the simultaneous operations of numerous shear bands.9–13 Furthermore, in a deformation map of BMGs, a homogeneous-deformation regime at high strain rates was proposed.11 The viewpoint seems to be opposite to the conventional notion on the strain-rate dependence of the plastic deformation of metallic glasses.7,20 We note that this viewpoint is mainly based on the work using nanoindentation, which has a different stress state from uniaxial loading. Therefore, a further investigation on the plastic deformation
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