Local temperature rises during mechanical testing of metallic glasses
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W.H. Wang Institute of Physics, Chinese Academy of Sciences, Beijing, 100080, People’s Republic of China
J.J. Lewandowskia) Department of Materials Science and Engineering, Case Western Reserve University, Cleveland, Ohio 44106
A.L. Greerb) Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB2 3QZ, United Kingdom (Received 2 August 2006; accepted 4 October 2006)
Under ambient conditions, plastic flow in metallic glasses is sharply localized into shear bands. The heat content of, and consequent temperature rise at, shear bands in three bulk metallic glasses are compared using a recently reported fusible coating method. The minimum shear offsets necessary to detect local heating are determined. It is shown that the dependence of heat content on offset is consistent with frictional heating in the band. The effective stress on the band undergoing shear is 50–70% of the macroscopic shear stress, a ratio compared with simulations of shear-band initiation and operation. It is also noted that frictional heating can occur not only at shear bands, but also at mixed-mode cracks.
I. INTRODUCTION
Bulk metallic glasses (BMGs) have attracted attention for their potential applications as structural materials.1–3 Compared to conventional engineering alloys, they show uniquely high values of elastic limit (yield stress) and yield strain.4 BMGs can show a very wide range of toughness.5 Even those with high toughness, associated with extensive local plasticity at the fracture surface, show essentially zero tensile ductility. This is the key factor limiting wider application of these otherwise very promising materials. Plastic flow under ambient conditions is very sharply localized into shear bands, the thickness of which is ∼10 nm.6,7 There is consequently much interest in better understanding of the operation of shear bands in metallic glasses. The localization of shear arises from worksoftening.8,9 The origin of this softening in shear bands a)
This author was an editor of this focus issue 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. b) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0068 J. Mater. Res., Vol. 22, No. 2, Feb 2007
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has remained controversial. As has been reviewed,10 some have argued that the softening is due to local heating, as for the shear bands in some conventional alloys subjected to high strain rates.11 Others have argued that the softening is due to structural changes (notably dilatation) in the shear band. From theory and experiment, estimates of the temperature rise at shear bands in metallic glasses have varied from less than 0.1 K to a few thousand K.10 Recently Lewandowski and Greer10 have applied a fusible coating method to determine the total heat content of individual shear bands and to estimate the maximum local temperature rise. Using fused zones o
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