Localized heating and fracture criterion for bulk metallic glasses
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T.G. Nieha) Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831; and Department of Materials Science and Engineering, The University of Tennessee, Knoxville, Tennessee 37996
M.L. Morrison, P.K. Liaw, and R.A. Buchanan Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee 37996 (Received 29 September 2005; accepted 9 January 2006)
In this study, we demonstrated that the failure of bulk metallic glasses (BMGs) results from a sudden temperature rise within a shear band. Using a shear transformation zone model, we successfully calculated the temperature within a shear band and found it consistent with the observation from an in situ infrared thermographic system. The instantaneous temperature within a shear band at fracture agrees remarkably well with the glass transition temperature (Tg), providing a new criterion to determine the strength of BMGs from their Tg. This agreement also discloses the fact that catastrophic failure of BMG is caused by the sudden drop in viscosity inside the shear band when the instantaneous temperature within a shear band approaches Tg. I. INTRODUCTION
The recent discovery of bulk metallic glasses (BMGs) has generated substantial interest in their unique combination of structural properties: ultra-high strengths (∼2 GPa), super-elasticity (∼2%), high fracture toughness, excellent wear, and corrosion resistances.1–7 However, because of the absence of dislocation and grain boundary structures, and thus work hardening, their ductility and toughness are largely limited by the catastrophic shear band propagation. Although the composite approach has been used to improve the ductility and toughness, an understanding of the shear band evolution and failure mechanism is still vital for optimizing the performance of BMGs. To model the shear band evolution mechanism in metallic glasses,8–10 one factor often overlooked is the local heating within shear bands. Local heating as a result of plastic energy dissipated within a shear band is expected to increase its temperature. Once the temperature is beyond the glass transition temperature (Tg), viscosity drops dramatically and significant softening occurs, which can lead to rapid shear band propagation and catastrophic failure in BMGs. Thus, it is conceivable that the temperature rise within shear bands during deformation may have a close correlation with the Tg of a BMG. Unfortunately, no direct measurements of shear-band temperature in BMGs have been made thus far, partially a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0124 J. Mater. Res., Vol. 21, No. 4, Apr 2006
because of the great difficulty of measuring temperature in extremely small distances (∼10 nm in shear bandwidth11,12) and short time scales (∼10−5 s for shear band propagation13). Owing to the lack of experimental verification, the theoretical efforts in predicting the shear band temperature are far from conclusive. Although several papers7,10,14 predicted the shear b
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