Rate-Dependent Temperature Increases in Shear Bands of a Bulk-Metallic Glass
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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 pct,[2] as well as good corrosion and fatigue resistances.[3–13] However, the poor ductility and subsequent catastrophic fracture severely limit their processing and application. At low temperatures (e.g., room temperature) and high strain rates, metallic glasses exhibit inhomogeneous deformation, which is confined to highly localized narrow shear bands.[14–16] Shear banding in metallic glasses significantly affects their macroscopic plastic flow and fracture. The serrated plastic flow during inhomogeneous deformation in metallic glasses was widely observed in various loading modes, such as compression,[17–25] bending,[26,27] tearing,[28,29] and nanoindentation.[18,30–35] Unlike crystalline materials, metallic glasses exhibit shear softening and the excessive propagation of individual shear bands may cause W.H. JIANG, Research Assistant Professor, F.X. LIU, Doctoral Student, and P.K. LIAW, Professor, are with the Department of Materials Science and Engineering, the University of Tennessee, Knoxville, TN 37996, USA. Contact e-mail: [email protected] H. CHOO, Professor, Department of Materials Science and Engineering, the University of Tennessee, Knoxville, USA, is Staff Scientist, Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831. H.H. LIAO, formerly Student, Department of Mechanical Engineering at University of Tennessee, is a Doctoral Student, with the Department of Mechanical Engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061. This article is based on a presentation given in the symposium entitled ‘‘Bulk Metallic Glasses IV,’’ which occurred February 25– March 1, 2007 during the TMS Annual Meeting in Orlando, Florida under the auspices of the TMS/ASM Mechanical Behavior of Materials Committee. Article published online October 5, 2007 1822—VOLUME 39A, AUGUST 2008
premature fracture. Shear banding may also lead to the local structural changes within a shear band, such as nanocrystallization,[36–45] nanovoids,[41–45] and excessive free volume.[46,47] However, up until now, it has not been explained conclusively why shear banding can cause such changes to the structure and property of metallic glasses. More than three decades ago, it was proposed that the shear-banding behaviors may be closely related to the local temperature increase in a shear band.[48] The stoppage of the propagation of a shear band may be attributed to a temperature increase that can dissipate rapidly excess free volume.[49,50] A serrated plastic flow may also result from a local temperature increase.[22] However, the calculations using a model for heat generated by dislocation
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