Serrated flow behavior induced by blunt mechanism of shear crack propagation in metallic glass
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S.X. Mao Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, People’s Republic of China; and Department of Mechanical Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania 15261
B.L. Shen Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo, 315201, People’s Republic of China
J. Eckertc) IFW Dresden, Institut fu¨r Komplexe Materialien, D-01171 Dresden, Germany; and Technische Universita¨t Dresden, Institute of Materials Science, D-01062 Dresden, Germany (Received 6 June 2008; accepted 13 October 2008)
We present a blunt mechanism to explain the serrated flow behavior and slight “work hardening” at the beginning of yielding during the compression of metallic glass, which is in line with the piling-up of parallel shear bands on the fracture surface with a gradually increasing space from the edge of surface to inside. Meanwhile, two intrinsic parameters, i.e., strength intensity of blunt behavior, K , and global work-hardening sensitivity exponent, n, are introduced to characterize the blunt effect on the net increase in flow stress or work-hardening behavior of metallic glass. I. INTRODUCTION
Because the unique plastic deformation in bulk metallic glasses (BMGs) is only confined to the localized narrow regions (called shear bands and characterized by the excess free volume),1,2 shear bands become a close, concerned subject and eternal problem to be investigated.1–3 Transmission electron microscope (TEM) observations on shear bands showed that the thickness of shear bands is in the range of 10 to 60 nm, depending on the testing conditions.3 In a recent study, Li et al.4 observed some nanometer scale voids within the shear bands, correlated with the coalescence of excess free volume following the plastic flow, with quantitative high-resolution transmission electron microscope (HRTEM). In addition, the deformation-induced in situ nanocrystallization was observed by HRTEM to further block plastic flow along some shear bands.5 Moreover, many investigations indicate that the formation of shear Address all correspondence to these authors: a) e-mail: [email protected] b) e-mail: [email protected] c) 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.2009.0059 436
http://journals.cambridge.org
J. Mater. Res., Vol. 24, No. 2, Feb 2009 Downloaded: 19 Mar 2015
bands was affected by the ambient temperature, stress state, annealing condition, sample geometry, second phase, particle or fiber, and so on, which led to the different deformation and fracture behavior.6–10 Zhang and Eckert11 proposed the unified tensile fracture criterion to soundly explain the observed three different failure modes of BMGs.12 In addition, a fracture mode factor a ¼ t0 =s0 11 and fragmentation coefficient, Fn (Fn ¼ An =A0 ),13 were defined to describe the
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