Revelation of the effect of structural heterogeneity on microplasticity in bulk metallic-glasses
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Qing Wang The Department of Mechanical Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, People’s Republic of China; and The Institute of Materials Science, The University of Shanghai, 200072 Shanghai, People’s Republic of China
Peter K. Liaw The Department of Material Science and Engineering, The University of Tennessee, Knoxville, Tennessee 37996-2200 (Received 22 July 2009; accepted 2 November 2009)
In this article, the shear-banding behavior in bulk metallic-glasses (BMGs) is studied using a focused ion beam (FIB)-based nanoindentation method, which involves cylindrical nanoindentation of a FIB-milled BMG microlamella and is capable of revealing the subsurface shear-band patterns down to the submicron scale. The results of the current study on a Zr-based BMG clearly show that short shear bands, with the lengths of a few hundred nanometers, could be severely kinked before growing into a longer one, which implies that structural heterogeneity plays an important role in the microplasticity of BMGs. Furthermore, through the three-dimensional finite-element simulation combined with the theoretical calculation based on the Mohr–Coulomb law, it is found that the yield strengths exhibit a large scatter as a consequence of the structural heterogeneity when microplasticity occurs in the Zr-based BMG, which is consistent with our recent findings obtained from the microcompression experiments. I. INTRODUCTION
Bulk metallic glasses (BMGs) have been attracting a great deal of interest of materials scientists since 1960,1 when it was first demonstrated that amorphous metals could be synthesized through the rapid quenching of supercooled liquids. In the absence of crystalline defects, such as dislocations and grain boundaries, BMGs are superior to their crystalline counterparts in yield strengths and corrosion resistance.2,3 The unique combination of the mechanical properties of BMGs make them an excellent class of materials for a variety of applications, including sports and luxury goods, electronics, medical devices, and national defense.4 However, this class of materials usually exhibits limited ductility at room temperature. When plasticity sets in, the lack of strain hardening gives rise to the strain localization into a few narrow shear bands within the BMGs. The subsequent catastrophic propagation of the shear bands ultimately results in the brittle-like fracture. The inelastic deformation in BMGs originates from the existence of the atomic clusters in an amorphous structure, which can undergo shear transformations much a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2010.0058 J. Mater. Res., Vol. 25, No. 3, Mar 2010
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easier than their surroundings under mechanical loadings. In the BMG literature, such atomic clusters are termed as the shear transformation zones (STZs) as an analog of dislocations in crystalline materials to initiate plastic flows in BMGs.5 According to the unified yieldin
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