The more shearing, the thicker shear band and heat-affected zone in bulk metallic glass
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N.M. Xiao Department for Special Environment Materials, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
Z.F. Zhang and M.L. Suia) Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China (Received 17 January 2008; accepted 18 April 2008)
In a compression test for a Zr-based bulk metallic glass, a dominant shear band was preserved before fracture by a cylindrical stopper. A heat-affected zone (HAZ) ∼10 m thick together with shear band was discovered in the center of the deformed sample by preferential ion milling. By using a low aspect ratio sample for compression, diverse micron-scaled HAZs among multiple shear bands were also revealed. Based on above experimental results and the isothermal source model, it was found that the thickness of shear band and its HAZ increased continuously with the progression of shear deformation.
I. INTRODUCTION
Without slip system and dislocation-free structure in metallic glasses, shear bands often become the important plastic deformation mechanism at room temperature in bulk metallic glasses (BMGs).1–8 The formation of free volume3 and/or local heating4 can cause a decrease in viscosity, leading to low plasticity and abrupt failure by shearing off along the dominant shear band. Recently, large compressive plasticity was achieved in more and more fully amorphous alloys, coming from the alloys themselves5,6 or the external confinement condition.7,8 In these cases, multiple shear bands are activated and the rapid propagation of shear bands is confined. These create new chances to further understand some primary properties of shear bands, including dimensions, heat release, structural change, and so on. It is acknowledged that shear band thickness is on the order of 10 nm according to observations9,10 and modeling.11,12 However, the shear bands investigated in either bending9 or in situ tension10 experiments were created in the amorphous foils with nanometer-scaled offsets, which reflect the initial shear deformation process. Although the well-developed shear bands could be seen
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Address all correspondence to this author. email: [email protected] DOI: 10.1557/JMR.2008.0258 J. Mater. Res., Vol. 23, No. 8, Aug 2008
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on surface for BMGs under loading, the offsets make it impossible to measure the thickness of shear bands exactly. So, it is still a question what is the true dimension for the well-developed shear bands in the interior of bulk samples (at least after wiping off the offset on the surface). The local heating accompanied with the severe plastic deformation within shear bands has been observed frequently.13–16 A large amount of heat generated will necessarily transfer from the shear band into the surrounding matrix, forming a heat-affected zone (HAZ). As important boundary conditions, the dimension and temperature of HAZ could be used to estimate the temperature rise of shear band. Using infrared imaging, Yang
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