Shear band multiplication aided by free volume underthree-point bending
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.5Cu17.9Ni14.6Al10Ti5 bulk metallic glass (BMG) alloy samples in both rod and plate geometry were prepared. Different free volume states were obtained through thermal treatment. The plastic deformation ability of the BMGs was investigated through both a three-point bending test and compression test. The three-point bending results reveal the important role of free volume content on the formation of multiple shear bands, as the shear band propagation can be efficiently stopped due to the existence of the stress gradient from the surface to the neutral plane. In compression, the sample size rather than free volume controls the shear banding behavior. I. INTRODUCTION
As a class of advanced materials, bulk metallic glasses (BMGs) have gained considerable attention due to their unique properties, including high strength, corrosion resistance, wear resistance, and relatively low elastic modulus, which are rarely found in crystalline materials.1–3 However, the strain softening effect often causes catastrophic failure along a single shear band with almost zero plasticity under tension and limited plasticity under compression,4–6 restricting the application of BMGs as structural materials. Therefore, how to improve the plasticity of BMGs has become one of the most pressing issues in the community in recent years. A number of reports of plasticity improvement have appeared for some Zr-,7 Pd-,8 Pt-,9 and Cu–Zr-10–12 based BMGs. In particular, several groups have attempted to control the internal structure of BMGs for better plastic behavior. A frequently invoked concept is the presence and variation of “free volume.” For example, Chen et al.13 obtained large compressive plastic strain up to 32.5% in Cu45Zr46Al7Ti2 metallic glass rod with 2 mm diameter, and the authors attributed this to the large amount of randomly distributed free volume induced by minor Ti alloying. However, exactly how the small addition of Ti changed the internal glass structure was not resolved. Huang et al.14 reported that the BMG sample with 1 mm diameter showed superior plasticity than the one with 6 mm diameter at the same composition, and this “smaller is softer” trend was ascribed to the different amounts of free volume induced by the different cooling rate experienced by these samples. However, the sizea)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2010.0028 J. Mater. Res., Vol. 25, No. 2, Feb 2010
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dependent plasticity may not be solely due to the free volume effect, as Li’s group already explained15 using a shear band instability index involving test machine stiffness and sample size.16 Xie et al.17 also thought that the test machine elastic springback related to sample size can be an influencing factor, rather than the free volume. While many previous reports,18–20 including our own preliminary account in a previous communication on Cu46Zr47Al7 BMGs,21 have established the connection between the mechanical behaviors of the metallic glasses
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