Effect of Sn Additions on the Damage Tolerance of a ZrCuNiAl Bulk Metallic Glass
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INTRODUCTION
BULK metallic glasses (BMGs) have great potential as structural materials due to their high specific strength.[1] Nevertheless, such applications are rather limited because of their low tensile plasticity under ambient conditions[2,3] even though early work has shown high fracture toughness is possible.[4,5] Extensive investigations have been focused on improving the damage tolerance of BMG materials both extrinsically and intrinsically.[6–13] In addition to the former practices through introducing favored stress states or structural heterogeneities at multiple length scales, considerable efforts have been devoted to improving toughness by microalloying and processing.[14–16] Mechanical properties of monolithic BMGs can be tailored by alloying under the guidance of Poisson’s ratio (m), shear modulus, fraction of excess free volumes or shear transformation zone size.[7–9,17–22] In particular, Poisson’s ratio, m reflects the competition between the resistance of change of interatomic bond angle (shear) and the variation of bond length (dilatation) and it was originally linked to fracture energy by Pugh based on his empirical analysis of dislocation controlled plastic deformation mechanisms in pure metals.[23] In practice, v has been successfully used to characterize tough (v > 0.3) vs brittle (v < 0.3) properties for specific BMG systems such as Mg-based, Ti-based, and Fe-based BMGs.[21,24,25] Unfortunately, v yields limited success as a general criterion (i.e., v = 0.3) for brittle-to-ductile transition (BDT) among various BMGs.[6,26,27] For example, some Pd-based BMGs exhibit unusual brittle behaviors even though v > 0.3.[28] According to the Griffith equation assuming no thermal relaxation takes place,[29] to C.C. YUAN and X.X. XIA, Graduate Students, D.Q. ZHAO, Senior Research Professor, and X.K. XI, Associate Professor, are with the Institute of Physics, Chinese Academy of Sciences, Beijing 100190, P.R. China. Contact e-mail: [email protected] K.H. JIANG, Senior Research Professor, is with the School of Mechanics, Beihang University, Beijing, P.R. China. Manuscript submitted April 6, 2012. Article published online October 10, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
estimate fracture energy of quasi-brittle materials, a length scale at crack tip (like dislocation Burgers vector and core widths in crystals) might have to be introduced in addition to elastic constants. However, systematic experimentation toward this direction, which is relevant for the design of tough metallic glasses, is still lacking.[3] In this work, we first assume that fine tuning the intrinsic toughness/plasticity of metallic glasses upon microalloying might affect both elastic constants and experimentally accessible characteristic length scales related to fracture energy dissipation at crack tip. Minor alloying effects on the damage resistance of a typical Zr-based BMG are then evaluated with (1) compressive plasticity, (2) notched and (3) fatigue precracked fracture toughness. The result shows that the observed fracture energy is c
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