In situ transmission electron microscopy studies of shear bands in a bulk metallic glass based composite
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In situ straining transmission electron microscopy (TEM) experiments were performed to study the propagation of the shear bands in the Zr56.3Ti13.8Cu6.9Ni5.6Nb5.0Be12.5 bulk metallic glass based composite. Contrast in TEM images produced by shear bands in metallic glass and quantitative parameters of the shear bands were analyzed. It was determined that, at a large amount of shear in the glass, the localization of deformation occurs in the crystalline phase, where formation of dislocations within the narrow bands are observed.
I. INTRODUCTION
A new class of materials emerged with the discovery of the multicomponent systems with exceptional glassforming ability, which made possible the synthesis of metallic glasses at large thicknesses.1,2 Bulk metallic glasses have unusual mechanical properties, such as high elastic limit, up to 2%, and very high yield stress of 0.02Y (Y is Young’s modulus) but exhibit almost no global plasticity.3 Two regimes of plastic deformation have been observed, viscous or homogeneous deformation, which occurs at high temperatures and low strain rates, and inhomogeneous flow at low temperatures and high strain rates.4 Inhomogeneous deformation manifests itself in the formation of shear bands at a 45° angle with respect to the applied stress in uniaxial loading. Deformation mechanisms of metallic glasses attracted a lot of attention from both theoretical and experimental sides.4–10 Most of the experimental work, however, was done using scanning electron microscopy (SEM). Transmission electron microscopy (TEM) studies of shear bands were not very successful so far due to the relatively small structural changes in the shear bands, frequently undetectable by TEM. The most successful approach in studying shear bands in metallic glasses turned out to be in situ straining experiments or deformation of thin films of material without successive thinning.11,12 Propagation of a shear band in a thin film creates a step at the surface and changes the mass-thickness contrast in the TEM in the region of the shear band. This paper reports the TEM analysis of shear bands in a two-phase material, Zr56.3Ti13.8Cu6.9Ni5.6Nb5.0Be12.5, which consists of amorphous and crystalline phases. Such multiphase materials and composites based on a glassy matrix have been developed in attempt to improve ductility and toughness of metallic glasses.13,14 The J. Mater. Res., Vol. 16, No. 9, Sep 2001
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presence of the second phase particles of crystalline phase was found to hinder shear band propagation and promote formation of multiple shear bands.13 The goal of our study was to understand the deformation mechanisms of such multiphase material using TEM. II. EXPERIMENTAL
The alloy was prepared by the arc melting of a mixture of constituent elements, with purity of 99.7 at.% or better, in a Ti-gettered argon atmosphere on a water-cooled Cu crucible. The specimens were flipped over and remelted several times to promote homogeneity. The specimens for TEM were prepared by io
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