Anisotropic atomic structure in a homogeneously deformed metallic glass
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R.T. Ott and D.J. Sordelet Materials and Engineering Physics Program, Ames Laboratory (USDOE), Ames, Iowa 50011 (Received 15 June 2006; accepted 4 October 2006)
The anisotropic atomic structure in a Zr41.2Ti13.8Cu12.5Ni10Be22.5 metallic glass strained during uniaxial tensile creep at 598 K was studied at room temperature using high-energy x-ray diffraction. Changes in the atomic structure were examined by comparing the total scattering function [S(Q)] and the reduced pair distribution function [G(r)] of the creep to that of a companion specimen subjected to the same heat treatment only. Two-dimensional maps of the ⌬S(Q) and its Fourier transformation demonstrate the distribution in the bond orientation anisotropy increases with increasing total strain. A fit of the reduced pair distribution function using a simplified two-component model suggests that the bond length changes in the deformed creep samples are not uniform.
I. INTRODUCTION
At high temperatures and lower applied stresses, metallic glasses deform homogeneously whereas at lower temperatures and higher stresses, the strain is localized into shear bands in which shear-induced dilation occurs.1–4 The observations of a drop in the flow stress for constant strain rate creep experiments5,6 and accelerating strain rates for constant-stress creep experiments7 support strain-induced structural disordering during homogeneous flow, which leads to an increase in the excess free volume compared to the relaxed structure. At elevated temperatures a competing ordering process, structural relaxation, also occurs, which acts to annihilate excess free volume.6,8 To understand the competing effects of free volume creation and structural relaxation, a better picture of the structural anisotropy in homogeneously deformed metallic glasses is required.9,10 Egami and coworkers have found that for the scattering vector parallel and perpendicular to the tensile loading axis, S(Q) is anisotropic for the two directions.10 The pair distribution function calculated from the anisotropic structure factor revealed that the number of bonds in the direction normal to the loading axis increases whereas the number of bonds in the direction parallel to the loading axis decreases. This anisotropic atomic structure, or bond
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0044 382 J. Mater. Res., Vol. 22, No. 2, Feb 2007 http://journals.cambridge.org Downloaded: 16 Mar 2015
orientational anisotropy (BOA), was also simulated using molecular dynamics.11 These results suggest that metallic glasses retain memory of the directionality of their homogenous plastic deformation. Using a new methodology to analyze high-energy synchrotron x-ray diffraction (HEXRD) results, we describe the structural anisotropy in a Zr-based metallic glass that evolved after isothermal creep deformation. This study expands on earlier work12 to include a detailed twodimensional (2D) mapping of the structural anisotropy along the gauge length of a deformed creep sample as
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