Observation of structural anisotropy in metallic glasses induced by mechanical deformation
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Takeshi Egami Department of Materials Science and Engineering, The University of Tennessee, Knoxville, Tennessee 37996-2200; and Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6376 (Received 4 July 2006; accepted 4 October 2006)
We have investigated atomic structure of a Fe81B13Si4C2 metallic glass after mechanical creep deformation. We determined the structure function and pair density function resolved for azimuthal angle using x-ray scattering and a two-dimensional detector. The results are analyzed by the spherical harmonics expansion, and are compared to the often-used simple analysis of the anisotropic pair density function determined by measuring the structure function along two directions with respect to the stress. We observed uniaxial structural anisotropy in a sample deformed during creep experiment. The observed macroscopic shear strain is explained in terms of local bond anisotropy induced by deformation at elevated temperature. The bond anisotropy is a “memory” of this deformation after load was removed. We showed that use of sine-Fourier transformation to anisotropic glass results in systematic errors in the atomic pair distribution function.
I. INTRODUCTION
Metallic glasses have been under vigorous investigation since first reported back in 1960,1 particularly after the recent development of bulk metallic glasses (BMGs).2–5 Metallic glasses exhibit a unique collection of promising properties such as high strength,6 high elasticity,7,8 excellent magnetic properties,9 and good corrosion resistance,10,11 among others. BMGs have additional merit of near-net-shape formability.12–14 These attractive properties have made BMGs the focus of intensive research among a number of groups around the world. Upon cooling through the glass-transition temperature (Tg), the viscosity of the melt increases by many orders of magnitude, and the supercooled liquid forms a glass. The resultant glass is metastable: it can transform to the crystalline phase but also can undergo subtle structural changes if annealed at low temperatures. Glass retains the disordered atomic structure of liquid, and ideally is an isotropic solid. Frequently because of processing conditions, such as directional heat flow, some structural anisotropy is produced during quenching, and has been observed by structural investigations. Generally, if the a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0043 412 J. Mater. Res., Vol. 22, No. 2, Feb 2007 http://journals.cambridge.org Downloaded: 18 Mar 2015
crystalline inclusions are absent, annealing at high temperatures results in an isotropic structure. However, upon mechanical deformation the glass becomes anisotropic, and the conventional method of structural analysis that assumes isotropic structure is no longer valid. In this article we discuss how the structure of such glasses can be properly analyzed. II. STRUCTURAL ANALYSIS OF DEFORMED GLASSES
The atomic structure of a glass is typically studied by x-ray, ne
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