http://journals.cambridge.org Downloaded: 01 Oct 2012 IP address: 160.36.231.126 Observation of structural anisotropy in metallic glasses induced by mechanical deformation Wojtek Dmowski a) Department of Materials Science and Engineering, The University of Tennessee, Knoxville, Tennessee 37996-2200 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 Fe 81 B 13 Si 4 C 2 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 investiga- tion 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 elas- ticity, 7,8 excellent magnetic properties, 9 and good corro- sion resistance, 10,11 among others. BMGs have addi- tional merit of near-net-shape formability. 12–14 These at- tractive properties have made BMGs the focus of intensive research among a number of groups around the world. Upon cooling through the glass-transition temperature (T g ), 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 condi- tions, such as directional heat flow, some structural an- isotropy is produced during quenching, and has been observed by structural investigations. Generally, if the crystalline inclusions are absent, annealing at high tem- peratures 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, neutron, or electron diffraction. To determine the structural function, the measured scattering intensity is processed to correct for artifacts, such as background, absorption, multiple, and inelastic scattering depending on the scattering probe, atomic scattering factor, and the detector efficiency 15 and then normalized to absolute units. The normalized structure function, S(Q), Q  4sin/, where  is the scattering angle and  denotes wavelength, is Fourier transformed to obtain pair density function (PDF),  0 g(r),  0 gr =  0 + 1 2 2 r  0  SQ - 1sinQrQdQ , (1) where g(r) is a pair distribution function, and  0 denotes the atomic density. With the exception of inelastic a) Address all correspondence to this author. e-mail: wdmowski@utk.edu DOI: 10.1557/JMR.2007.0043 J. Mater. Res., Vol. 22, No. 2, Feb 2007 © 2007 Materials Research Society 412