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Structural anisotropy in metallic glasses induced by mechanical deformation

Wojtek Dmowski 1,  Takeshi Egami 1,2

1. University of Tennessee (UTK), Knoxville, TN, United States
2. Oak Ridge National Laboratory (ORNL), One bethel Valley Road, Oak Ridge, TN 37932, United States

Abstract
Advanced X-ray facilities allow use of the pair distribution function to study new aspects of local atomic structure. The anisotropy in a metallic glass is usually ignored because it is small and difficult to measure. However, the use of an area detector and high flux/high energy X-ray sources makes such studies practical. In particular it is interesting to examine structural changes induced by a mechanical deformation. There is general consensus that glass deformation must be accompanied by some local rearrangement of atoms to accommodate shear strain. However, disordered nature of a glass and small deformation volumes make it difficult to observe such atomic rearrangement experimentally. In addition the elastic strain induces structural anisotropy. Consequently, spherically averaged Fourier transformation cannot be used to obtain the pair distribution function, and for example, strain-stress analysis becomes confounded. However it is feasible, in case of high symmetry deformation, to perform expansion in terms of the spherical harmonics of both structure and pair distribution functions, whose anisotropic components are now related by the spherical Bessel transformation. We studied in-situ structure of a glass under macroscopic external stress in an elastic regime and during, and after homogenous plastic deformation (high temperature creep). The experiment was carried out at APS using high energy X-ray setup (~ 100 keV). We examined structural anisotropy and processed the data using the expansion in terms of Legendre polynomials ( l=2, uniaxial anisotropy). We found that mechanical deformation involved rearrangement in clusters of atoms by local bond exchange. Such events are the atomistic mechanism of anelastic and plastic deformation and supports structural anisotropy in the deformed state. Figure 1 shows evidence of a structural anisotropy after the creep deformation.

This work was supported by the U.S. DOE under DE-AC05-00OR-22725.

 

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Presentation: Poster at 11th European Powder Diffraction Conference, Poster session, by Wojtek Dmowski
See On-line Journal of 11th European Powder Diffraction Conference

Submitted: 2008-04-30 22:44
Revised:   2009-06-07 00:48