Short and medium range structure in elastic deformation of metallic and covalent glasses.

We present a concise methodology to analyze structural response to the applied stress in amorphous solids, including metallic glasses, glassy selenium, silica and polycarbonate, using high energy X-ray diffraction and atomic pair distribution function (PDF) analysis. To assess the structural anisotropy induced by applied axial stress, diffraction data were expanded into spherical harmonics. Using Bessel transformation, components of the structure function were converted into isotropic and anisotropic PDFs. The PDFs were compared to the expected model behavior for ideal elastic deformation to separate homogeneous affine strain from local non-affine strains. In metallic glass the range of non-affine deformation is limited to the nearest neighbor shell, suggesting local strain relaxation under stress that occurs even in the elastic regime. Beyond the second atomic shell strain is uniform. However, in glassy silica, polycarbonate and selenium strong local bonding inhibits local displacements and strain in short range order (SRO) is accommodated by rotation of local units. Interestingly, beyond a molecular unit, deformation in covalent systems is similar to metallic glasses, and response of the medium range order (MRO) scales with the macroscopic stress.