2, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Lack of ductility is one of the major shortcomings of bulk metallic glasses which hamper their wide application as structural material. Ductility is a complex mechanical property which is difficult to characterize precisely. In a sense metallic glass is always microscopically ductile, because applied shear stress can locally liquefy glass. But it has no work-hardening, thus often local yielding results in catastrophic shear failure. In order to achieve macroscopic ductility glass must be able to relax local stress concentration before it starts macroscopic shear band or crack. In our view the key is the residual liquidity in glass. The structure of supercooled liquid is heterogeneous, and the frozen-in structure at the glass transition contains weak liquid-like and strong solid-like regions. It is difficult to assess such heterogeneity directly from the structure itself, but it is possible to characterize it through the structural response to applied stress. We determined the anisotropic pair-density function (PDF) of various metallic glass samples under uniaxial stress by high-energy x-ray diffraction using the spherical harmonics expansion of the structure function S(Q) and the PDF. The measured anisotropic PDF at large distances agrees with the one expected for affine (uniform) deformation which determines the long-range strain e∞. However, at short distances it deviates from the affine deformation, and at the first neighbour the local strain, e1, is smaller than e∞. The deviation from the affine deformation occurs because of local liquid-like regions, so that the ratio G = e0/e∞ , or ΔG = 1 - G, characterizes the strength of residual liquidity in glass. We found that the ratio e0/e∞ is closely related to ductility. In particular, Gc = 0.77 is the threshold which separates brittle and ductile behaviors. If G > Gc the samples are brittle, whereas if G < Gc the samples are ductile. Thus we suggest that the percolation of the liquid-like regions results in ductile behaviour. This new parameter is compared to other criteria for ductility.