Shear processes in bulk metallic glasses

We investigated Newtonian and non-Newtonian flow in bulk metallic glasses by compressing cube-shape specimens that have a central hole. The hole localized deformation along well-defined shear planes, which enabled us to plastically deform the same volume of the glass specimen along differently oriented glide directions. We found that the glass has a memory of its previous plastic deformation, but this memory is largely independent of the previous glide direction. The transition from Newtonian to non-Newtonian flow is abrupt and occurs at a Deborah number, De=γ˙·τ, of 0.5, where γ˙ is the plastic shear strain rate and τ is the relaxation time for the annihilation of the flow defects. This value of De is consistent with that observed at the onset of flow instabilities in Newtonian liquids, which occurs at De ≈ 1.