Shear Bands in Metallic Glasses: Atomic Transport, Propagation – and Relaxation Behavior

Abstract

Plastic deformation of metallic glasses, if exerted at low homologous temperatures with respect to the glass transition temperature, is mostly localized in plate-like mesoscopic defects, so-called shear bands. Although the occurrence of shear bands is well known and often determines the mechanical performance of the material, their actual physical properties remain fairly unknown. Additionally, it is widely accepted that localized regions, so-called shear transformation zones, undergo plastic yielding through a shear transformation at low strains. Yet, how those localized regions are distributed, in what way they are structurally distinct from the surrounding matrix and how they couple to evolve into shear bands with macroscopic lengths at higher strains is also unclear. This contribution addresses those coupled questions and issues by combining complementing experimental methods and – for selected situations – results from atomistic simulations [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]. Here, experimental data on the rate of atomic diffusion within shear bands have been obtained using the radiotracer method on post-deformed specimens. Those measurements that had been performed at temperatures below Tg on samples after they underwent plastic deformation, showed a drastic increase of the atomic mobility inside the shear bands. In fact, as indicated in Fig. 1, the diffusion coefficient inside the shear bands was found to be more than six orders of magnitude larger than the diffusion coefficient of the undeformed matrix of the same material. Open image in new window Fig. 1. Diffusion coefficients of radioactive Ag and Au isotopes in Pd40Ni40P20 bulk metallic glass. The diffusion coefficients in the shear band (blue) were measured after deformation by cold rolling. Remarkably, the activation enthalpy for diffusion inside the shear bands amounts to only one third of the activation enthalpy for bulk diffusion. This large difference indicates large modifications of the atomic structure inside the shear bands.