  The usefulness of glasses, and particularly of metallic glasses, in
technological applications is often limited by their toughness, which is
defined as the area under the stress vs. strain curve before plastic yielding.
Recently toughness was found to increase significantly by the addition of small
concentrations of foreign atoms that act as pinning centers. We model this
phenomenon at zero temperature and quasi-static straining with randomly
positioned particles that participate in the elastic deformation but are pinned
in the non-affine return to mechanical equilibrium. We find a very strong
effect on toughness via the increase of both the shear modulus and the yield
stress as a function of the density of pinned particles. Understanding the
results calls for analyzing separately the elastic, or "Born term" and the
contributions of the "excess modes" that result from glassy disorder. Finally
we present a scaling theory that collapses the data on one universal curve as a
function of rescaled variables.
