  Traditionally, the formation of amorphous shear bands (SBs) in crystalline
materials has been undesirable, because SBs can nucleate voids and act as
precursors to fracture. They also form as a final stage of accumulated damage.
Only recently SBs were found to form in undefected crystals, where they serve
as the primary driver of plasticity without nucleating voids. Here, we have
discovered trends in materials properties that determine when amorphous shear
bands will form and whether they will drive plasticity or lead to fracture. We
have identified the materials systems that exhibit SB deformation, and by
varying the composition, we were able to switch from ductile to brittle
behavior. Our findings are based on a combination of experimental
characterization and atomistic simulations, and they provide a potential
strategy for increasing toughness of nominally brittle materials.
