  Shear banding, or localization of intense strains along narrow bands, is a
plastic instability in solids with important implications for material failure
in a wide range of materials and across length-scales. In this paper, we report
on a series of experiments on the nucleation of single isolated shear bands in
three model alloys. Nucleation kinetics of isolated bands and characteristic
stresses are studied using high-speed in situ imaging and parallel force
measurements. The results demonstrate the existence of a critical shear stress
required for band nucleation. The nucleation stress bears little dependence on
the normal stress and is proportional to the shear modulus. These properties
are quite akin to those governing the onset of dislocation slip in crystalline
solids. A change in the flow mode from shear banding to homogeneous plastic
flow occurs at stress levels below the nucleation stress. Phase diagrams
delineating the strain, strain rate and temperature domains where these two
contrasting flow modes occur are presented. Our work enables interpretation of
shear band nucleation as a crystal lattice instability due to (stress-assisted)
breakdown of dislocation barriers, with quantitative experimental support in
terms of stresses and the activation energy.
