  Recent computational and laboratory experiments have shown that the
brittle-ductile transitions in metallic glasses such as Vitreloy1 are strongly
sensitive to the initial effective disorder (or "fictive") temperature. Glasses
with lower effective temperatures are weak and brittle; those with higher
effective temperatures are strong and ductile. The analysis of this phenomenon
presented here examines the onset of fracture at the tip of a slightly rounded
notch as predicted by the shear-transformation-zone (STZ) theory of spatially
varying plastic deformation. The central ingredient of this analysis is an
approximation for the dynamics of the plastic zone formed by stress
concentration at the notch tip. This zone first shields the tip but then breaks
down suddenly producing a discontinuous transition between brittle and ductile
failure, in agreement with the numerical and experimental observations.
