  Understanding ductility or brittleness of monolithic bulk metallic glasses
(BMGs) requires detailed knowledge of the amorphous structure. The medium range
order (MRO) of ductile Pd$_{40}$Ni$_{40}$P$_{20}$ and brittle
Zr$_{52.5}$Cu$_{17.9}$Ni$_{14.6}$Al$_{10}$Ti$_5$ (Vit105) was characterized
prior to and after notched 3-point bending tests using variable-resolution
fuctuation electron microscopy. Here we show the presence of a second larger
MRO correlation length in the ductile material which is not present in the
brittle material. A comparison with literature suggests that the larger
correlation length accounts for larger shear transformation zones (STZs) which
increase the heterogeneity. This enables an easier shear band formation and
thus explains the difference in deformability.
