  Micro- or minor alloying of metallic glasses is of technological interest. An
originally ductile Pd-based monolithic bulk metallic glass
(Pd$_{40}$Ni$_{40}$P$_{20}$) was selectively manipulated by additions of Fe or
Co. The alloying effects were extreme, showing either exceptional ductility
upon Co addition or immediate catastrophic failure upon Fe addition when tested
under uniaxial compression or 3-point bending. The amorphous structure was
characterized prior to deformation with respect to its medium-range order (MRO)
using variable resolution fluctuation electron microscopy (VR-FEM). We observe
striking differences in the MRO between the ductile and brittle metallic
glasses, with the ductile glasses exhibiting a rich structural diversity and
MRO correlation lengths up to 6 nm. The MRO heterogeneity seems to enable
easier shear banding and hence enhance the deformability.
