Brittle-to-Ductile Transition in Metallic Glass Nanowires

When reducing the size of metallic
glass samples down to the nanoscale regime, experimental studies on
the plasticity under uniaxial tension show a wide range of failure
modes ranging from brittle to ductile ones. Simulations on the deformation
behavior of nanoscaled metallic glasses report an unusual extended
strain softening and are not able to reproduce the brittle-like fracture
deformation as found in experiments. Using large-scale molecular dynamics
simulations we provide an atomistic understanding of the deformation
mechanisms of metallic glass nanowires and differentiate the extrinsic
size effects and aspect ratio contribution to plasticity. A model
for predicting the critical nanowire aspect ratio for the ductile-to-brittle
transition is developed. Furthermore, the structure of brittle nanowires
can be tuned to a softer phase characterized by a defective short-range
order and an excess free volume upon systematic structural rejuvenation,
leading to enhanced tensile ductility. The presented results shed
light on the fundamental deformation mechanisms of nanoscaled metallic
glasses and demarcate ductile and catastrophic failure.