Bulk metallic glasses (BMGs) are an interesting class of materials noted for their wide variety of mechanical properties, associated most notably with their lack of long-range crystallographic order1. BMGs include alloys that exhibit extremely high strength in excess of most engineering materials2,3, as well as low stiffness and high elastic strain limits4,5. Because of these wide-ranging properties, BMGs are attractive alloys for future applications as they offer a potential for the development of stronger and tougher structural materials6,7,8,9.
One of the main impedances to the adoption of high-strength BMGs is their limited ductility, which can be restricted by single shear band formation and rapid propagation at low strains, which often results in catastrophic failure10,11,12,13,14. Accordingly, of key importance to further alloy development is understanding how such shear bands originate at the nanoscale because, although single shear-band formation can cause BMGs to fail at near-zero tensile ductilities, multiple shear-band formation represents the fundamental essence of plasticity in these alloys. As BMGs invariably display high strength, the creation of tensile ductility — via multiple shear banding — is thus essential to their fracture toughness, and hence damage-tolerance, in terms of their potential role as future structural materials.
Several mechanisms have been proposed for the initiation and propagation for shear bands, the predominant hypotheses being free volume softening15,16,17,18,19,20, adiabatic heating softening21,22,23, and shear transformation zones (STZs)20,24,25. Recently, with advancements in both modeling26,27,28 and experimentation29, STZ formation has emerged as the prevailing mechanism by which shear bands form and propagate10,11. In this theory, a STZ is a cluster of atoms which plastically rearranges under mechanical stress. As the stress to transform many STZs homogeneously is very high, in a real material this is hypothesized to preferentially occur at stress concentrations10,30. Once a high enough density of activated STZs have formed, a shear band develops and can propagate10,11.
To date, observing the mechanisms of BMG shear band formation, while possible in molecular dynamics (MD) simulations, has been experimentally challenging due to the high rate of the catastrophic shear band propagation and the current experimental limits of electron microscopy. However, observing shear band nucleation and dynamics at the scales possible in transmission electron microscopy (TEM) is crucial to linking our understanding of deformation mechanisms provided by MD simulations to the macroscale mechanical behavior. Previous TEM experiments in bulk metallic glasses have largely been limited to ex situ qualitative imaging studies with high enough resolution to resolve shear bands, but have difficulties in quantitative interpretation31,32,33,34, or more quantitative fluctuation electron microscopy (FEM) studies on the structure of BMGs35,36,37,38,39 that fall below the local spatial resolution needed for individual shear band characterization. In situ experiments to date have been qualitative, too slow during acquisition, hard to interpret due to a lack of understanding of the contrast mechanisms in shear bands, or at too low of a spatial resolution to be comparable to MD models40. Recent advancements in techniques and hardware have, however, allowed for the observation of strain41 and as we will show here, the evolution of locally resolved atomic short and medium range order, with nanometer resolution during in situ deformation, providing much more comparable information to the significant modeling efforts which have been performed.
In this study, we design an in situ sample to study the coupling of local atomic order and strain during tensile deformation. The BMG used in this study is a member of the model glass family CuxZryAl100−(x+y)42,43, which has been extensively studied for its high glass-forming ability44, and relative ease of computational modeling. These glasses have local clusters of atoms that pack into icosohedral structures45,46,47,48,49, which due to their two-, three- and five-fold symmetry axes in projection, have characteristic symmetric diffraction patterns50. We directly observe a change in structural order correlated with strain as measured from the NBED patterns acquired during in situ deformation.