The size reduction of metallic glasses into nanoscale has been proposed as a promising route to overcome limited plasticity of metallic glasses and to develop novel nanostructured materials with exceptional mechanical properties. However, a lack of comprehensive understanding about mechanical behavior of nanoscale metallic glasses is attenuating both academic and industrial values of metallic glasses. Hence, this work aimed at addressing remaining issues on size effect and providing a basic guideline for designing desirable nanomechanical behaviors. With these ends in view, the mechanical response of metallic glass particles was thoroughly analyzed to clarify the effect of size on the mechanical behavior of metallic glasses and to investigate the effect of intrinsic and extrinsic factors on nanomechanical behaviors. To address the effect of size on not only mechanical properties but also shear avalanches which require uniform sample shapes and large plastic deformation range, metallic glass specimens examined in this study were prepared into spherical shape via gas atomization to have wide diameter ranges (300 nm to 4 μm). A Pd42.5Cu30Ni7.5P20 MG (Pd-MG), with outstanding glass forming ability and thermal stability, is chosen as a model system. Mechanical properties of Pd-MG particles were measured based on contact mechanics and sample size dependences of each mechanical property are confirmed. This finding indicates that conflicting results reported in regard to the size dependence of yield strength may have been originated from extrinsic factors that obscure size effects such as dissimilarities in the shape of nanoscale MG specimens. Moreover, in order to investigate the effect of size on shear avalanches of Pd-MG particles, a statistical analysis on the size distribution of strain bursts is carried out. The complementary cumulative distribution functions (CCDFs) of strain burst sizes revealed that strain localization is weakened and self-organized critical behavior extends up to larger strain bursts upon sample size reduction. The results demonstrate that the self-organized critical behavior of plastic carriers (shear transformation zones or shear bands) is strengthened with the decrease of sample size. Then, to elucidate the origin of inhomogeneous deformation of Pd-MG particles and to figure out intrinsic properties which have influences on nanomechanical behaviors, the deformation map for nanoscale Pd-MG is constructed. The map suggests that Pd-MG particles deform inhomogeneously at room temperature because critical boundaries which involve in the determination of deformation behavior intersect at a diameter range near the diameter of the smallest particle tested. Moreover, notable correlations between nanomechanical behaviors and intrinsic properties are found based on deformation map: (1) normalized critical stress curves for inhomogeneous deformation rely more heavily on sample size when Poisson’s ratio is large, (2) iso-viscosity contour locates at lower stress level when activation energy is small and shear transformation zone (STZ) volume is large, and thus, it depends on Poisson’s ratio, glass transition temperature, and elastic modulus. The findings demonstrate that not only Poisson’s ratio but also glass transition temperature and elastic modulus are controlling factors for nanomechanical behaviors. Another fruitful insight that the deformation map provides is that deformation behavior of nanoscale MGs can be tuned by manipulating extrinsic factors that can shift critical boundaries. From this point of view, the potential of electron beam irradiation as a candidate for extrinsic controlling factors is investigated via in situ compression tests inside a TEM with a high accelerating voltage of 300 keV. The result reveals that the deformation behavior of nanoscale MGs can change from inhomogeneous deformation to homogeneous deformation with viscosity value comparable to that of supercooled liquid under external stress with the aid of electron beam irradiation. To sum up, controversial issues regarding the size dependence of mechanical properties and shear avalanches are addressed through unprecedented systematic investigations on the inhomogeneous deformation behavior of MG particles. Moreover, intrinsic and extrinsic factors influencing the deformation behavior of nanoscale MGs are figured out based on the deformation map for nanoscale MGs. These results suggest that the tailor-made design of nanoscale MGs with desirable mechanical behaviors can be practicable by manipulating intrinsic and extrinsic factors.
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Analysis on inhomogeneous deformation behavior of metallic glass particle
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