Sample size matters for Al ₈₈Fe ₇Gd ₅ metallic glass: Smaller is stronger

For metallic single crystals with dimensions in the micrometer and sub-micrometer regime, systematic studies have established that sample size has an obvious influence on the apparent strength, following a "smaller is stronger" trend. For amorphous metals, several metallic glasses (MG) appear to exhibit a similar trend, while a few others do not. Here, another MG is examined, Al ₈₈Fe ₇Gd ₅, using quantitative in situ tensile and compression tests inside electron microscopes, with sample effective diameter covering a wide range (100 nm to 3 μm). A clearly elevated strength is observed, as high as about twice the value of bulk samples, for samples with diameters approaching 100 nm. A size regime is proposed, where the strength is controlled by the nucleation of the shear band, starting from its embryonic stage: the smaller the sample size, the more difficult this nucleation becomes. The size dependence is also discussed from an energy balance perspective: the resulting simple power law fits the data as well as other published strength data for a number of MG systems.