The excellent combination of high strength and low stiffness make bulk-metallic glasses (BMGs) candidate materials for many structural applications. Their fracture toughness, however, can vary markedly, and their ductility, particularly in tension/compression, is rather limited, although in bending they are quite ductile. Here, we report on a systematic study on Zr and Pd-based glasses to investigate the influence of sample size and loading condition on the fracture toughness of BMGs. Results show that with decreasing sample size the fracture behavior changes from brittle failure with low fracture toughness, via a semi-brittle failure regime, to fully ductile fracture and non-catastrophic failure with sub-critical crack growth, i.e., R-curve behavior. Our tests on samples subjected to different stress-states (three-point bending vs. tension loading) result in highly variable data which brings into question the extent of validity of nonlinear elastic fracture mechanics to characterize the toughness of BMGs.
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