On the fracture behavior of bulk metallic glasses

Abstract

High strength in combination with low stiffness, high hardness, large elastic strain limits and near net-shape castability make bulk-metallic glasses (BMGs) candidate materials for many structural applications. Major drawbacks for their use in engineering service, however, are highly variable fracture toughness values and ductilities which can be entirely different for loading in tension, compression or bending. Specifically, whereas ductility is rather limited in tension/compression, BMGs can be quite ductile in bending. Due to the often-limited dimensions of cast BMGs, standard fracture-toughness tests are generally performed on smaller-sized samples with dimensions often comparable to the critical bending thickness of a glass. This critical bending thickness is defined as the dimension below which a glass can achieve the relevant number of shear bands to demonstrate significant bending ductility. To date, however, it is not clear how BMGs would behave in fracture toughness tests evaluated on samples with dimensions that are either below, above, or comparable to a glass’s critical bending thickness. Furthermore, while fracture toughness tests are often performed with “bending” geometries (three-point bending, compact-tension specimens), it has yet to be determined how the behavior of BMGs under these constrained stress-states relates to that in tension. 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