Interface Constraints on Shear Band Patterns in Bonded Metallic Glass Films Under Microindentation

When using the bonded interface technique for indentation tests, the semicircular and radial shear bands can be observed on the top surfaces and bonded interfaces in bulk metallic glasses (BMGs). In addition to the stress relaxation effects at the bonded interface, indentation tests on bonded BMG films on the steel platen further demonstrate the effects of the film/substrate interface on shear band patterns. The understanding of these shear band patterns will help design internal constraints to confine shear bands and thus to prevent brittle failure of BMGs. In contrast to previous studies, which connect shear band directions to principal shear stress or effective stress, as in the Mohr-Coulomb model, this article adopts the Rudnick-Rice instability theory-shear bands are a result of loss of material stability but are not a yield phenomenon. Shear band directions depend on material constitutive parameters (including Poisson's ratio, coefficient of internal friction, and dilatancy factor) and principal stresses. Consequently, internal constraints such as the bonded interface and film/substrate interface may redistribute the stress fields and thus affect the shear band propagation directions. Finite element simulations were performed to determine the contact stress fields using continuum plasticity model. It is found that semicircular shear bands on the bonded interface follow the direction of the second principal stress, while radial shear band patterns depend on the two in-plane principal stresses. With the presence of film/substrate interfaces, the radial shear bands will be "reflected" at the interface, and the semicircular shear bands change directions and end at the interface. It should be noted that the actual stress field differs from the continuum plasticity simulations because of the strain localizations associated with shear bands. To this end, an explicit history of shear band nucleation and propagation is simulated by the free volume model, which reproduces the change from radial to semicircular shear bands when interface relaxation is introduced. These predictions agree well with our experimental observations of microindentation tests on two Zr-based BMG films laterally bonded and placed on a steel platen.