Observation of shear band formation in nanocrystalline PdaAu alloy during in situ SEM compression testing

Understanding the deformation behavior of nanocrystalline (nc) materials is important because of their possible application as structural materials. The investigation of macroscopic nc samples during in situ mechanical tests can shed light on operating deformation mechanisms. For example, observations during a compression test in a scanning electron microscope (SEM) or results obtained by more localized methods like transmission electron microscopy can extend our knowledge on this subject. In this study, we present the results of in situ SEM compression tests on nc Pda10 at.% Au alloy with a mean grain size of 23 nm produced using the combination of inert gas condensation with subsequent high-pressure torsion. We show that plastic flow in this material is very inhomogeneous on both the macro and the mesoscale as it is localized in one area of the compression sample and within shear bands. The formation of shear bands is accompanied by strain softening. We propose that such behavior can be explained by the activation of grain boundary mediated deformation mechanisms instead of dislocation-based plasticity. This conclusion was supported by in situ synchrotron XRD measurements which revealed that no preferential grain orientation is forming during the compression of identically prepared samples of the same material.