Shear instability in heterogeneous nanolayered Cu/Zr composites

Ultrastrong nanolayered metallic composites are usually subjected to low ductility due to plastic instability during deformation. Here we investigated the shear instability of a newly designed heterogeneous nanolayered Cu/Zr composites by microindentation. The heterogeneity in size was generated by inserting a few thin Cu-Zr bilayers with an individual layer thickness of 2.5 - 10 nm into the interface region of the Cu/Zr layered composites with an individual layer thickness of 100 nm. The microindentation tests showed that multiple shear bands appeared in the heterogeneous composite with one bilayer, whereas only a single shear band was formed in that with two or three bilayers. Most importantly, the layer strain in the multi-shear band region is much smaller than that in the single-shear band area. For example, the strain of the 100 nm layers within the shear band in the composite with one 10 nm bilayer could reach as low as 2.8, which was less than half of that in the composite with three 10 nm bilayers, i.e., 6.1. These findings demonstrated that strain delocalization can be achieved through shear band multiplication if an appropriate number of thin bilayers were used as interlayers in the 100 nm Cu/Zr composites. Besides, compared with the homogeneous composite with an individual layer thickness of 100 nm and the bimodal composite which is composed of alternating one 100 nm Cu-Zr bilayer and two 10 nm Cu-Zr bilayers, the heterogeneous composite with one bilayer displayed a higher strength (2.15 GPa) and a favorable resistance to strain localization.