Hierarchical strain band formation and mechanical behavior of a heterostructured dual-phase material

Dispersive strain bands have been reported as a characteristic deformation feature of heterostructured materials, which helps to improve ductility. However, their formation mechanism is still not well understood. Here we report the formation of dispersed strain bands through dual-level hierarchical strain banding and its effect on the mechanical behavior of a heterostructured Fe-40Cu model material. Specifically, deformation started by the formation and propagation of dispersed microscale strain bands in the heterostructured Fe-40Cu material. High strain gradient was generated within the microscale strain bands during their propagation and was accommodated by the accumulation of geometrically necessary dislocations (GNDs). The dispersed microscale strain bands were not uniformly distributed, but instead grouped together to form macroscale strain bands that were uniformly distributed over the entire gage section to accommodate the majority of the applied strain. The formation of this dual-level hierarchical strain bands prevented the formation of large strain localization to fail the sample prematurely. It was also found that increasing the strain hardening capacity of soft copper zones provides more room for the accumulation of GNDs, resulting in higher constraint to microscale strain band propagation and consequently higher ductility. These observations suggest the possibility of tailoring microscale strain bands to optimize tensile performance of heterostructured materials.