Additional hardening in harmonic structured materials by strain partitioning and back stress

Deformation behavior of a harmonic structured material (HSM), core–shell 304L stainless steel, is investigated using micro-digital image correlation (micro-DIC). High strain-partitioning between core and shell is observed. Because the grain boundaries with a grain-size gradient in HSM induce high deformation-incompatibility, strain peaks are detected near core–shell boundaries and in grain boundaries of cores. This incompatibility is compensated by geometrically necessary dislocations, generating back stress. The back stress is measured using tensile unloading-reloading testing. This investigation demonstrates higher back stress and strain hardening rate in HSMs than homogeneous materials, resulting in enhanced ductility.