Mechanics of accelerated strain hardening in harmonic-structure materials

Harmonic-structure (HS) design is one of the most efficient in the family of architected gradient-structure materials recently attracting increasing attention in global material science community due to leading structural performance characteristics. This work studies in situ the mechanics of plastic deformation in HS materials on the example of commercially pure nickel (Ni) during uniaxial tensile loading. It reveals that strain partitioning between ultrafine and coarse grain fractions in HS Ni commences already at early stages of plastic deformation. Slow accumulation of strain in the ultrafine-grain regions leads to the slow consumption of ductility resource, while accelerated accumulation of strain in the coarse-grain regions leads to an accelerated strain hardening. These effects combined with evolving variations of strain tensor components have synergetic effect leading to the unique plastic behaviour and excellent structural performance of HS materials.