Size effect on the subsequent yield and hardening behavior of metal foil

Abstract

With the growing demand for miniaturized and integrated products, the metal foils have been extensively applied in the field of aerospace, electronics, medical equipment, and microsystem technology. However, the deformation process is usually under the complex stress status for the fabrication of the micropart. In addition, size effect dramatically impact the microforming process and therefore the well-developed metal forming theory established at macroscale is inapplicable at the grain level. To further investigate the size effect on the subsequent yield and hardening behavior, the biaxial tests of stainless-steel foils with diverse grain sizes and thicknesses were conducted under different stress states. The initial and subsequent yield loci are computed under various loading histories according to the plastic work per unit volume principle. It is found that the evolution of yield loci from initial to the subsequent state shows a remarkable difference for metal foils with different grain sizes. Based on Yld2000-2d yield criterion, the theoretical yield loci based on three different hardening models, including isotropic, kinetic, and isotropic-kinetic mixed hardening models, were acquired to explore and evaluate their applicability at microscale. Results indicated that the isotropic-kinetic mixed hardening model can better describe the evolution of subsequent yield locus than the individual hardening model. In addition, the back stress is shifted down with the increase of grain size for the foils with a certain thickness. Based on the Taylor-Bishop-Hill (TBH) polycrystal plasticity model, the evolution of subsequent yield surface is interpreted by the transformation of grain orientation.