ABSTRACT
This chapter reports significant hetero-deformation-induce (HDI) strengthening and strain hardening in gradient structured (GS) IF steel. The nature of plastic deformation in the gradient structure is still not very clear. In fact, the gradient structure can be approximately regarded as the integration of many thin layers with increasing grain sizes. Back stress has been reported to play a crucial role in strain hardening, strengthening and mechanical properties. It is a type of long-range stress exerted by geometrically necessary dislocations that are accumulated and piled up against barriers. Unloading-reloading was performed at varying tensile strains to investigate the evolution of HDI stress during tensile test. The GS IF steel developed strong HDI strengthening and HDI strain-hardening during tensile testing, which arise from the plastic incompatibilities due to its microstructural heterogeneity. The equation derived and the procedure proposed in this work for calculating the HDI stress from the unloading-reloading hysteresis loop produces more consistent HDI stress value than what is previously reported.
