ABSTRACT
This chapter reports the discovery of a hitherto unknown strain hardening mechanism, which is intrinsic to the gradient structure in an engineering material. The gradient structure shows a surprising extra strain hardening along with an up-turn and subsequent good retention of strain hardening rate. Strain hardening is critical for increasing the material ductility. Cross-sectional Transmission Electron Microscopy (TEM) observations were conducted to investigate the microstructual evolution and grain size distribution in the gradient structure. TEM observations provide us with information on the evolution of dislocation structures in the nano-grains. At very small strain of 0.008, the tangled high-density dislocations are visible either in their interior or at boundaries and sub-boundaries. Due to the extra strain hardening, the gradient structure provides for an effective route to a superior combination of good ductility and high strength. When the homogeneous IF-steel is deformed to increase strength, its ductility usually drops dramatically, especially when the strength is above 400 MPa.
