LAUSR

Abstract Although twinning induced plasticity (TWIP) steels have achieved a satisfactory combination of high strength and large plasticity, surface nanocrystallization realizes a further improvement of yield stress in TWIP steels without sacrificing much ductility via gradient microstructures. Experimental investigations have already revealed the excellent mechanical properties and deformation mechanisms of the gradient nanostructured (GNS) TWIP steels. But the prediction and optimization of their mechanical properties are limited due to the lack of a constitutive model. Here we establish a size-dependent crystal plasticity model containing dislocation slipping and deformation twinning, which can describe the tensile response of TWIP steels with different grain sizes. After that, this model is applied to simulate the tensile deformation behavior of the GNS TWIP steel with three kinds of gradient microstructures, namely gradient grain size, dislocation density and twin fraction. The modeling predictions are in agreement with the existing experimental data. Through the analysis of deformation contours and microstructural evolutions, the intrinsic reason for the balance of strength and ductility in the GNS TWIP steel is discussed, and the contribution of each gradient microstructure is quantized. It is found that the surface gradient region containing fine grains, high densities of dislocations and twins improves the yield stress. The homogeneous region in the core helps maintain the strain hardening ability, but the gradient region has lower strain hardening ability, which causes surface notches and slight loss of the ductility. This study offers valuable insights into predicting and further optimizing the mechanical behavior of GNS materials.