Study of the influence of δ-ferrite on the TRIP effect of a low-density high aluminum steel by using full-field crystal plasticity modeling

Abstract

Low-density high aluminum transformation induced plasticity (TRIP) steel is a new developed light-weight structural material. In the conventional casting (CC) TRIP steel, however, the presence of high Al content results in the formation of coarse dendritic δ-ferrite, which seriously deteriorates the performance of the material. The spray forming process show good potential to refine and homogenize the microstructure. It was found that the δ-ferrite in the spray formed (SF) TRIP steel is refined remarkably and its distribution is more homogeneous. Uniaxial tension tests demonstrate that the ductility of the SF TRIP steel is increased by 5% compared with the CC TRIP steel. The difference of their properties is mainly due to the different morphologies of the δ-ferrite. We further employed a full-field crystal plasticity (CP) modeling to probe the micromechanical mechanisms in details, and a new simplified model was proposed to describe the TRIP effect and phase transformation. The material parameters of three individual phases δ-ferrite (δ), austenite (γ), and martensite (α′) for the CP model were identified from the loading curves of nanoindentation tests. Through the simulation, it was found that compared with SF TRIP steel, there are large deformation bands formed in the CC TRIP steel during the deformation. These areas are the regions in which austenite has the priority to transform into martensite. It is the initial heterogeneity of deformation in the CC TRIP steel that results in the heterogeneous phase transformation, and thus intensifies the heterogeneous deformation at the phase boundaries, which ultimately accelerates the generation of cracks.