Microplasticity behavior of multiphase high-strength nanobainitic steel based on a modified law of mixtures

Abstract

The transformation of a metastable phase plays an important role in guaranteeing the excellent property of a material. However, this transformation introduces a new phase that contributes to the mechanical behavior, thereby making the calculation of the contribution of each phase to the material's stress and strain more complex. In this study, a typical two-phase nanobainitic steel with 20.5 vol% retained austenite in the as-transformed microstructure was investigated. The retained austenite was gradually transformed into martensite during the deformation process. A modified law of mixtures was used to partition the macrodeformation behavior of each phase. A reverse analysis method based on the Koistinen-Marburger equation was used to estimate the microstress and microstrain evolution of the retained austenite. The block and film of retained austenite were considered separate phases. The detailed calculations of the parameters of each phase and their stress and strain were described in detail. Bainitic ferrite, as the major phase, contributed most to the deformation behavior, including the stress and strain. Its contribution to the stress gradually decreased with an increasing amount of martensite, and its contribution to the strain gradually increased when the retained austenite was consumed. The contribution of retained austenite focused on strain, with the blocky retained austenite contributing the most during the early deformation. The gradually formed martensite possessed an ultrahigh strength and only contributed to the stress. The coordination deformation behavior of each phase was further refined.