Micromechanical behavior of TRIP-assisted multiphase steels studied with in situ high-energy X-ray diffraction

The stress partitions among multiple phases for two cold-rolled transformation-induced-plasticity (TRIP)-assisted C-Mn-Al-Si steels, with different carbon content levels of 0.1% and 0.2% (mass fractions), were investigated using in situ high-energy X-ray diffraction at ambient and low temperatures (-40 degrees C) under uniaxial tensile loading. Based on the evolution of stresses for various phases during plastic deformation, a modified constitutive model was established for describing the micromechanical behavior of TRIP-assisted multiphase steels, based on a Gladman-type mixture law (GTML) embedded with the Mecking-Kocks work-hardening formula. The index n, an important parameter in the GTML for characterizing the accommodation of loading stresses among different phases, was determined to be severely affected by the transformation kinetics of retained austenite. The quantitative relationship between n and the transformation rate of retained austenite was also clearly revealed in the investigated alloys. The modified model thus correlates the mismatch of stresses for multiple phases with the transformation kinetics of metastable phases during plastic deformation and is suitable for all advanced steels with multi-scale microstructures. This model forms the basis for microstructure-based numerical simulations of micromechanical behavior and greatly benefits the design of a new type of high-strength and high-plasticity steel. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.