Microstructure-based Finite Element Simulation Of Strain-induced Martensitic Transformation

In the21stcentury, steels are still the irreplaceable materials whichplay very important role in human life. However, due to the urgent needof saving energy and emission reduction and environmental protection,developing advanced high strength steels has become a main tendency.However, the strength and ductility of these steels are usually mutuallyexcluding, that is, ductility decreases with raising strength. Therefore, howto improve the ductility of advanced high strength steels (AHSSs) becomesa vital issue that we face in developing new generation AHSSs. SinceZackay proposed the concept of Transformation Induced Plasticity (TRIP)in1976, so many researches on it has been conducted so far. However, themechanism of TRIP effect can be only qualitatively explained, and has notbeen experimentally and theoretically verified so far. Because experimentscan hardly be performed to verify the quantitative contributions of bothstress relaxation and stress redistribution from martensitic transformationto plasticity. Thus, computer simulation becomes an important fool toreveal the micro-mechanism of TRIP effect. In this study, we simulated theeffects of stress relaxation and stress-free relaxation respectively onstrain-induced martensitic transformation and marco-plasticity.In recent years, finite element analysis has been applied to theresearch of multiphase steels. Finite element analysis is to use mathematicalapproximation method to simulate the real physical system. In this article, afinite model was developed based on the actual microstructure of a novelquenching-partitioning-tempering (Q-P-T) steel obtained from electronbackscattering diffraction (EBSD). Moreover, a one-dimensional strain equivalent model was built to quantitatively introduce the effect of stressrelaxation from the strain-induced martensitic transformation. With themodel, the TRIP effect under the condition of uniaxial tension wassimulated successfully. Therefore, we develop the finite element model inwhich stress relaxation effect is introduced.The microstructure-based simulation results reveal themicro-mechanism of TRIP effect. Stress relaxation from TRIP relieves thestress of both the untransformed retained austenite and its adjacentmartensite and blocks the formation of cracks, meanwhile, a considerableaustenite still exists at a higher strain level, which is the origin of TRIPeffect. Compared to original (thermal-induced) martensite, fresh(strain-induced) martensite bears higher stress. Therefore, it is predicted thatcracks form at first in fresh martensite or its boundaries. Moreover, stressrelaxation make strain-induced martensite form in intermittent and slowway, which is consistent with experimental results. However, in stress-freerelaxation state fresh martensite appears in successive and quick way, whichis not consistent with experiments, and thus this verifies in opposite waythat TRIP effect inevitably produces stress relaxation. Moreover, thecomparison of effects of stress relaxation and stress-free relaxation onmacroscopic responses of the representative volume element (RVE)quantitatively shows that the contribution of stress relaxation tomacro-plasticity. Ultimately, the simulation result verifies that the stressrelaxation from strain-induced martensite is of significant mechanism oftransformation induced plasticity.