FINITE ELEMENT SIMULATION OF THE EFFECT OF STRESS RELAXATION ON STRAIN-INDUCED MARTENSITIC TRANSFORMATION

Near 50 years ago,transformation induced plasticity(TRIP) effect was proposed and TRIP steels as an advanced high strength one are widely investigated.However,the mechanism of TRIP effect can be only qualitatively explained,and has not been experimentally and theoretically verified so far.In this work,a strain equivalent model for strain-induced martensitic transformation was built in a microstructure-based finite element model of novel quenching-partitioning-tempering(Q-P-T) steel.With the model,the TRIP effect under the condition of uniaxial tension was simulated,from which the micro-mechanism of TRIP effect is revealed.Stress relaxation from TRIP relieves the stresses within untransformed retained austenite and its adjacent martensite and blocks the formation of cracks,meanwhile,a considerable retained austenite still exists at higher strain level,which is the origin of TRIP effect.Compared with original(thermal-induced) martensite,fresh(strain-induced) martensite bears higher stress.Therefore,it could be predicted that cracks form at first in fresh martensite or its boundaries.Moreover,stress relaxation makes strain-induced martensite formed in intermittent and slow way,and this is consistent with experimental results.However,in stress-free relaxation state fresh martensite appears in successive and quick way,not consistent with experiments,and thus this verifies in opposite way that TRIP effect inevitably produces stress relaxation.