应力松弛对应变诱发马氏体相变影响的有限元模拟

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

在基于新型淬火-分配-回火(Q-P-T)钢微观组织的有限元模型中,建立了产生马氏体相变的一维应变等效模型,模拟了单轴拉伸条件下的相变诱发塑性(TRIP)效应,由此揭示了该效应的微观机制.TRIP效应产生的应力松弛有效地缓解了未转变的残余奥氏体和邻近马氏体的应力,阻止了裂纹的形成,并使较多的残余奥氏体在较大的应变下存在,这是TRIP效应的起因;模拟结果还显示,相变形成的新(应变诱发)马氏体比原始(热诱发)马氏体承载更大的应力,由此预测裂纹首先在新马氏体中或其边界处形成.应力松弛效应使应变诱发马氏体断续缓慢地生成,这与实验观察结果相符.通过比较有应力松弛效应和无应力松弛效应的有限元模拟结果发现,无应力松弛效应使应变诱发马氏体相继快速地生成,这与实验不符,由此反证TRIP效应必然产生应力松弛.

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 retaiaed 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.