Crystal Plasticity Modeling Of Plastic Deformation Mechanism For High Manganese Steel And Its Application

TWIP steel has made full use of Twinning/Transformation Induced Plasticity(TWIP/TRIP)effect to realize the characteristics of enhanced strength and enlarged plasticity,which was induced by slip,twinning and transformation.It is urgent to grasp the coupled mechanisms and the coordination of TWIP and TRIP effects in order to realize the high strength and perfect ductility.In this dissertation,a crystal plasticity constitutive model considering TWIP/TRIP effect will be proposed to analyzed the competition and coupling of different mechanisms and its influence on macroscopic response.An application consists of micro deep drawing will be conducted at microscopic level to investigate the earing evolution.The main research of this dissertation are as follows:Through the microstructural characterization experiments of twinning induced plastic deformation behavior of the Fe-Mn-Si-Al TWIP steel,the evolution of twinning during plastic deformation process was revealed and the location of twin lamellae were nucleated at HAGBs.According to the analysis of the texture components stability,it is founded that twinning had a great effect on Cu-and Brass-type textures.In view of the coexistence mechanism of TWIP and TRIP effects,a crystal plasticity based model considering slip,twinning and transformation was proposed,The crystallographic structure and kinetics of martensitic transformation were considered and introduced into plastic deformation gradient.The activation of transformation systems are judged by the martensitic transformation force and the flow rule of tranformation was determined by the martensitic volume fraction.The strain hardening behavior was reproduced by this proposed model.Then the dominated effect of martensitic transformation on the strain hardening are revealed and it was noted that the martensitic tranformation induced the stress drop first and then caused by twinning.According to the deformation characteristics of strain hardening under large plastic deformation induced by TWIP effect,an expanded hardening law embedded into crystal plasticity model has been established.The evolutions of slip resistance initial hardening,twinning reorientation and secondary slip hardening had been proposed.The staged strain hardening behavior was reproduced by this model.It is founded that the dominated hardening mechanism of initial hardening stage is determined by the slip and twinning increments,the value of twin volume fraction induced the sequences of stress drop and the activation of slip systems in twinned region caused the hardening strengthening of secondary slip stage.On the basis of the effects of grain and geometry size on plastic slip,a crystal plasticity based model coupling size effect for micro forming was proposed through the correlation of the size scaling factor and slip resistance.By using this model,the size effect and the orientation effect of the earing evolution for the micro parts during the micro deep drawing experiment of the FCC-type TWIP steel sheet was investigated.It is founded that the earing profile is related to the size scaling factor positively.And the influence of Goss orientation on earing height was larger than that of Brass and Cubic orientations.And the symmetric earing profile was caused by the symmetric distributions of activated slip systems.