Mechanical Response Characteristics And Microscopic Mechanisms Of TRIP-Assisted Duplex Stainless Steel Under Cyclic Loading

TRIP-assisted duplex stainless steel,which is fabricated by replacing Ni with Mn and N,exhibits a good combination of low costs and excellent properties,leading to wide application prospects in engineering construction,rail transit and other fields.These key components show obvious cyclic plastic deformation characteristics in the stamping and seismic loading process.However,previous studies on TRIP-assisted duplex stainless steel mainly focused on the mechanical properties and microscopic mechanisms under monotonic tension,and paid little attention to the plastic deformation characteristics under cyclic loading.The linkage among deformation,microstructure and property is still unclear.In this paper,the monotonic tensile and symmetric cyclic(with the strain amplitudes of 0.3%≤ε_a≤2.0%)loading tests of a TRIP-assisted duplex stainless steel were carried out,the mechanical response characteristics were investigated from various perspectives.The microstructure evolution under cyclic loading,as well as its relation and difference with monotonic loading,were also systematically analyzed by means of EBSD,TEM,XRD,and magnetic measurement.The correlation mechanism among deformation,microstructure and properties was elucidated through considering the essential characteristics of TRIP-assisted duplex stainless steel(e.g.,multiphase,metastable).Based on the plastic response under cyclic loading,the cyclic plasticity partitioning and deformation coordination between constituent phases were clarified,the action mechanism was revealed.The kinetics model of martensitic transformation and the cyclic constitutive model considering martensitic transformation were established.The main results are as follows:(1)The results from thermodynamic calculation and microstructure observation showed that the austenite phase is metastable in the test steel,in which martensitic transformation ofγ→ε→α’andγ→α’can occur during monotonic tensile and compressive deformation,leading to"three-stage"work hardening behavior and excellent comprehensive mechanical properties.However,there were obvious differences in some aspects under tensile and compressive conditions,including the rotation mode of grain orientation,the acting mechanism and the kinetics law ofα′martensitic transformation.The combination of these mechanisms led to the asymmetric characteristics of tensile and compressive mechanical behavior.(2)The cyclic mechanical response characteristics of the test steel were investigated involving the aspects of cyclic hysteresis properties,cyclic softening/hardening response and cyclic stress-strain behavior,which were found to be sensitive to strain amplitude.Combined with microstructure observations,it was found that planar slipping occurred in both ferrite and austenite phases during the initial loading cycles,leading to initial cyclic hardening of test steel.With the increase of cyclic number or strain amplitude,dislocation rearrangement occurred in the ferrite,leading to cyclic softening of test steel.While martensitic transformation gradually occurred in austenite,leading to abnormal mechanical responses compared with conventional duplex stainless steels.Meanwhile,the orientation of ferritic grain changed from cubic to rotating cubic and rotating Goss,while the orientation of austenitic grain was gradually randomized.The introduction of tensile predeformation promoted the development of dislocation rearrangement and martensitic transformation during subsequent cyclic deformation,and preserved the tensile texture in ferrite and austenite after cyclic deformation.At the same time,weakened initial cyclic hardening and enhanced cyclic softening were shown.(3)The cyclic deformation coordination between austenite and ferrite was studied based on the statistical analysis of hysteresis loops,the stage characteristics of cyclic strain hardening behavior and the distribution characteristics of local misorientation.With the increase of cyclic number or strain amplitude,ferrite gradually carried more cyclic plasticity than austenite,resulting in the cyclic deformation of the two phases from coordinated to uncoordinated,and theα/γinterface to be the main nucleation site of fatigue crack.Substructure observations showed that martensitic transformation can influence the deformation coordination by changing the cyclic plasticity partitioned into ferrite phase,and the occurrence ofαˊmartensite can promote the nucleation of fatigue crack inside theαˊmartensite phase and atαˊ/γinterface.The plasticity partitioning difference between the two phases increased with the increase of prestrain level,leading to uncoordinated deformation from the initial cyclic deformation.(4)A unified kinetics model of martensitic transformation under cyclic loading was established through regression calculation.The cyclic constitutive model considering martensitic transformation was established by combining the transformation-modified Voce isotropic strengthening model and the Chaboche kinetics strengthening model.This model can well simulate the stress/strain response of test steel under different strain amplitudes studied.