Interphase Heterogeneous Deformation Behavior And Damage Of TRIP-Assisted Duplex Stainless Steel

Mn-N alloyed TRIP-assisted duplex stainless steel not only saves Ni but also effectively utilizes the martensitic transformation of metastable austenite to greatly enhance the plasticity and strength of the material during deformation,becoming a new low-cost and high-performance structural material with great development potential.However,as martensite transformation progresses,a complex microstructure forms from the intermixing of three phases(i.e.,austenite,ferrite,martensite)in the steel,which results in complex inhomogeneous deformation and damage behavior.The volume fraction of martensite has been dynamically evolving to make the heterogeneous deformation more complex.The heterogeneous deformation behavior is an important factor leading to damage of materials.However,the correlation between the heterogeneous deformation behavior and material damage is unclear currently.This paper takes a self-developed TRIP-assisted duplex stainless steel as the research object.The characteristics of martensitic transformation were characterized and analyzed.A martensitic transformation model of crystallography was established,and martensitic transformation simulation was achieved by using the crystal plasticity finite element model(CPFEM).The characterization parameters of heterogeneous deformation were established for the heterogeneous deformation of grains on both sides of grain and phase boundaries.The influence of martensitic transformation was clarified on the strain distribution law between phases.The mechanism of boundary damage and failure was discussed using the characterization parameters of heterogeneous deformation.In the application of micro deep drawing,the influence of organizational characteristics was explored on forming results.The mechanical properties and work hardening behavior of "three-stage" characteristics were studied by conducting uniaxial tensile tests under different loading directions and deformation temperatures.The martensitic transformation kinetics were established under different loading directions and deformation temperatures.The evolution law of microstructure and evolution characteristics of kernal average misorientation(KAM)during the martensitic transformation process were analyzed by characterizing the martensitic transformation at different deformation stages.The nucleation pattern of martensite was determined at the intersection position of two fault bands or at a single fault band.A martensitic nucleation criterion was established based on crystal plasticity theory,and a simulation of martensitic transformation was achieved using CPFEM.Comparing simulated and experimental results from three aspects: tensile stress-strain response,martensitic volume fraction evolution,and microscopic martensitic transformation location characteristics,it is shown that the martensitic transformation model and calibrated material parameters are accurate and reliable.The model was used to analyze the improvement effect of martensitic transformation evolution on deformation heterogeneity and the influence of martensitic volume fraction evolution on work hardening.At the micro-scale,based on the velocity gradient tensor and the slip amount of slip system,the deformation degree coordination parameter and slip transfer parameter were established to analyze the coordinated deformation behavior of grains on both sides of grain and phase boundaries.The effect of martensitic transformation on the strain distribution was investigated by comparing crystal plasticity finite element simulation with in-situ tensile experiments,and the influence of loading direction on the strain partitioned behavior of various phases was discussed.The distribution characteristics of microcracks in the sample under large deformation were investigated.It was found that the interface between martensite and ferrite was the main location for microcrack nucleation.Herein,three types of microcrack nucleation locations were statistically categorized: the microcrack nucleation is located at the intersection of the three phases of martensite,ferrite,and austenite,at the intersection of martensite/ferrite phase boundary and ferrite grain boundary,and at the intersection of the martensite/ferrite phase boundary and the original austenite grain boundary.Microcracks might propagate along the original austenite/ferrite phase boundary or ferrite grain boundary with a smaller angle to the phase boundary.The behavior characteristics of damage(microcracks nucleation and propagation)were analyzed by analyzing the deformation degree coordination parameters and slip transfer parameters of different boundaries.A micro-deep drawing experiment for cylindrical cups was designed and implemented.By comparing the simulated and experimental results,based on the analysis of deformation heterogeneity and the research on the damage and failure of TRIP-assisted duplex stainless steel,it was found that there was a significant risk of crack failure in the TD direction of the cylindrical cups(the drawing direction is perpendicular to the direction of banded structure).It was pointed out that the deformation heterogeneity caused by organizational distribution characteristics was the main factor affecting the shape and damage of the formed parts.