Investigation On Multiscale Mechanical Behavior Induced By Slipping And Twinning For TWIP Steel

Twinning induced plasticity(TWIP)steel exhibits excellent strength,plasticity and formability.which meets the dual standards of high strength and high plasticity for the modern automotive steel.The deformation mechanisms of TWIP steel are dominated by slip and twinning.It is urgent to grasp the interaction of micro-mechanisms and their influence on macro-strengthening and plasticizing during plastic deformation.In order to reveal the micro-evolution characteristics of all deformation mechanisms and the macro-strengthening and plasticizing mechanism,a multiscale characterization for the dislocation and twinning of TWIP steel is taken as a breakthrough.In this dissertation,the discrete dislocation dynamics(DDD),phenomenological dislocation dynamics(DD)and crystal plasticity finite element(CPFE)methods corresponding to microscopic,meso and macro scales are developed respectively,and multiscale model with non direct coupling of DDD and CPFE is developed to systematically investigate the micro,micro and macro-scale plastic mechanical behavior of TWIP steel during deformation.The main achievements are shown as follows:The dislocation reactions at the twin boundary and grain boundary are determined and the corresponding criterias for topology reaction processes are formulated.A three-dimensional discrete dislocation dynamics(3D-DDD)model of TWIP steel polycrystal coupling twins is developed.The influence of twins on dislocation evolution and their contribution to flow stress during deformation of TWIP steel are studied quantitatively.The results show that a decomposition occurs to form twin dislocations to coordinate plastic deformation when the dislocation moves to the twin boundaries with the favorable orientation,twins have almost no effect on flow stress.The critical twin stresses of twin surface and internal source nucleation and growth are quantitatively calculated by dislocation theory coupled with integrating twin-energy pathways,and the activation evolution modes of twin source nucleation and surface source growth in TWIP steel single crystal is determined.A dislocation-density-based phenomenal dislocation dynamics constitutive model considering twinning softening effect is proposed to describe the evolution of the twin nucleation propagation and growth process.Then it is employed to reflect the deformation mechanisms of TWIP steel single crystal during plastic deformation.The results show the twinning induced softening is caused by different critical shear stress for twinning(CSST)nucleation and growth.Furthermore,different leading partial dislocations are obtained under tension and compression even with the same orientation,which results in the tension-compression asymmetry in twinning nucleation.Based on crystal plasticity theory,dislocation density is introduced to describe the effect of dislocation interaction on hardening behavior in slip resistance model,a polycrystal homogenization method is used to keep geometry coordination and stress balance between adjacent grains.A crystal plasticity constitutive model of TWIP steel coupling slip and twinning is developed based on the crystal plasticity theory.It is applied to simulate the plastic deformation process of single crystal for typical orientation microstructures under simply loading condition and predict the texture evolution under diff'erent loading conditions.The results show that when the volume fraction of twin reaches saturation value,new slip systems be activated as a result of stress dropping suddenly.In addition,with the increasing of the strain,the strain hardening phenomenon and texture density enhanced during the tensile process.Although texture types changed,texture density unchanged during the compression process.Owing to deformation increasing along the diameter direction,there is no obvious texture inside the cylinder when torsion deformation is small,texture emerged and enhanced gradually with the increasing of strain.The evolution of dislocation at twin and grain boundary during tensile process of TWIP steel is studied by 3D-DDD from micro-scale simulation.The hindrance strength of twin and grain boundary to dislocation slip is analyzed by statistical method,and the conclusion is transmitted to CPFE model.Thus,a multiscale model with non direct coupling of DDD and CPFE is developed,which is used to simulate the mechanism of plastic deformation induced by dislocation and twinning in micro region and the influence of macro strain hardening behavior in the plastic deformation of TWIP steel.The results show that the increase of twin volume fraction leads to the increase of dislocation density than the strain hardening rate curve begins to rise when dynamic recovery occurs.The smaller the grain size,the smaller the peak value of strain hardening rate.