Multiscale Investigation On Mechanical Behavior And Microstructure Evolution Of TWIP Steel Under Dynamic Loading Conditions

Modern automobile puts forward higher requirements for lightweight,energy saving and safety.Twinning induced plasticity(TWIP)steel is a new generation of automobile steel with high strength,high plasticity and high strain hardening properties,and its strong plastic product can reach 50 GPa·%.Automotive steel needs not only high strength to maintain the overall body weight and excellent fracture toughness to achieve structural stability,but also high impact resistance to absorb the impact energy during body collision.It is urgent to grasp the mechanical behavior and microstructure evolution characteristics of TWIP steel under dynamic loading.Therefore,in this dissertation,based on the macro and micro experiments,the discrete dislocation dynamics(DDD),the multiscale model through the coupling of discrete dislocation dynamics and finite element(FE)and the dynamic physically-based model are developed,and they deeply study the strain rate related mechanical behavior and microstructure evolution laws of TWIP steel.The main work of this paper is as follows:A controllable strain fixture paired with split Hopkinson device is designed to realize the dynamic loading experiment of TWIP steel with the sequential strains.Combined with quasi-static tensile test,scanning electron microscope(SEM)and electron backscatter diffraction(EBSD),the macroscopic mechanical behavior and microstructure characteristics of TWIP steel in the range of large strain rate of 0.001-3000 s-1,interrupted strain of 0.1,0.2 and 0.3 and fracture were studied and analyzed.The results show that the flow stress of TWIP steel under dynamic loading is significantly higher than that under quasi-static loading.The strain rate sensitivity of flow stress changes at about 100 s-1 in yield stage,and a negative strain hardening rate sensitivity occurs in the later stage of deformation;Twinning is more active at high strain rate,which is characterized by smaller twinning strain,higher proportion of twin boundary,higher geometric necessary dislocation density,more uniform dislocation distribution and stronger twin dislocation interaction,so that the strain hardening rate in the initial stage of dynamic loading is higher than that of quasi-static loading;The weakening of twin related hardening behavior and the resulting negative strain hardening rate sensitivity are attributed to the decrease of twin volume fraction growth rate in the later stage of deformation.A DDD model describing the dynamic evolution of twins of TWIP steel single crystal micropillars in the plastic deformation process is established.By analyzing the change of free energy of two deformation twin nucleation modes before and after twinning,the stress and geometric criteria of partial dislocation from the surface emission and internal decomposition are introduced into the DDD model.The model can accurately describe the stress response of single crystal micropillars experiment,reproduce the evolution of twin morphology and capture the transformation of twin orientation.The results show that internal sources with lower density,closer spacing and more uneven distribution will induce fewer twin nucleation sites,and closer twin spacing to merge via the surface sources used for twin thickening,which are responsible for the larger average twin thickness and higher flow stress.The analytical model depicts the competition between new twin activation and near-twin merging for the evolution of twin thickness based on the kinetics of the newly formed twin embryos.Based on the superposition principle,a multiscale model of DDD and FE coupling in the polycrystalline plastic deformation of TWIP steel is established.The model describes the nucleation,slip,interaction and annihilation mechanism of dislocations.The polycrystalline model with grain boundary is introduced through Voronoi polygon generation algorithm.The grain boundary can describe not only the obstruction for dislocations,but also the dislocation transfers across grain boundary driven by stress concentration,Further considering the time scale dependent dislocation source activation mechanism and the viscous damping mechanism of dislocation movement,the strengthening mechanism of grain boundary under dynamic loading is analyzed.The results show that in the strain rate range of 102-104 s-1,the strain rate sensitivity of yield strength changes from low to high,the grain size does not change the strain rate sensitivity behavior of yield strength,and the grain refinement will reduce the strain rate sensitivity after the steady-state deformation stage.Aiming at the negative strain rate dependent hardening behavior of TWIP steel under dynamic loading,a phenomenological constitutive model describing the high strain rate and temperature dependent dynamic mechanical behavior is established.By considering the interaction between dislocation and twin,the model introduces dynamic twin dynamics and dynamic recovery factor to modify the classical kocks mecking relationship.Combined with the characteristic parameters of macro mechanical behavior and microstructure evolution,the micro mechanism of the effects of strain rate and adiabatic temperature rise on the dynamic mechanical behavior of TWIP steel was analyzed and obtained.The results show that the negative strain rate hardening at the initial stage of deformation is due to the strengthened twinning at high strain rate,which delays the proliferation of dislocations;The negative strain rate hardening in the later stage of deformation is the result of the dynamic adiabatic temperature rise effect promoting the dynamic recovery of dislocations.