| Due to the coexistence of austenite and ferrite,the duplex stainless steel performs an excellent combination of the mechanical properties and corrosion resistance,and has been widely employed in the petrochemical,nuclear-electrical and oceanographic engineering.However,the fatigue still threatens the structural integrity of engineering components with duplex stainless steel.As a multiscale failure model,the initiation and propagation of fatigue cracks depend on not only the macroscopic mechanical response but only microscopic deformation,particularly for the multiphase materials,which have a severely incompatible distribution of multiscale deformation due to the mismatch of mechanical properties between two phases.Thus,it is of great significance for the fatigue strength evaluation to investigate the multiscale cyclic deformation behavior and constitutive models.The most studies are limited to the cyclic mechanical responses of the duplex stainless steel under different cyclic amplitudes with a constant loading rates.And the complete theories about the cyclic deformation behavior and its physical mechanical for the duplex stainless steel under complex cyclic loading have been not established yet,and the multiscale cyclic constitutive models which combines the physical mechanisms are still required.Therefore,this paper investigated the time-and amplitude-dependence of cyclic deformation behavior of 2205,and reveled its cyclic physical mechanism.A multi-mechanism macroscopic cyclic viscoplastic constitutive model and a crystal plasticity-based mesoscopic cyclic model were established to predict its multiscale cyclic deformation behavior.The main conclusions are summarized as follows:(1)A series of low cycle fatigue tests with different amplitudes,rate and tensile holding were performed.The macroscopic mechanical behavior of the duplex stainless steel can be described as the primary cyclic hardening followed by cyclic softening up to the final fracture regardless of the fatigue amplitudes and loading rate,but the hardening factor decreased with the increase of strain amplitude,while the softening factor was opposite.In addition,the cyclic softening behavior was significantly inhibited by the slow loading rates or tensile holding.The stress relaxation behavior was found under the tensile holding,and it is restrained significantly during the cyclic softening.(2)The electron back scattered diffraction observation was performed to identify the mesoscopic distribution of cyclic deformation within two phases of 2205 under different amplitudes,and the interaction mechanism model between nitrogen and dislocation movement was proposed.The main deformation-bearing phase of duplex stainless steel changed with the external cyclic loads,i.e.,the austenite as the soft phase performs preferentially plastic deformation at the lower amplitudes,then more plastic deformation occurred within ferrite with the increasing of external loads and deformation-induced hardening of austenite.And the plastic deformation within the austenite was preferentially formed at the phase boundaries,while it mainly concentrated at the grain boundaries for the ferrite.The interaction between the solution nitrogen atom and the dislocation motion,including the pinning,unpinning and repinning,leads to a significantly time-dependent cyclic softening behavior.(3)In the framework of the classical macroscopic phenomenological cyclic viscoplastic constitutive model,the cyclic hardening/softening equations for the duplex stainless steel was proposed by combining its cyclic microscopic physical mechanism.And a multi-mechanisms macroscopic cyclic viscoplastic constitutive mode was established by considering the strain-induced hardening related to bearing phase,the viscous softening caused by dynamic recovery of microstructure and the interaction between nitrogen atom and dislocation motion.The macroscopic cyclic deformation behaviors of material under various cyclic loading were predicted accurately.(4)The crystal plasticity theory for two constitutive phases within duplex stainless steel was developed based on the classical single crystal plasticity model by considering the cyclic hardening and softening mechanism of two phases,and the micromechanical material parameters of the two phase were calibrated accurately.Furthermore,the proposed crystal plastic constitutive model was embedded into the finite element software,and the polycrystalline duplex stainless steel finite element model was established.The macroscopic cyclic deformation behaviors of duplex stainless steel under different cyclic amplitudes and loading rates were calculated accurately,and the predicted mesoscopic phase deformation behavior was verified by the electron back scatter diffraction observation.In addition,the multiscale cyclic deformation behavior of duplex stainless steel under different phase ratios was also predicted by the crystal plasticity model.(5)The multiscale cyclic deformation behavior of a three-point bending sample was predicted by the combination of proposed multi-mechanism macroscopic cyclic model and crystal plasticity-based mesoscopic cyclic model by mean of the submodel technology.And the digital image correlation technology has been employed to prove the accuracy of simulated results.The macroscopic and mesoscopic cyclic deformations have been predicted accurately,which provides an important theoretical basis for the multiscale fatigue analysis of engineering components. |
