Study On The Plastic Deformation Mechanism Of HCP Materials With Elastic Visoplastic Self-consistent Model

Relative to cubic crystal structure(steel,aluminum and its alloy,etc.),metallic material with HCP crystal structure has low asymmetry and less independent slips,leading to significant role of twinning on plastic mechanisms,related research are less than cubic metal.As typical HCP materials,magnesium alloy and titanium alloy have more excellent performances than metals with cubic crystal structure,with wide application in national defense industry and the lightweight technology of engineering structure.By means of experiments and numerical simulations,this paper studied systemically plastic mechanisms of magnesium alloy and titanium alloy under monotonic loading and loading-unloading-reverse loading with the finite strain elastic visco-plastic self-consistent model(EVPSC-TDT),which has been updated by implementing the twinning and de-twinning(TDT)model.The main achievements are as follows:(1)Monotonic loading tests with large strain of MgAZ31 rolled plate are conducted to measure the stress-strain curves of tension and compression along rolling direction(RD),transverse direction(TD),normal direction(ND)and 45oin RD-ND plane,respectively.The compression-unloading-tension along RD tests of MgAZ31 rolled plate are conducted by MTS809 fatigue test system,and three stress-strain curves with different pre-compressions are obtained.In addition,texture evolutions of MgAZ31 during compression-unloading-tension are measured by electron back-scattered diffraction(EBSD).(2)Numerical simulations of MgAZ31 rolled plate under monotonic loading with large strain are carried out to analyze the change laws of stress-strain curve,texture evolution,twin volume fraction of tension and compression along RD,TD,ND and 45oRD-ND plane.The results indicate that basal slip is the most easily activated to be the predominant plastic deformation mechanism,while{10(?)2} tensile twin is the main reason of in-plane compression anisotropy and asymmetry between compression and tension.The change laws of number of grain with twinning exhaustion and working hardening are consistent: both increase to peak then decrease gradually and the peak strain are also same,revealing twin mechanism in hardening of Mg alloy.Special analysis software is developed to calculate volume fraction of each twin variant ordering by schmid factor.By using this special software,distribution characteristics of volume fraction of each twin variant could be obtained to analysis the texture evolution mechanisms of Mg under in-plane compression and ND tension.The simulation results we get are good agreement with experiment measured results.(3)Based on experiment analysis result that de-twinning has lower CRSS compared to twinning,one parameter k is introduced to weaken CRSS of de-twinning,improving EVPSC-TDT model.By the improved model,simulated stress-strain curve,twin volume fraction and slips/twinning relative activity under reverse tension loading with three different pre-compressions.The simulation results indicate that the higher pre-compression,the stronger hardening effect of basal slip on {10(?)2} tensile twinning and the higher yield stress in reverse tension.The simulated yield stress in reverse tension with improved EVPSC-TDT model is in good agreement with experiment result.(4)Numerical simulations of titanium alloy rolled plate under monotonic tension and compression with large strain are carried out to predict change laws of the stress-strain curve,work hardening,texture evolution,twin volume fraction of tension and compression along RD,TD and ND.The results indicate that prismatic slip is the most easily activated to be the predominant plastic deformation mechanism in titanium alloy.Comparing with monotonic tension,monotonic compression presents larger flow stress.In addition,the greater difference in twin volume fraction of tension and compression has,the more obvious asymmetry is.By comparing simulation results with random texture and high-symmetry basal texture,in-plane anisotropy mechanism of titanium alloy is reveled.The simulation results we get are good agreement with experiment measured results.(5)The stress-strain curves and texture evolution of titanium alloy rolled plate under monotonic loading,loading-unloading-reverse loading along RD,TD and 45oin RD-TD plane are predicted by numerical simulations.The simulation results are good agreement with experiment measured results.Through analyzing twin volume fraction and slips/twinning relative activity under monotonic loading,it could be indicated that twinning is the main reason of in-plane anisotropy under monotonic loading for titanium alloy.The more larger twin volume fraction is,the more significant in-plane anisotropy is.Mechanisms of slips,twinning and de-twinning during compression-unloading-tension deformation are uncovered by analyzing twin volume fraction and slips/twinning relative activity under loading-unloading-reverse loading.The results indicate that eh prismatic slip is dominant plastic deformation mechanism during reverse loading and twin volume fraction evolution law proves the characteristics of twinning and de-twinning behavior.The research results provide important references for optimizing plastic processing technology and developing magnesium alloy and titanium alloy with high performance.