| Magnesium and magnesium alloys have a series of advantages such as low density,high specific strength,high specific rigidity,non-toxic,environmental protection,large reserves and so on.They are known as"new green structural materials in the 21stcentury"and have been widely used in national defense,automobile,3C,medical and other fields.The plastic deformation behavior of magnesium alloys directly affects the high temperature application of cast magnesium alloy and the plastic ability of wrought magnesium alloy,which greatly limits the application and development of magnesium alloys.In this paper,Mg-Al magnesium alloys,which are the most widely used magnesium alloys,are selected as the research objects.Two typical plastic deformation scenarios of cast magnesium alloy under high temperature creep condition and wrought magnesium alloy under electroplastic processing condition are selected,and two parallel research main lines of cast magnesium alloy AZ91 and wrought magnesium alloy AZ61 are opened up.The effects of alloying elements,microstructure and deformation conditions during the plastic deformation behavior of Mg-Al magnesium alloys were investigated using non-contact measurement technology,microscopic observation technology and digital image processing technology.The main research contents of this paper are as follows:1.The effect of alkaline earth modification on plastic deformation of cast magnesium alloy.A new Mg-9Al-1Zn-XSr alloy was designed,melted and manufactured based on cast magnesium alloy AZ91.The effects of Sr addition on the microstructure and mechanical properties of cast magnesium alloy AZ91 were studied.The results show that with the increase of Sr addition,the yield strength of AZ91 alloy increases,but the tensile strength and elongation increase at first and then decrease.When the content of Sr is 1%,AZ91-Sr（AZJ911）exhibits the best comprehensive mechanical properties,with tensile strength of 248MPa,yield strength of 156MPa and elongation of 6.8%.The addition of Sr produces a new phase Al4Sr,which can refine the grain size of AZ91 alloy and show a second phase reinforcement.However,the addition of Sr leads to the decrease of Al content,weakens the solid solution strengthening effect of Al atom and reduces the content ofβ-Mg17Al12eutectic phase.The addition of Sr has little effect on the fracture mechanism of the alloy.By creep analysis of AZJ911 alloy,the creep properties and microstructure evolution of AZJ911 alloy with Sr addition were observed,and the effect and mechanism of Sr addition on the creep properties of AZJ911 alloy at high temperature were explored.The results show that both stress and temperature have significant effects on creep.The addition of Sr leads to the formation of thermally stable Al4Sr phase,which reduces the content of Mg17Al12 eutectic phase,thus contributing to the improvement of creep properties.The dynamic precipitation ofγ-Mg17Al12 phase during the creep process contributes to the creep deformation of AZJ911 alloy.Based on the conventional power law relationship,the stress index and creep activation energy are calculated to be 8.4 and 102 k J/mol respectively.The high stress index indicates that the power law relationship fails,which is caused by the Orowan strengthening effect caused by the dynamic precipitation ofγ-Mg17Al12 phase and the threshold stress.The creep deformation of AZJ911 alloy is mainly controlled by dislocation climbing,which is consistent with the results of screw dislocation observed in creep samples.2.The effect of electroplastic effect on plastic deformation of wrought magnesium alloy.The orthogonal test of L25（5^6）was designed with AZ61commercial pipe as the research object.The effects of tensile speed（0.1-7.2mm/min）,current intensity（40-100a）,pulse frequency（200-600Hz）and duty cycle（20-100%）on the electroplasticity of wrought magnesium alloy were investigated.The results show that under pulse current,the tensile strength of the wrought magnesium alloy decreases obviously,and the fracture elongation increases significantly,up to 40.5%.The fracture of the sample appeared necking phenomenon,and the surface of the sample appeared"orange peel"fold in the tensile process.The microstructure analysis results from a large field of vision show that there are a large number of twins and cracks near the fracture under the action of pulse current,and the incidence of twins decreases with the distance from the fracture.The grain size varies from 86μm2 to238μm2.The closer to the fault direction,the smaller the grain size is.Dynamic recrystallization was observed in metallographic images.The effects of current intensity（100-130A）,pulse frequency（200-400Hz）and duty cycle（20-100%）on mechanical properties and microstructure of wrought magnesium alloy AZ61 pipe at a certain tensile speed were analyzed.The results show that the pulse frequency has an important effect on fracture elongation and can reduce the tendency of twinning generation.The current intensity plays a significant role in the decrease of tensile strength and promotes dynamic recrystallization.When other factors are constant,duty cycle ratio plays an amplifier role in current intensity,and the smaller the duty cycle is,the more obvious the amplification effect on current intensity is.The plastic instability（PLC）phenomenon during the plastic deformation of wrought magnesium alloy under pulse current was studied,and the electroplastic mechanism of wrought magnesium alloy was explored.The results show that PLC phenomenon occurs in the tensile process of wrought magnesium alloy under the action of pulse current.According to the general classification method,serrated rheology belongs to type A.PLC phenomenon may be mainly which are caused by the electronic wind and magnetic effect caused by dislocation and slip system suddenly starting influence,and unlocked dislocation to the second phase of the cutting and local plastic deformation of the material are also important reasons for PLC phenomenon.Through the analysis of strain hardening,dislocation density and substructure,it is concluded that there are both thermal and non-thermal effects in the plastic deformation of wrought magnesium alloy by pulse current,but the non-thermal effect is the fundamental reason for the increase of plastic capacity.In the AZ61 alloy with pulse current assisted tensile deformation,most<c+a>dislocations are movable and undergo significant cross-slip,probably as a result of the pulsed current.Therefore,it is speculated that the pulse current can pass through the suppression<c+a>dislocation base plane decomposes to facilitate<c+a>dislocation cross slip,thus resulting in high plasticity.In addition to the reconstruction of dislocation configuration in deformed microstructure by pulse current,electronic wind effect and magnetic effect also play a positive role in the improvement of plastic capacity.Using Field-Backofen as the basic model and using the product rule,the three factors of pulse current--frequency,current intensity and duty cycle--were introduced into the model to construct the constitutive equation of extruded magnesium alloy under the electroplastic effect:σ=1495ε0.331ε0.168·exp（-0.0003·f-0.0087·I-0.0004·n）This equation can accurately simulate curves in f=100-400Hz,I=60-110A and N=40-80%.3.Application of digital image processing technology in the study of magnesium alloy plastic deformation.MATLAB and other mathematical analysis softwares are used to develop a new algorithm based on mathematical morphology.Compared with other algorithms,mathematical morphology algorithm has more advantages in processing digital images with uneven light and uneven image quality.The realization process of extracting key information of digital image and establishing digital image analysis model by using mathematical morphology algorithm are explored.Combining with the statistics principle,complete the digital image information extraction,statistical analysis,visualization and relevant content are completed in the digital image,microstructure analysis from microcosmic to macroscopic scale across is realized and developing a large quantitative analysis method under broad vision,providing a solid technical foundation for the study of alkaline earth metal modification and electroplastic effect on the plastic deformation of magnesium alloys provides a solid technical foundation. |
