| At present,magnesium alloys are the lightest structural metallic materials in use.Since magnesium alloys have significant effects in light weight construction,reducing energy consumption,energy saving and emission reduction,their applications in aircraft,automobiles,rail transit and other fields are gradually promoted,but they are still mainly used as non-load bearing structural parts.In recent years,Mg-Zn-Ca alloys have received extensive attention from scientific researchers for their good mechanical properties,room temperature formability,and heat and corrosion resistance.However,there are very few studies on the fracture toughness that is required for load-bearing structural parts for Mg-Zn-Ca series alloys.Moreover,the preparation of Mg-Zn-Ca series wrought alloys with high Zn content is still a bottleneck.Therefore,using Ca content as the variable,the thermal compression deformation behavior,dynamic recrystallization behavior during thermal compression,and hot processing and deformation ability and microstructure characteristics of Mg-4 wt.%Zn-x wt.%Ca(x=0,0.2,0.5,0.8)alloys were investigated in this thesis.From these investigations,the optimal hot processing parameters of Mg-4Zn-xCa alloys were found and then used to prepare wrought Mg-4Zn-xCa alloys with isothermal forging process.Finally,the fracture toughness and propagation behavior of the fatigue crack of wrought Mg-4Zn-xCa alloys were investigated.In order to reveal the deformation rules and predict the flow stress during the thermal compression process so as to direct the hot preparation process,the thermal compression deformation behavior of Mg-4Zn-xCa alloys was investigated.Firstly,the flow stress behavior of Mg-4Zn-xCa alloys was investigated,and the constitutive equation of hot compression deformation for predicting the flow stress of Mg-4Zn-xCa alloys was established.Secondly,the influence of various Ca content on the hot compression deformation constitutive equation of Mg-4Zn-xCa alloys was investigated,and then the influence of Ca element on the material constant n of the constitutive equation was further revealed.Finally,based on this,the hot compression deformation constitutive equation of Mg-4Zn-xCa alloys were further optimized.The results show that with increasing Ca content,the predicting accuracy of the flow stress of Mg-4Zn-xCa alloys by the constitutive equation gradually decreases.When Mg-4Zn alloy is deformed at 200?350?,its deformation mechanism is dominated by basal slip of<a>dislocation.Therefore,the material constant n doesn't obviously change.Ca addition results in the activation of the pyramidal<c+a>dislocations of Mg-4Zn-xCa alloys,which leads to obvious reduction of the n value,and the activation temperature of the pyramidal<c+a>dislocations decreases with increasing Ca addition.The easier activation of the pyramidal<c+a>dislocations of the Mg-4Zn-xCa alloys is mainly related to the decrease of axial ratio(c/a),stacking fault energy(SFE)and grain size caused by Ca addition.Finally,based on the influencing rules of Ca on the material constant n,the constitutive equations of Mg-4Zn-xCa alloys were optimized in different temperature ranges,and the predicting accuracy of the optimized constitutive equation is significantly improved.In order to reveal the microstructure refinement rules of Mg-4Zn-xCa alloys during hot compression deformation so as to improve the strength and toughness using hot processing methods,the dynamic recrystallization behavior of Mg-4Zn-xCa alloys during hot compression deformation was investigated.Firstly,the Sellars model was established to characterize the relationship between the critical strain(?c)and the thermal deformation conditions(Z parameter)of the dynamic recrystallization of Mg-4Zn-xCa alloys.Secondly,the effects of different thermal deformation conditions(T,?)and various Ca contents on the microstructure of Mg-4Zn-xCa alloys were investigated,and then the quantitative relationship between the grain size(dDRX)as well as the volume fraction(fDRX)and the related Z parameter of Mg-4Zn-xCa alloys were established.Finally,the nucleation mechanism of the dynamic recrystallization process of Mg-4Zn-xCa alloys was discussed.The results show that the Sellars models between the critical strain(?c)and the thermal deformation conditions(Z parameter)of Mg-4Zn-xCa alloys could be expressed as ?c=aZd where a and b are constants.The dynamic recrystallized grain size(dDRX)and volume fraction(fDRX)of Mg-4Zn-xCa alloys both increase with the increase of deformation temperature and decrease of strain rate(decrease of Z parameter).Specifically,the quantitative relationship between the dynamic recrystallized grain size dDRX and the Z parameter of the Mg-4Zn-xCa alloys could be expressed as dDRX=AZ" where A and n are constants.The quantitative relationship between the dynamic recrystallized volume fraction(fDRX)and the Z parameter of the Mg-4Zn-xCa alloys could be expressed as fDRX=B+ClnZ where B and C are constants.At the same strain rate and temperature,the dynamic recrystallized grain size(dDRX)and volume fraction(fDRX)of Mg-4Zn-xCa alloys decrease with increasing Ca content,and the main reason is that the precipitation of the fine(200?300 nm)second phases located at the grain boundaries in the Mg-4Zn-(0.2/0.5/0.8)Ca alloys can effectively hinder the growth of dynamic recrystallized grains,and at the same time,the calcium atoms dissolved in the matrix and the fine calcium-containing second phase precipitates will pin the dislocations,hinder the movement of the dislocations,and inhibit the occurrence of dynamic recrystallization.Besides,with increasing Ca addition and strain rate,the microstructures of the hot compressed Mg-4Zn-xCa alloys tend to show shear band and twin induced nucleation characteristics.Meanwhile,with increasing Ca content,the size and quantity of the second phases gradually increase,which makes the particle stimulated nucleation(PSN)mechanism gradually become the main dynamic recrystallization nucleation mechanism in Mg-4Zn-xCa alloys.In order to obtain the optimal thermal processing window for Mg-4Zn-xCa alloys with a high Zn content to help work out the hot working process and further elaborate the effect of Ca in the hot working process of Mg-4Zn-xCa alloys,the hot processing and deformation ability and microstructure characteristics of Mg-4Zn-xCa alloys were investigated.Firstly,the processing maps of Mg-4Zn-xCa alloys were constructed,and both the stability and instability domains were obtained.Secondly,through microstructure analysis of the typical domains(instability domain,stability domain and peak power dissipation efficiency domain)of the processing map,the optimal hot processing intervals of Mg-4Zn-xCa alloys were determined.Finally,the hot working ability of Mg-4Zn-xCa alloys was further investigated using high temperature thermoplastic test,and the reason why Ca addition deteriorates the hot working ability of Mg-4Zn-xCa alloys was further explained through analyzing the fracture surface of the samples after high temperature thermoplastic test.The results show that Ca addition enlarges the instability domain and narrows the optimal thermal processing window.The power dissipation efficiency of the instability domains of the processing map is generally lower than 0.15,and the microstructure of the instability domains is characterized by elongated as-cast grains,un-DRXed regions,twins,deformation bands or minor cracks,while stability domains mainly exhibit DRXed microstructure.Meanwhile,Ca addition also deteriorates the high temperature thermo-plasticity and the reason is that the brittle Ca-containing second phases(Ca2Mg6Zn3)tend to become the cracking sources of Mg-4Zn-(0.2/0.5/0.8)Ca alloys during high temperature tensile process,especially when the tensile temperature is 350? which is close to the melting point of Ca2Mg6Zn3.In order to successfully prepare wrought Mg-4Zn-xCa alloys,providing a reference for the hot processing of Mg-Zn-Ca alloys with high Zn content,and further reveal the influence of Ca addition on the fracture toughness of Mg-4Zn-xCa alloys,isothermal forging was used to prepare wrought Mg-4Zn-xCa alloys and then the fracture toughness and fatigue crack propagation behavior of wrought Mg-4Zn-xCa alloys were investigated.Firstly,based on the optimal processing interval of Mg-4Zn-xCa alloys,wrought Mg-4Zn-xCa alloys were successfully prepared by isothermal forging process.Secondly,the microstructure characteristics of both isothermal forged and isothermal forged and annealed Mg-4Zn-xCa alloys,together with the fracture toughness of isothermal forged Mg-4Zn-xCa alloy were investigated.Finally,the fatigue crack propagation test was carried out to investigate the fatigue crack propagation behavior of Mg-4Zn-xCa alloys,and the mechanism of improving the fracture toughness and hindering the fatigue crack propagation of Mg-4Zn-xCa alloys through Ca addition was explained.The results show that with increasing Ca addition,the grain size of the isothermal forged and isothermal forged and annealed Mg-4Zn-xCa alloys gradually decreases,the amount of the second phases(Ca2Mg6Zn3)gradually increases,and the macro texture is gradually weakened.With the increase of Ca content,the yield strength of the isothermal forged Mg-4Zn-xCa alloys gradually increases,and their elongation gradually decreases,the plane strain fracture toughness(KIC)of Mg-4Zn-xCa alloys is gradually improved,the size of the plastic zone close to stretch zone(SZ)gradually increases,and the crack propagation resistance of Mg-4Zn-xCa alloys gradually increases as well.With the increase of Ca content,the fatigue crack propagation rate of Mg-4Zn-xCa alloys gradually decreases,and the fatigue crack propagation resistance of Mg-4Zn-xCa alloys gradually increases.This is mainly attributed to the grain refinement caused by Ca addition,because grain refinement can activate the non-basal slip ahead of the fatigue crack tip,release the stress at the crack tip and inhibit the formation of deformation twins,which could reduce the probability of fatigue cracks propagation along the twin boundaries and therefore improving the material's ability to hinder fatigue crack propagation.In addition,the increasing proportion of large-angle grain boundaries also helps increase the material's ability to hinder fatigue crack propagation. |
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