Development And Study On Strengthening Mechanism Of High-strength Mg-Gd-Zn Series Alloys

Magnesium(Mg)is the lightest metal structure material,which shows great application potential and prospect in automobile and aircraft industries due to its excellent specific strength and potential.Mg alloys are called as “the 21 st century green structural material.The application of the Mg alloys is significant for lightweighting,energy-saving and emission-reduction,which can produce great social value and economic benefits.However,their relatively weak strength greatly limits widespread applications.Developing high-strength Mg alloys is of great theoretical and engineering value for the application of Mg alloys.Mg-Gd series alloys are one of the most promising series for developing highstrength Mg alloys.Researchers have done a great deal of work on developing highstrength Mg-Gd series alloys,covering aging behavior,recrystallization and so on.However,the work is not systematic enough and the processes and strength cannot both satisfy industry requirement till now.This paper selected Mg-Gd alloys as the base alloys and systematically control microstructure and properties by adjusting microalloying elements,extrusion processes and heat treatments based on previous work.This thesis consists of 7 chapters.The 1st chapter introduces some properties and research statues of Mg alloys.The 2nd chapter introduces the experimental procedures in this work.The 3rd ~ 6th chapters are the main results.The 3rd chapter studied the effects of microalloying elements Zn,Zr and Mn on the cast Mg-Gd(-Zn)alloys and the 4th chapter studied the how Zr and Mn affect recrystallization of the wrought Mg-Gd-Zn alloys.Both above chapters provide guidance for the compositions.Then the 5th chapter studied the relationship between extrusion temperature versus microstructure and properties,to point out the target microstructure.The 6th chapter shows work on the heat treatment(namely the effects of strain aging)after extrusion.Combining the above results,we finally developed super high-strength Mg alloys.The 7th chapter shows the main conclusions and innovation of the above work.The strengthening mechanisms of the composite precipitate phases,adjusting recrystallization behavior,composite grain structure,and novel strain aging in Mg alloys are studied through the aspects of composition,technology and microstructure.Based on the results,the ultra-high strength(TYS > 500 MPa,UTS > 550 MPa)wrought Mg alloy was successfully developed by taking full use of fine grain strengthening,texture strengthening,LPSO structure strengthening,precipitation strengthening and work hardening.Furthermore,the strengthening mechanisms of composite precipitation strengthening,bimodal-grained structure strengthening and strain aging strengthening were also studied to provide guidance for further development of high-strength Mg alloys.The main work and conclusions in this paper are as follows:1.The effects of Zn addition on grain size,precipitates,ageing behavior and mechanical properties in casting Mg-14Gd-0.4Zr(wt.%)alloy were studied.The results indicate that the Zn addition can reduce the eutectic temperature and solid solubility,which leads to insoluble eutectic phase inhibiting grain growth during solution treatment.More importantly,the addition of Zn can introduce the basal ?? phase.The ?? and ?? precipitates with their perpendicular distribution can provide stronger precipitation strengthening and more significantly strengthen the sample by inhibiting slips and twining,compared with only the ?? precipitates.Due to the combination of fine grains and the composite strengthening effects of ?? and ?? precipitates,the casting Mg-Gd-ZnZr alloys exhibited excellent mechanical properties including ultimate tensile strength of 403 MPa,tensile yield strength of 288 MPa and elongation of 2.9%.2.The effects of Zr and Mn additions on the phases of casting Mg-15Gd-1Zn(wt.%)alloy were studied.The results indicate that the Zr addition will hinder he formation of the X phase with the LPSO structure along grain boundaries by reacting with Zn and reducing the Gd/Zn ratio,while the addition of Mn can promote the formation of the X phase along grain boundaries and the LPSO structure within grains.When with similar grain sizes,transforming the hard and fragile(Mg,Zn)3Gd phase into the ductile X phase can improve ultimate tensile strength and elongation;3.The effects of Zr,Y and Mn additions on the recrystallization behavior of wrought Mg-Gd-Zn alloys were studied.The Zr addition introduce fine Zn-Zr and Mg(Zn,Zr)particles which sufficiently prevent the grain growth during dynamic recrystallization.The additions of Y and Mn inhibit recrystallization by promoting the formation of the LPSO structure within grains,which results in the formation of un-fully recrystallized bimodal-grained structure;4.The effects of differential-thermal extrusion compared with isothermal extrusion on the microstructures and properties of Mg-15Gd-1Zn-0.4Zr(wt.%)alloy were investigated.Compared with the isothermal extrusion,the differential-thermal extrusion was found to provide faster cooling rates after deformation,which results in finer recrystallized grains and fewer LPSO structure within grains.Meanwhile,the higher preheat temperature before differential-thermal extrusion reduced precipitation during the preheat treatment,which reserved stronger precipitation strengthening ability for the samples aged after extrusion.For the above reasons,the differential-thermal extruded samples have higher strength and better ductility than the isothermal extruded samples;5.The bimodal-grained and equiaxed structures were prepared with different extrusion temperatures to study the effects of the bimodal-grained and equiaxed structures on the mechanical properties including the strength and yield asymmetry.It was found that the bimodal-grained structure can significantly improve the tensile strength.The strengthening mechanism is that the texture strengthening of the unrecrystallized grains and the fine grain strengthening of the recrystallized grains.Meanwhile,the fine recrystallized grains in the bimodal-grained structure inhibit twinning when strain is small,which decreases the yield asymmetry.As a result,the bimodal-grained sample exhibited high tensile yield strength(465 MPa),ultimate tensile strength(524 MPa)and very low yield asymmetry(0.99).In contrast,the equiaxed structure is harmful for tensile properties and yield asymmetry,but it can significantly enhance compressive strength.The reason is that its texture is favorable for tensile twining in tension and unfavorable for that in compression.The aged sample with equiaxed grains showed very high compressive yield strength(500 MPa),ultimate compressive strength(520 MPa)and higher yield asymmetry(1.39).6.The effects of strain aging on the precipitation behavior and mechanical properties of Mg-13Gd-1Y-1Zn-0.4Zr/Mn(wt.%)alloys were investigated.It was found that the strain aging provides stronger aging strengthening than conventional aging,both in the equiaxed and bimodal-grained samples.Meanwhile,the strain aging also has significant effects on the precipitation strengthening ability.Besides promoting precipitation hardening,the strain aging with both cold rolling and pre-stretching provided a stronger work-hardening effect than the strain aging with only cold rolling,thereby significantly increasing the tensile yield strength.Finally,we developed the Mg-13Gd-1Y-1Zn-0.4Mn(wt.%)alloy with ultra-high tensile yield strength(543 MPa)and ultimate tensile strength(564 MPa)by applying the strain aging with both cold rolling and pre-stretching.