Refinement Of Grains, Plastic Deformation Recrystallization, Aging Precipitation And Weldability Study Of The Magnesium Alloys Containing Rare Earth

Magnesium alloys containing rare-earth (RE) are among the bestlightweight structural materials with a good castablity, significant latentplastic deformability, high strength-to-weight ratio and excellentmechanical properties. The author did a lot of researches in rare-earthalloying with Mg-Al-Zn (AZ31 AZ61 AZ91), ZK60 and Mg-RE alloys.The intermetallic phase was identified by X-rays diffraction analysis(XRD) and scan electron microscopy (SEM). The refinement of grainswas carried out by adding RE and Zr. The hot-force compressionsimulation experiment of the investigated alloys was performed beforerolling or extrusion plastic deformation. The aging precipitation andweldability of the investigated alloys was studied and the microstructurewere took observation by optical microscopy (OM), SEM andtransmission electron microscopy (TEM) at casting, during plasticdeformation, before and after heat-treatment. The experimental resultswere analysed and generalized.1. The author made three series of Mg-Al-Zn-Ce; ZK60-RE andMg-0.5%Zn-0.5%Zr-RE alloys. The intermetallic phase has Mg-REcompounds besidesα-Mg, Mg₁₇Al₁₂ of the Mg-Al-Zn alloys,α-Mg, MgZn of the ZK60-RE andα-Mg of the Mg-0.5%Zn-0.5%Zr-RE alloys.The intermetallic phase distributed on the crystal boundary as-casting and aligned along extrusion direction after extruding. The crystal-size ofAZ31Ce as-casting was refined by adding Ce. The crystal-size of theMg-0.5%Zn-0.5%Zr and ZK60 alloys were refined by adding Nd and Y.The essences of refinement of grains of the Mg alloy were systematicallystudied. It revealed that adding RE can improve the refinement of grains.Through principle of solute elements division principle, the mechanism ofrefinement of grains of Mg alloys containing RE is caused by thecomposition undercooling as solute elements (RE) division again.2. After rolling, significant strengthening was emerged fromMg-Al-Zn-Ce alloys. The ultimate tensile strength was improved. Theinvestigated alloys were strengthened chiefly by Al₄Ce andβ-Mg₁₇Al₁₂broken during rolling. The mechanism of plastic deformation of theMg-Al-Zn-Ce alloys is versatile. The slip of dislocation and twinningwere carried on large-size grains. The deformation of large-size grainswere coordinated by the small-size grains. A lot of small-size: dynamicrecrystallization grains were produced during rolling since addingadequate Ce. The deformability of the investigated alloys was increasedmassively.3. The deformation activation energy of the AZ31, AZ61, AZ91containing 1.0%Ce are lower than that of AZ31, AZ61, AZ91 alloyswithout Ce. The deformability of the former was improved at 673K. AfterMg-0.5%Zn-0.5%Zr alloys added Nd and Y, ZK60 added Y, the compounds containing RE distributed pervasively on the Mg matrix. Theslip of dislocation was hindered by the compounds particle during plasticdeformation. The energy of dislocation slip increased and the dynamicrecrystallization temperature increased.4. Mg-0.5%Zn-0.5%Zr-2.2%Nd-4.0%Y alloy possesses goodextrusion deformability and strengthening ability after heat-treatment.The mechanical properties of this alloy at T5-treatment after extrusion areσ_b=293.5MPa,σ_0.2=281.2MPa,δ=11%. The ultimate tensile strengthat 250℃areσ_b=228.78MPa,δ=14.8% andσ_b=211.2MPa,δ=16.4% at 330℃. Mg-0.5%Zn-0.5%Zr-2.2%Nd-4.0%Y alloy possesseshigh mechanical properties comprehensively.5. The equilibriumβ-Mg₁₇Al₁₂ precipitated directly fromsupersaturated solid solution in Mg-Al-Zn-Ce alloys. Theβphaseprecipitated continuously within crystal grain and precipitateduncontinuously on the crystal boundary. It was found that ZK60 alloysprecipitated beforehand. The transition phase MgZn′(β′) precipitated firstat 150℃treatment. The strengthening of ZK60 alloys originates fromthat hindrance ofβ′to dislocation. The refinement of grains since Zrand RE Nd, Y are beneficial to increase the density ofβ′in Mg matrix.The value of strengthening improved markedly. The precipitation order ofZK60-RE alloys are SSSS→β′→βat 150℃and that ofMg-0.5%Zn-0.5%Zr-RE alloys are SSSS→β″→β′→βat 200℃. 6. After doing gas tungsten arc welding (GTAW/MIG) experimentwith AZ-Ce, ZK60-RE alloy, the formation of crystal nucleus andcrystalline growth of the welding line melted metal of the investigatedalloy were found. In other word, the new nucleus growth was allied to thegrain of mother stuff crystal, and the heterogeneous nucleation occured inthe centre of the welding line melted metal. RE can improve thetransverse ultimate tensile strength of AZ31 and ZK60 alloys. Theeffectual coefficient of the welding connect increased obviously. Thestrengthening mechanism of the welding connect of AZ31Ce alloys is thatCe and Al combined Al₄Ce compound. Nd and Y reduced the harm of Znto grain boundary while Zn and Mg compound of bulky MgZn phaseduring crystallizing.