Experimental And Theoretical Investigation Of Solid Solution Strengthening And Second Phase Strengthening In Mg-Zn-Zr-Y Alloys

Magnesium alloys were a promised environmentally friendly candidate for thedevelopment of lightweight in automotive and aerospace field due to the advantage oflow density high specific strength and high recovery rate. Mg-Zn-Zr（ZK） system alloysare the widely commercial used wrought magnesium alloys at present. Addition of Yhad great effects on the microstructures and mechanical properties of ZK system alloys.The existing forms of alloying element in magnesium alloys can be divided into twotypes. One was as solute in the α-Mg solid solution, which resulted in solid solutionstrengthening.The other was to form the second phase, which resulted in second phasestrengthening. As for the solid solution strengthening, however, the solid strengtheningbehavior for Y is still unclear until now. Therefore, it was necessary to study themechanism for solid solution strengthening of Y in magnesium alloys. For the secondphase strengthening, it was necessary to determine the relationship between the alloyingelement and the corresponding constituent phases because of the diversity of the secondphase in Mg–Zn–Y–Zr alloys. For the second phase in Mg–Zn–Y–Zr alloys, it has beenreported that long period stacking ordered structure （LPSO） can improve obviously thestrength and toughness of the alloys. However, the formation mechanism of LPSO andthe strengthening and toughening mechanism of the alloys containing LPSO wereunrevealed. So it was necessary to investigate the mechanism of strengthening andtoughening of the alloys containing LPSO.The quantitative relationship between the alloying element and the correspondingconstituent phases, the mechanism of solid solution strengthening for Y in magnesiumalloys, the formation mechanism of LPSO and the strengthening and tougheningmechanism of the alloys containing LPSO were investigated by experimental analysisof OM, SEM, TEM, HRTEM, DSC and XRD and by theoretical calculations such asphase diagram calculations and first-principles calculations in this work.Firstly, the quantitative relationship between the alloying element and thecorresponding constituent phases was investigated by phase diagram calculations andexperimental verification. The results showed that the formation of the secondaryphases in Mg–Zn–Y–Zr alloys strictly depended on the Y/Zn atom ratio and X(Mg₁₂YZn)-phase, W （Mg₃Y2Zn₃）-phase and I （Mg₃YZn₆）-phase were precipitated in turn with the Y/Zn atom ratio decreased. The Y/Zn atom ratio with the correspondingconstituent phases were suggested quantitatively as follows: the phase constituent wasα-Mg+I,when Y/Zn ratio was about0.164; and α-Mg+I-phase+W, when Y/Zn ratiowas in the range of0.164⁰.33; and α-Mg+W, when Y/Zn ratio was about0.33; andα-Mg+W+X, when Y/Zn ratio was in the range of0.33¹.32; and α-Mg+X, whenY/Zn ratio was about1.32. Our results can offer a guideline for alloy selection and alloydesign in Mg–Zn–Y–Zr system.Secondly, as to the contradictory between the solid solution strengthening behaviorof Y in Mg solid solution and the classic theory of elastic interaction, the solid solutionstrengthening behavior of Y in Mg solid solution was investigated by experimentalmeasurement of the size misfit of Y and by comparing the solid solution strengtheningefficiency of Y with Al and Zn. The results showed that the solid solution strengtheningof Y was consistent with the classical Labusch theoretical model. The misfit of Y in Mgsolid solution was about+27.1％. The efficiency of the solid solution strengtheningfollowed this sequence: Y＞Zn＞Al. Previous report pointed out that the solid solutionstrengthening of Y has an inconsistent with the classical Labusch model, which could beexplained by the estimated value of misfit for Y in the published paper had a big errorcompared with the experimental value.Thirdly, the formation mechanism of LPSO was investigated by SEM、TEM、HRTEM and first-principles calculations. The results showed that stacking faults playeda crucial role in formation of LPSO structure and the formation of LPSO can bedescribed by stacking fault mechanics. The formation of LPSO was observed to beaccompanied by stacking faults and the lamellar LPSO structure and stacking faultswere both formed on （0001）α-Mghabit plane and grew or extended along [011-0]α-Mgdirection in Mg-Zn-Y-Zr alloy in the as-cast Mg-3.67Y-2.03Zn-0.26Zr alloy prepared bymetallic mould casting method. First-principles calculation showed that addition of Y cansharply decrease the stacking fault energy of the Mg-Zn-Y-Zr alloy, while Zn slightlyincreased the stacking fault energy of the alloy. It was showed the important reason thatLPSO was formed was because of the sharply decreased stacking fault energy byaddition of Y to the Mg-Zn-Zr alloys. Therefore, the influence of stacking fault energyon the formation of LPSO should be covered into the element features and criterion ofthe formation of LPSO. The element features and criterion of formation of LPSO in Mg–RE–X system was suggested to be modified as follows: the element RE and X can decrease the stacking fault energy of magnesium alloys and the algebraic expression forthe sum of chemical misfit of RE and chemical misfit of X was negative with a biggernumerical value.Fourthly, the influence of LPSO on the strength and toughness of Mg-Zn-Zr-Yalloys was analyzed by observations on the microstructure of the as-extrudedZK20+3.67Y alloy. The results demonstrated the capability of LPSO to accommodatethe deformation, which greatly increased the workability and the toughness of thealloys.The block-like LPSO phase took part in a great extent of the deformation duringthe extrusion process. The deformation structure evolution which depended on thedeformation exten was origined from the high density of deformation kinks anddeveloped to the sub-grains and finally developed to DRX-ed grain with increaseddeformation extent. LPSO had great effect on the DRX behavior during hot extrusion.LPSO with a certain size and volume fraction acted as a role to retard DRX during hotextrusion, which resulted in work hardening in the α-Mg matrix and positive effect onthe strength of the as extruded alloys. The low stacking fault energy of Mg-Zn-Zr-Yalloy containing LPSO had a positive effect on the high strength and good plasticity ofthe alloys.Fifthly, the microstructure and mechanical properties of Mg-Zn-Zr-Y alloycontaining W （Mg3Y2Zn3） phase was investigated. The results showed that W phase hadpositive effect on the mechanical properties of the as-extruded Mg-Zn-Zr-Y alloy. Wphase （Mg3Y2Zn3） was introduced by addition of Y in the ZK51alloy. The W phase wasfragmented and dispersed during hot extrusion, which improved the mechanicalproperties of the extruded alloy greatly due to the extremely fine grains with size of2-3μm. The yield strength of the extruded alloy exhibited about310,250and237MPa at25,150and200°C, respectively, which were remarkably better than those of WE54commercial alloy used at elevated temperature.Finally， the as-cast microstructure, the as-extruded microstructure and themechanical properties of the Mg-Zn-Zr-Y alloys containing the ductile second phase（LPSO） and the brittle second phase （W phase） respectively were compared. The effectsof second phase on the behavior of extrusion deformation were analyzed. The resultsshowed that the fragment extent of the ductile second phase was less than that of thebrittle second phase, which resulted in the larger grain size of the extruded alloyscontaining the ductile second phase than that containing the brittle second phase. However the ductility of the extruded alloys containing the ductile second phase washigher than that containing the brittle second phase due to the ductility of the secondphase.