Effect Of The Pulsed Magnetic Field On The Microstructures And Mechanical Properties Of Mg-Gd-Zn-(Zr) Alloys

Magnesium alloys are the lightest structural materials with low density, highspecific strength and stiffness, good electric conduction and thermal conduction,electromagnetic shielding, good formability, as well as easy recycled, and become theworld’s fastest-growing alloy. The low mechanical properties of magnesium alloy wasan important reason to limited their development and application. The mechanicalproperty of the material depends on the microstructure of alloy. Alloying andsolidification control technology can improve the microstructure of alloy. This articlebased on the binary Mg-Gd alloy, choose Zn element for alloying element and the bestmechanical properties of alloy was prepared. Then, a pulsed magnetic field was appliedon the solidification process of alloy to improve the microstructure, in order to enhancethe mechanical properties of alloy.The effects of different Zn contents on microstructures and mechanical propertiesof Mg_98-xGd₂Zn_x（x=0,0.5,1,1.5） alloys were mainly investigated, by opticalmicroscope（OM）, X-ray diffraction（XRD）, scanning electron microscopy（SEM） withenergy dispersive spectrum（EDS）, hardness testing and compressive testing machine.The strengthening mechanism of the alloy was analyzed and discussed. Then, theoptimum composition of Mg₉₇Gd₂Zn₁alloy was prepared. The effect of the frequency ofthe pulsed magnetic field on microstructures and mechanical properties ofMg-Gd-Zn-（Zr） alloys were investigated, the refining and strengthening mechanism ofthe alloys was analyzed and discussed. The main conclusions can be summarized asfollows:1. The Zn element added obvious changed and refined the microstructure ofMg_98-xGd₂Zn_xalloys through the research of different Zn in Mg_98-xGd₂Zn_xalloys. Withthe increase of the content of Zn, the refined effect refinement first, then coarsening.The microstructures of Mg₉₈Gd2alloy are composed of the dendrite primary Mgphase and the semi continuous eutectic （Mg,Zn）3Gd phase. When the content of Zn wasincreased to1%, the effect of refinement was significant. The microstructures ofMg₉₈Gd2alloys are composed of the rosettes primary Mg phase and the reticulateeutectic （Mg,Zn）3Gd phase, also appears X phase.The mechanical properties testing show that the vickers hardness、tensile strengthand elongation of Mg_98-xGd₂Zn_xalloys increases at first and then decreases with the increase of the content of Zn. When the Zn content is1.0%at.%, the vickers hardness、yield strength、compressive strength and elongation of alloys reaches to maximum valueis76.6Hv、127MPa、281.3MPa、12.7%, respectively. When Zn content is1.0%at.%, thefine grain strengthening was strongest, and also a lot of strengthening phase（X phase）was appears, which was the main reason for the mechanical properties increase of thealloy.2. The microstructure of Mg₉₇Gd₂Zn₁alloy was obvious refined with the pulsedmagnetic field through the research of different pulsed magnetic field in Mg₉₇Gd₂Zn₁alloy. With the frequency of pulsed magnetic field increase, the morphology of primaryphase was change from dendrite into spherical or rosettes. The morphology of theprimary phase change into spherical or rosettes crystal with the pulsed magnetic field of10Hz, and the refinement effect was most.The mechanical properties testing show that the vickers hardness、tensile strengthand elongation of Mg₉₇Gd₂Zn₁alloy increases at first and then decreases with theincrease of the vibration frequency of pulsed magnetic field. When the vibrationfrequency of pulsed magnetic field is10Hz, the vickers hardness、elongation、yieldstrength and compressive strength of alloys reaches to maximum value is83Hv,13.8%,129MPa,326MPa, respectively.3. Add Zr element to Mg₉₇Gd₂Zn₁alloy, the percentage composition of X phase ingrain boundary decrease. Because with the Zr addition, the microstructure of alloy wasobvious refined, made the distance of the matrix and the grain boundary decrease. Theenrichment of the solute atoms in the grain boundary was slow down. Then, the moreGd and Zn elements dissolved in the matrix lead to X phase formation reduced.Under the pulsed magnetic field, the solidified microstructures of Mg₉₇Gd₂Zn₁alloy refinement effect higher than Mg₉₇Gd₂Zn₁Zr_0.18alloy. The dendrite growth patternof Mg₉₇Gd₂Zn₁was significant effect of pulsed magnetic field and the refinement effectwas most. The equiaxed crystal growth pattern of Mg₉₇Gd₂Zn₁Zr_0.18was week effect ofpulsed magnetic field and the refinement effect was little.