| With the growing emphasis on green development,there is an increasing demand for magnesium alloys,which are considered as the lightest metal structural material.The majority of high-strength magnesium alloys are made with rare earths.Mg-Gd-Y-Zn based alloys have been widely studied in recent years due to their excellent mechanical properties and the presence of long period stacking order(LPSO)strengthening phases.The type of LPSO phase in Mg-Gd-Y-Zn alloy varies depending on the relative content of RE and Zn.In addition to controlling the second phase from its elemental composition,different casting processes can also be used to adjust the content and morphology of the second phase.Electromagnetic Stirring(EMS)is a method that involves applying a magnetic field around the melt and using the Lorentz force generated by electromagnetic induction to force the metal melt to flow,resulting in the effect of stirring the melt.The method of electromagnetic stirring semi-solid processing has been successfully applied to industrial production of iron,copper,and aluminum alloys.However,the effect of electromagnetic stirring on the solidification structure of magnesium alloys is still being researched,and some scholars have studied the effects of stirring frequency and current on commercial magnesium alloys.The effects of electromagnetic stirring on the microstructure and properties of rare earth magnesium alloys containing LPSO structural phases are not yet well understood,and there are limited reports both domestically and internationally.This thesis investigates Mg-6Gd-3Y-x Zn-Zr alloys(with x=1,1.5,2,3)using metal mold casting and electromagnetic stirring-assisted solidification to prepare ingots.The study analyzes the microstructure and properties of the alloy after casting and heat treatments(solid solution treatment and aging treatment),and summarizes the effects of electromagnetic stirring on rare earth magnesium alloys with different Zn content.The findings of this study provide theoretical basis and technical guidance for the industrial production of the alloy.The as-cast 1Zn alloy without electromagnetic stirring did not show the formation of LPSO phase,while the 1Zn alloy with electromagnetic stirring produced LPSO phase.The percentage area of LPSO phase in the alloy was higher compared to the alloy with similar Zn content without electromagnetic stirring.Electromagnetic stirring promoted the diffusion of alloy elements and reduced the stacking fault energy of the matrix.This facilitated the entry of Zn and RE atoms into the stacking faults and formation of a 14 H LPSO phase.The LPSO phase structure in the alloy without electromagnetic stirring was 18 R.The hardness of the alloy with electromagnetic stirring was higher,and the hardness gradient within the grain was smaller compared to the alloy without electromagnetic stirring.After being held at 520 ℃ for 8 hours and water-cooled,the Mg-6Gd-3Y-x Zn-Zr alloys(x=1,1.5,2,3)were subjected to solid solution treatment.The 1Zn alloy without electromagnetic stirring did not show any presence of LPSO phase,and the second phase was observed to have dissolved into the matrix after the solid solution treatment,which led to significant increase in the grain size.With increased Zn content,the area fraction of residual LPSO phase increased following the solid solution treatment while the degree of grain growth decreased,and the type of LPSO phase transformed into 14-LPSO phase.However,the alloy with electromagnetic stirring still showed a 14 H structure of LPSO phase with larger quantity of residual LPSO phase.The aged hardening process of the alloy was studied by holding the alloys at 200 ℃ for10 min to 132 h after solid solution treatment.The aged hardening curve became flatter with an increase in Zn content,indicating that a larger amount of residual LPSO phase remained after solid solution treatment,and fewer β’ phases precipitated during aging.The 3Zn alloy demonstrated the weakest strengthening effect during aging,and formed a laminar LPSO phase.The optimal performance was observed in the 1.5Zn alloy with electromagnetic stirring,with a strength and elongation of 213.5 MPa and 9.7 %,respectively.On the other hand,the best performance was observed in the 2Zn alloy without electromagnetic stirring,with a strength and elongation of 202.8 MPa and 5.5 %,respectively. |
