Study On Microstructure Evolution And Room/High Temperature Mechanical Behavior Of Mg-Y-Sn Alloys With Different Alloying Contents

Mg alloys have been gradually applied in the fields of transportation,electronic 3C and biomedicine due to their low density and high specific strength.Among high temperature Mg alloys,Mg-RE alloys such as WE54 and GW103 exhibit excellent high temperature mechanical properties due to the existence of stable and finely dispersed precipitates and solute segregation at high temperature,but at the same time these alloys show high rare earth content,and the plasticity at high temperature also needs to be improved.After adding a certain amount of Sn element to the Mg-Y alloy,the high-density and dispersive coherent Sn₃Y₅ phase can effectively hinder the dislocation slip and grain boundary slip during the deformation process,thereby significantly improving the room temperature mechanical properties of the alloy.Therefore,in this paper,Mg-x Y-y Sn alloy containing single Sn₃Y₅ phase was prepared by adjusting the weigth percent ratio of Y and Sn.Combined with scanning electron microscope（SEM）,backscattered electron diffraction（EBSD）and room/high temperature tensile experiments,the evolution of precipitates,dynamic recrystallization behavior and mechanical properties with alloy content was analyzed.Finally,the high-temperature strengthening mechanism of Mg-6Y-1.5Sn alloy with excellent mechanical properties at high temperature was further analyzed by quasi-in-situ tensile experiments.The main research results are listed as follows:By adjusting the weigth percent ratios of Y and Sn（10 and 4）,Mg-xY-y Sn alloys containing a single Sn₃Y₅ phase were prepared,and the effects of alloying element content on the microstructure evolution and room/high temperature mechanical properties of Mg-x Y-y Sn alloys were studied.It is found that the as-extruded Mg-x Y-y Sn alloys all exhibit a bimodal structure,in which the recrystallized grain size is about 6～9μm.At the same Y and Sn ratio,the content of precipitates increases with the raise content of alloying elements.When the Y content reaches 6wt.%（Mg-6Y-0.6Sn and Mg-6Y-1.5Sn）,the density of dispersed particles reaches its peak value.After the total alloy content exceeds at 10 wt.%,the microsized Sn₃Y₅accumulate obviously and the nanosized precipitates disappears.Besides,at the same Y and Sn ratio,the room and high temperature mechanical properties of Mg-x Y-y Sn alloy increased first and then decreased with the increase of alloying element content.Among them,Mg-6Y-0.6Sn and Mg-6Y-1.5Sn alloy exhibits excellent high temperature strength.The yield strength of Mg-6Y-1.5Sn alloy at 200℃and 300℃reaches 193MPa and 186MPa,and the tensile strength reaches 337MPa and 328MPa,and its strength at 300℃is obviously better than that of commercial WE54 and most GW alloys with favourable strength at high temperature.In order to explore the difference in mechanical behavior of alloys with the same ratio of Y and Sn,and the reasons for the differences in high temperature mechanical properties of different alloy when the content of alloy elements is close to the same,the dynamic recrystallization behavior and work hardening behavior of Mg-Y-Sn alloys were analyzed.The results show that the Mg-x Y-y Sn alloy undergoes both continuous dynamic recrystallization and discontinuous dynamic recrystallization during the hot extrusion process.Under the same ratio of Y and Sn,the ratio of dynamic recrystallization first decreased and then increased with the increase of alloy element content.Among them,Mg-6Y-0.6Sn and Mg-6Y-1.5Sn alloys show the highest work hardening rate at room and high temperatures.Besides,when the alloy content is close,the dynamic recrystallization ratio of Y and Sn ratio of 4（Mg-8Y-2Sn,Mg-4Y-1Sn）is smaller than that of Y and Sn ratio of 10（Mg-10Y-1Sn,Mg-5Y-0.5Sn）alloys,since the Sn can significantly reduce the stacking fault energy of the alloy,and the nano-precipitate phase hinders dynamic recrystallization.Compared with other alloys,Mg-6Y-0.6Sn and Mg-6Y-1.5Sn alloys have the highest initial work hardening rate at room and high temperatures,which is presumed to be mainly related to the large number of nanosizedprecipitates in the alloy hindering dislocation and grain boundary movement.When the total alloy content is close,the alloy with the ratio between Y and Sn of 4（Mg-8Y-2Sn,Mg-4Y-1Sn）is compared to the alloy with the ratio of Y and Sn of 10（Mg-10Y-1Sn,Mg-5Y-0.5Sn）alloys exhibit a higher work hardening rate,mainly due to the lower dynamic recrystallization ratio of the former,resulting in a larger strain energy storage in the un-DRXed region,and which is not conducive to dislocation initiation during deformation,resulting in a lower dynamic recovery rate.Besides,more substructures in the unrecrystallized structure may also alleviate the softening caused by dynamic recovery.To further study the effect of Sn addition in Mg-Y alloys,the microstructure and room/high temperatures mechanical properties of Mg-6Y,Mg-6Y-0.6Sn and Mg-6Y-1.5Sn alloys were compared.It was found that the addition of Sn transformed the precipitates in the alloy from Y-rich phase to Sn₃Y₅ phase,and the dynamic recrystallized grain size was refined from～18μm to～7μm.In addition,nanosized precipitates appeared in the alloy after the addition of Sn.Besides,completely recrystallized structure has changed to biomidal structure,and the recrystallization ratio decreased with the increase of Sn content.Compared with the Mg-6Y alloy,the high temperature strength of the alloy is significantly improved after adding Sn element.After adding 0.6wt%Sn,the yield strength increased by 73%,71%and 85%,and the ultimate tensile strength increased by 27%,31%and 23%at room temperature,200°C and300°C,respectively.When the Sn content was further increased to 1.5 wt.%,the yield strength increased by 49%,66%and 90%,and the ultimate tensile strength increased by 40%,50%and81%at room temperature,200°C and 300°C,respectively.The Mg-6Y-1.5Sn alloy exhibits excellent strength and work hardening behavior at 300°C,and it is speculated that its high-temperature strengthening mechanism is mainly related to nanosized precipitates and un-DRXed grains.The results of quasi-in situ experiments combined with slip trace analysis show that during the deformation process of the alloy,the proportion of basal slip in the recrystallized region is suppressed to a certain extent,and more than 35%of non-basal slip has been activated.Besides,the un-DRX regions make deformation more difficult due to their hard orientation and high density of dislocations.It is speculated that the higher strength of the alloy is the result of the combined effect of precipitation strengthening and dislocation strengthening and texture strengthening in the unrecrystallized region.