Effect Of Sn On Microstructure And Mechanical Properties Of Mg-Gd-Zn Alloy

Magnesium alloys,as a new type of light metal structural materials,have high specific strength,high specific stiffness and other excellent characteristics,and have broad application prospects,and are widely used in aviation,electronic information,automotive and other fields.Mg-Gd-Zn alloys have attracted extensive attention from researchers at home and abroad because of their high strength,good properties,good aging response and special long-period structure.In this paper,Mg-15Gd-2Zn-xSn alloys were prepared by electromagnetic induction equipment.The microstructures and mechanical properties of these alloys were observed,tested and analyzed.The effects of Sn on the microstructures and mechanical properties of Mg-15Gd-2Zn alloys were studied,and the differences of LPSO structure and second phase in different alloys were compared to enrich magnesium.The theory of alloying and strengthening of alloys.The changes of microstructures and mechanical properties of Mg-15Gd-2Zn-xSn alloy in as-cast state were studied.The results show that the structure of as-cast Mg-15Gd-2Zn-xSn alloy is mainly dendrite,the addition of Sn can refine dendrite,and the secondary dendrite spacing decreases significantly.The long-period phase of the alloy is mainly 14H-LPSO phase with fine lamellar distribution in the grains,which tends to grow from near the grain boundary to in the grains.When a small amount of Sn??1.0 wt.%?is added to the alloy,the length of the long-period phase becomes longer and the density increases.The addition of excessive Sn??2.0 wt.%?will result in the formation of coarse bulk MgGdSn phase in the alloy.The mechanical properties of as-cast alloys increase first and then decrease with the increase of Sn content.The comprehensive mechanical properties of Mg-15Gd-2Zn-0.5Sn alloys are the best.The yield strength is 189 MPa,the tensile strength is 268 MPa and the elongation is 7.6%.The microstructures and mechanical properties of Mg-15Gd-2Zn-xSn alloy under different heat treatments were studied.The results show that the microstructures and properties of the alloys are quite different at different solution treatment temperatures.The structure of Mg-15Gd-2Zn-xSn alloy after solution treatment at 480?for 12 h is mainly composed of a-Mg matrix,a?Mg,Zn?₃Gd phase at grain boundary and a lamellar 14H-LPSO phase in grain.The addition of trace Sn element can significantly increase the number of 14H-LPSO phases in the alloy and make the lamellar more compact.The grains of Mg-15Gd-2Zn-xSn alloy treated by solid solution at 520?for12 h are all transformed into equiaxed grains.The structure of the alloy mainly consists of medium?-Mg matrix and reticulated?Mg,Zn?₃Gd phase.The lamellar LPSO phase inside the grains almost completely dissolves into the matrix.During the subsequent aging treatment,alloys with different initial solution temperatures exhibit different aging responses.Compared with the samples aged at 480?the aging response of the samples aged at 520?is more remarkable,and their performance is improved more remarkably,but the peak aging time is prolonged.The aging hardening effect of the alloy can be significantly improved by adding a small amount of Sn.At the same time,the addition of Sn can also prolong the peak aging time of the alloy.The Mg-15Gd-2Zn-0.5Sn alloy treated by solid solution at 520?for 12 h has the best properties after aging treatment,its peak yield strength reaches 264 MPa,its tensile strength reaches 392 MPa and its elongation is 3.1%.Varying degrees of dynamic recrystallization of Mg-15Gd-2Zn-xSn alloy occurred after hot extrusion.The grains are mainly fine equiaxed grains.The coarse?Mg,Zn?₃Gd phase and MgGdSn phase are also extruded and broken into fine blocks,which distributes along grain boundaries in a semi-continuous manner.When trace Sn was added,the number of fine grained second phases at grain boundaries increased significantly,and dispersed at grain boundaries.Meanwhile,the lamellar 14H-LPSO phase in the inner layer became slender and finer.The yield strength and tensile strength of the alloy first increased and then decreased with the increase of Sn content.When Sn content reaches 0.5 wt.%,the yield strength,tensile strength and elongation of the alloy reach the maximum value,which are 332 MPa and 454 MPa,respectively.At the same time,the elongation of the alloy decreases to 6.8%.When Sn content reaches 1.0 wt.%,the elongation of the alloy reaches the highest,11.4%.However,as Sn content continues to increase,the yield strength,tensile strength and elongation of the alloy begin to decrease.