The Influence Of Zn/Gd Content On The Microstructure And Mechanical Propertis Of Mg-8Li Alloy

Magnesium-lithium alloy is the lightest metal structure material in engineering application,which is called for ultra-light alloy,and has broad application prospect in aerospace,weapon equipment and other fields.However,Mg-Li alloy has the disadvantages of low absolute strength and poor thermal stability,which limits its wide application.Based on the present situation of low strength of Mg-Li alloy and insufficient research on strengthening mechanism,this paper explored the effects of Zn and Gd content and Zn/Gd ratio on microstructure and mechanical properties of as-cast and as-extruded Mg-Li-Zn-Gd alloys,with the aim of developing high performance new Mg-Li alloy materials,and providing theoretical and technical support for expanding the application of Mg-Li alloy.The main results are following:Mg-8Li-x Zn-yGd?x=1,2,3,4;y=1,2 wt.%?alloys were prepared by vacuum smelting with argon gas as a protective gas.The results showed that the cast Mg-8Li-xZn-yGd alloys mainly consisted of?-Mg phase,?-Li phase,W-phase?Mg₃Zn₃Gd₂?,LiMgZn and Mg₃Gd phases.With the increase of Zn content,the number of long strips of W-phase increased gradually,the number of block-like Mg₃Gd phase decreased gradually;The yield strength and ultimate tensile strength of Mg-8Li-xZn-1Gd?x=1,2,3,4 wt.%?alloy increased gradually with the increase of Zn content,and the elongation firstly increased and then decreased.It is found that the increase of the strength of the alloy is mainly due to the second phase enhancement of the small W-phase and the solid solubility enhancement of Zn.It is also found that the action mechanism of Gd is different from that of Zn,with the increase of Gd content,the number of Mg₃Gd phase increases,and W-phase is gradually coarsening and discontinuous network structure is formed.The yield strength and ultimate tensile strength of Mg-8Li-4Zn-yGd?y=1,1.5,2 wt.%?decreased gradually with the increase of Gd content,and the elongation increased gradually.The results show that Mg-Li-Zn-Gd alloy with the Zn/Gd ratio of 4:1 has the highest reinforcement effect.The effects of solid solubility treatment and aging treatment on the mechanical properties of Mg-8Li-xZn-yGd alloy were explored.The results showed that the strength of Mg-8Li-xZn-yGd alloys were not improved obviously after solid solubility treatment,and the elongation was obviously improved,more than 30%.With the increase of aging time,the hardness increased first and then decreased,and the peak aging time was 4 hours under the aging treatment of 75?and 125?.In contrast,the hardness of alloys with solid solubility treatment and aging treatment was greater than the hardness of as-cast alloys direct aging treatment.On the basis of the above studies,with the fixed Zn/Gd ratio of 2 and 4,the Mg-8Li-x?2Zn-Gd?and Mg-8Li-x?4Zn-Gd??x=1,1.5,2 wt.%?alloys were designed.The effects of the content of Zn and Gd on the microstructure and mechanical properties of as-cast and as-extruded Mg-Li alloys with fixed Zn/Gd ratio were studied.The results showed that when the Zn/Gd ratio is the same for as-cast Mg-Li alloys,the average size of?-Mg grains decreased gradually with the increase of Zn and Gd content.The average grain size of the Mg-8Li-8Zn-2Gd is minimum,about 36?m.The number of second phase increased gradually with the increase of Zn and Gd content.The coarse network-shaped Mg₃Gd phase distributed in the?-Mg matrix.In the as-extruded alloys,the Mg₃Gd phase appeared as hollow skeletal shape,the size of the LiMgZn particles were small,around 1?m,the size of W-phase was about 5¹0?m.After extrusion deformation,W-phase and LiMgZn phase were fully broken,but some skeletal Mg₃Gd phase were not fully broken.The second phases in the alloys were characterized by TEM,and it was found that the distribution of W was along crystal boundary,the LiMgZn phase mainly distributed in the?-Li matrix,and there was a certain orientation relationship with the?-Li matrix.TEM also found a very small spherical particle,the size of 30²00 nm,dispersed in the?-Li,and not in the?-Mg.The strengthening mechanism of as-extruded containing Zn and Gd Mg-Li alloys is revealed,that is,the strength of alloy increases with the increase of Zn and Gd content,which is mainly due to the fine crystal reinforcement of?-Li of dynamic recrystallization after extrusion process,the second phase reinforcement of extrusion crushed W-phase and LiMgZn,and the dispersion enhancement of spherical particles.On the basis of theoretical research,the Mg-8Li-8Zn-2Gd,new type Mg-Li alloy,with the optimal comprehensive mechanical properties was developed,with yield strength,ultimate tensile strength and elongation of 274 MPa,283 MPa and 39.9%,respectively.