Effect Of Cryogenic Treatment And Alternating Temperature Cycle On Microstructure And Properties Of Mg-Li Alloy

Because of the special structure of Mg-Li alloy,it has the characteristics of low density,high specific strength and specific stiffness,high elastic modulus and good seismic performance.The two-phase Mg-Li alloy has high strength and elongation and in recent years,its research has gradually increased.The workpiece in the space will receive the role of sustained low temperature,the temperature is lower than the liquid nitrogen temperature,while the cryogenic treatment is also a way to improve the organization and performance of the alloy,the satellite around the earth flight,the workpiece temperature from high temperature to low temperature cycle.In the space can reach 127 ? high temperature,low temperature below the liquid nitrogen temperature.Nowadays,there is no extensive study on the microstructure and properties of Mg-Li alloy in space environment.The research on the cryogenic treatment and the alternating temperature cycle of Mg-Li alloy has not been reported.In this paper,the effects of cryogenic treatment and alternating temperature cycle on the microstructure and properties of Mg-Li alloy were revealed.In this paper,the different orientations of Mg-Li alloy sheet in cold rolling state are analyzed,which consists of 5-phase phase,?-Mg,?-Li,AlLi,MgLiZn,Mg2 Si.There is no difference in the composition of the three faces,and the phase distribution is slightly different.ND surface is equiaxed crystal,the size is about 100?m,TD and RD surface grain is long strip,grain length is about 100?m.In the TD and RD surfaces,the ? phase and the AlLi are distributed in a certain direction,and the resulting tissue is similar to the fiber structure.At the same time,the AlLi phase is mainly distributed in the phase boundary of the ? phase and inside the ? phase,and the massive and short rod Mg2 Si exist.In order to study the influence of alloying conditions on the microstructure and properties of the alloy,the samples were placed in liquid nitrogen and kept for 5 days,10 days,15 days and 20 days.The results show that the composition of the alloy does not change after the cryogenic treatment,and the grain size does not change much,the second phase of the dispersion is obviously increased.The strength of the alloy before and after cryogenic treatment is not changed,the elongation is slightly increased,and found that the fracture mechanism of the alloy before and after cryogenic treatment is microporous aggregation type fracture.As the working temperature of the workpiece in the space environment,up to 127 ?,the lowest lower than the liquid nitrogen temperature,the cycle system used in this paper is low temperature-196 ?,high temperature 127 ?,the maximum cycle to 100 times.The experimental results show that the phase composition of the alloy does not change much after the alternating temperature,but the grain is obviously refined,and the grain orientation is not obvious.The three faces are equiaxed and have a size of about 30?m.The ?-Mg in the surface and the RD surface is spheroidized,and the structure of the alloy becomes uniform and stable.After the alternating temperature treatment,the dispersion of the second phase in the alloy is dissolved in the ?-Li matrix,which reduces the effect of the second phase strengthening and improves the effect of solid solution strengthening.At the same time,when the number of cycles is low,the second phase is more complex,will interact with the dislocation,strengthen the metal,reduce the elongation,when the high-order cycle,the alloy in the second phase of the basic solid solution to obtain pure ?-Li which is the heart structure.And after the high cycle,the strength of the alloy is slightly lower,but the elongation is significantly improved.