Research On Mechanisms In Martensitic Transformation And Properties Modulation Of Mg-based Lightweight Shape Memory Alloy

Shape memory alloys have excellent properties and have been widely used in aerospace,biomedicine and automotive industries.However,the density of traditional shape memory alloys is above 4.5 g/cm3,which cannot meet the needs of lightweight alloys.The recently discovered MgSc alloy has this memory effect,and its density is only 2 g/cm³.It is a promising lightweight memory alloy.However,the low martensite transformation temperature and high cost of this alloy severely limit its practical application.Using the first-principles calculation method,on the basis of revealing the martensite transformation mechanism of the alloy and the law of the phase transformation path,the strategy of using Ca and rare earth elements La and Ce to replace is proposed to solve the low transformation temperature and high cost of MgSc alloy.The calculation results show that the Sc content is 15.625-18.750 at.%,MgSc alloy can undergo martensitic transformation.The density of states results show that electron-phonon coupling is the fundamental cause of lattice instability in the MgSc alloy system.At the same time,the phase transition temperature of the alloy increases as the Sc content decreases,when the Sc content decreases to 15.625 at.%,the martensite transformation temperature of Mg₂₇Sc₅ alloy reaches 400K.The calculation results of mechanical properties show that the G/B value of Mg₂₇Sc₅alloy is less than 0.57,and v is about 0.31 greater than that of Mg₂₆Sc₆ alloy.Therefore,Mg₂₇Sc₅alloy has better plasticity and ductility.Taking Mg₂₇Sc₅ alloy as the research object,the method of replacing Ca element is adopted.The total energy difference between the austenite phase and the martensite phase of the alloy increases,indicating that the martensite transformation temperature increases.When the replacement calcium content is 6.25 at.%,the martensitic transformation temperature of Mg₂₇Sc₃Ca₂ alloy reaches 442 K.At the same time,the density of Mg₂₇Sc₃Ca₂ alloy can increase by about 1.84 g/cm3.The calculation results of mechanical properties show that the G/B value of Mg₂₇Sc₄Ca₁alloy is less than 0.57,the v value is 0.29.Therefore,it has better ductility and plasticity.In addition,in order to further increase the martensitic transformation temperature of the alloy,the rare earth elements La and Ce are used here instead of methods.As a result,it is found that the addition of the rare earth elements La and Ce can significantly increase the martensite transformation temperature of the alloy.When replacing the miscellaneous La content to 6.25 at.%,the martensite transformation temperature of Mg₂₇Sc₃La₂ alloy is as high as 468 K.The calculation results of mechanical properties show that the G/B values of the replacement of rare earth are all less than 0.57,and the v values are all greater than 0.3.Therefore,MgSc RE(RE=La,Ce)alloy has excellent ductility and plasticity.