Theoretical Investigation Of The Martensitic Transformation Mechanism And Property Optimization In Mg-Sc Based Lightweight Shape Memory Alloys

Mg-Sc shape memory alloy is the lightest shape memory alloy with excellent superelastic property,corrosion resistance and ductility.It is a new intelligent material with broad prospects.However,the martensitic transformation temperature of Mg-Sc shape memory alloy is extremely low,the cost is very high and the hardness is poor.These problems seriously restrict the engineering application and development of this kind of lightweight shape memory alloy and have not been solved yet.The fundamental reason is that the martensitic transformation mechanism of Mg-Sc shape memory alloy is still not clear,leading to the lack of effective solutions to these problems.In this thesis,the martensitic phase structure,martensitic transformation mechanism and mechanical properties of Mg-Sc shape memory alloy have been systematically studied based on first-principles calculation method.Martensitic transformation temperatures,mechanical properties and physical nature of them were clarified in rare earth element（Y,La,Ce,Eu,Gd,Dy,Yb）doped Mg-Sc shape memory alloy.The optimal doping elements and content were predicted to improve the transformation temperature and hardness and save cost.It is found that Mg and Sc elements tend to form solid solution alloy,and the atomic distribution of Mg-Sc shape memory alloy is random.In this thesis,special quasirandom structures（SQS）method is used to generate body-centered cubic austenite,and martensites with orthorhombic and hcp structures are build according to Burgers distortion.From the perspective of energy and lattice dynamics,martensite of Mg-Sc shape memory alloy is determined to be orthorhombic structure,which solves the problem of unclear martensite structure in Mg-Sc shape memory alloy,and lays a foundation for revealing the mechanism of martensite transformation.In order to reveal the mechanism of martensitic transformation of Mg-Sc alloy,the phonon dispersion curves of Mg-18.75 at.%Sc shape memory alloy were analyzed based on the Sc content in experimental studies.It is shown that the main reason of martensitic transformation is the instability of austenite structure caused by softening of the acoustic branch atΓpoint.Further analysis from the level of electronic structure shows that the electro-phonon coupling effect causes the overlap between the p_z orbital of Mg and the d_xy orbital of Sc,which results in the Fermi surface nesting and softening of the acoustic branch atΓpoint.In addition,the martensitic transformation path of Mg-18.75 at.%Sc shape memory alloy was studied from the perspective of energy.The results show that there is no barrier in the martensitic transformation path of Mg-18.75 at.%Sc alloy.In order to further reveal the effect of temperature on the stability of martensitic phase,phonon dispersion curve at finite temperature were calculated by introducing anharmonic effect.The results show that the instability of austenitic structure of Mg-18.75 at.%Sc alloy is caused by softening of the acoustic branch atΓ-R.With the decrease of temperature,the stability of austenite decreases gradually.The instability rangeΓ-R corresponds to two adjacent（1 0 1）crystal planes close to each other,and this kind of crystal plane movement makes the austenite structure transform into martensite structure.In addition,the phase stabilities and martensitic transformation path of Mg-15.625 at.%Sc and Mg-21.875 at.%Sc alloys were studied.It is found that the martensitic transformation occur around 18.75 at.%Sc.Based on the effect of Sc element on phase stability in martensitic transformation mechanism,doping rare earth elements（Y,La,Ce,Eu,Gd,Dy,Yb）replacing Sc was proposed to improve the martensitic transformation temperature,improve hardness and reduce the cost of Mg-Sc shape memory alloy.Through the calculation of defect formation energy,it is found that rare earth elements doping in Mg-Sc system will preferentially replace the position of Sc atom.In addition,based on the Helmholtz free energy,a fast and accurate calculation method of martensitic transformation temperature was designed.The martensitic transformation temperature of Mg-18.75 at.%Sc shape memory alloy was calculated to be 161 K,which is close to the experimental results（183 K）.The results show that the doping of Y,Ce,Eu and Dy reduces the martensitic transformation temperature of Mg-Sc shape memory alloy,while La,Gd and Yb increase the martensitic transformation temperature.In particular,the martensitic transformation temperature of 1.25 at.%Gd doped was increased by nearly 30 K and hardness also increased.However,the martensitic transformation can be inhibited by increasing Gd content,and the optimal doping content of Gd element is around 1.25 at.%.The results indicate that proper amount of Gd doping can simultaneously improve the martensitic transformation temperature and hardness of Mg-Sc shape memory alloy,and reduce the cost.This thesis lays a foundation for the performance optimization of Mg-Sc shape memory alloy,and provides reference for the research and development of new lightweight shape memory alloys.