First-principles Investigation Of The Martensitic Transition And Magnetic Properties In Mg-based Heusler Alloys

Heusler alloys have many functional properties,among which the magnetic shape memory effect has attracted much attention due to its large strain range,high response frequency and high energy density.Some Heusler alloys with magnetic shape memory effect also possess other functional properties,such as half-metallicity,giant magnetoresistance and giant magnetocaloric effect.They are widely used in sensors,actuators,magnetic refrigeration and data storage.In this paper,Mg-Y-Z?Y=Sc,Ti,V,Z=Al,Ga,In?alloys have been systematically studied by the first-principles calculation method within the framework of the Density Functional Theory,including the martensitic transition,magnetism,atomic occupancy in defect structure,and the effect of the fourth component on transition and magnetism,etc.We predicte a new class of light-weight shape memory materials,reveal the law and origin of the composition dependence on phase transition temperature and magnetic properties,and provide theoretical foundation for further experimental research.Firstly,the equilibrium lattice constants,magnetic properties,tetragonal distortion,electronic structures,mechanical properties and phonon spectra for Mg₂YZ?Y=Sc,Ti,V,Z=Al,Ga,In?with different structures and magnetic configuration were calculated.The results show that the cubic phases of all alloys are inverse Heusler structure,and the densities range from 2.45 to 4.41 g/cm³.The alloys containing Ti and V are ferromagnetism and have the possibility to undergo martensitic transition.The magnetic moments of Mg₂VZ decrease considerably in the process of tetragonal distortion.Y atom plays a major role in the influence of electronic density of states,followed by Z atom,which directly affect the magnetic properties and the relative stability of two phases.The tendency for tetragonal distortion induced by John-Teller effect is closely related to the shear constant of cubic phase,and the mechanical properties of these alloys need improvement.Secondly,the defect structures of Mg-Y-Z?Y=Ti,V,Z=Al,Ga,In?alloy were studied.We calculated the formation energies of several defects.The Mg-rich Y-poor,Y-rich Mg-poor and V-rich In-poor alloys tend to replace the occupancy of deficient atoms directly by extra atoms,while the other off-stochiometirc alloys tend to form indirect antisite defects.The indirect antisite defect pairs consisting of Mg _Y and Y_Mg1are the easiest to form in stoichiometric alloys.In terms of the magnetic moment,the defect structures for Mg-V-Z alloys have obvious regularity,rather than that of Mg-Ti-Z alloy.Thirdly,the effect of composition on martensitic transition and magnetic properties for Mg₈V_xGa_8-x?x=2,3,4,5,6?alloy were studied.The results of structural relaxation show that substitute extra atoms for Mg and drive Mg to deficient atomic sites are energetically favorable for off-stoichiometric alloys.The structures with the lowest energy of the alloy are determined by the strength and number of interatomic bonds.Further studies indicated that the variation in the lattice constants is related to the shifting of the accumulated electrons in the covalent bonds.The magnetic moments of the extra V are antiparallel to those of V at the normal C sites.The magnetic moments of V are not necessarily affected most by the nearest V atoms.Lastly,the structures of Mg₈Sc_xTi_4-xIn₄?x=0,1,3,4?quaternary alloys were optimized,and its magnetic properties and tetragonal distortion were calculated.The atomic occupancies of alloys with different compositions are similar to that of ternary alloys,but the symmetry is higher.Mg₈ScTi₃In₄ will undergo martensitic transformation with c/a=1.45.The magnetic moment of the tetragonal phase increases considerably relative to that of the cubic phase and the volume increases slightly.Ti atoms close to Sc play a major role in the tetragonal deformation.The splitting and moving of d orbits in Ti atoms far from Sc cause a great change in its magnetic moment.