First-principles Calculations Of Electronic Structure,elastic Properties And Martensite Phase Transition Of Ni₂MnGa Alloys Doped By Point Defects

NiMn based alloy is a kind of ferromagnetic shape memory alloy,which has many excellent magnetic control functions,such as magnetocaloric effect,elastothermal effect,and magnetoresistance effect,etc.And this kind of material can produce large output strain as high as 10%under the action of a magnetic field and has a high response frequency of kHz,which is expected to become a new magnetic driving and intelligent sensing material.It has aroused wide interest in NiMn based alloys.But at the same time,most of NiMn based alloys are intermetallic compounds,which have disadvantages of poor mechanical properties,high brittleness,and easy fatigue damage.In addition,the stoichiometric NiMn-based alloys tends to have lower phase transition temperatures and weaker magnetic properties,which greatly limits the application range of the alloy.The latest experimental studies show that point defects have an important effect on the mechanical properties and phase stability of NiMn-based alloys,but the physical mechanism is still unclear.This phenomenon greatly restricts the progress of people using point defects to optimize the properties of NiMn-based alloys.Based on this,this paper adopts the first-principles calculation method to systematically study the effects of different types of point defects(replacement atoms,interstitial atoms and vacancies)on the crystal structure,magnetic-structure transition,mechanical properties and electronic structure of Ni2MnGa alloy.The internal mechanism of different point defect doping affecting the above properties of Ni2MnGa alloy.The main conclusions are as follows:We found that the abnormal structural stability of the studied alloy Ni2MnGa1-xAlx is an intrinsic effect caused by the change of chemical composition.Compared with Ga,Al element has a stronger ability to stabilize austenite.In terms of electronic structure,the usual driving force for structural transformation,Fermi surface nested states and few spin electronic states near the Fermi level are not sensitive to Al substitution.On the contrary,with the addition of Al,the covalent hybridization strength between Ni and main group elements monotonically decreases,which may be the reason for the abnormal relationship between the structural stability of the studied alloy and the lattice volume.The results of this work clarified the abnormal phase stability related to the lattice volume,thereby laying a theoretical foundation for the design of advanced magnetic shape memory alloys.In addition,we systematically studied the properties of optimal atomic position,magnetic properties,structural transformation,electronic structure and elastic modulus of Ni2MnGa alloys with different contents of In.It is found that compared to Ga,In improves the magnetic properties of NiMn-based alloys significantly.But In has the effect of stabilizing austenite and reducing the phase transition temperature.This phenomenon may be related to the alloy volume effect.The calculation of Ni2MnGa1-xInx alloy will guide us in the design of magnetic shape memory alloy and magnetocaloric material.In addition to the above research on the point defects of replacement atoms,we also calculated the effects of interstitial atoms and vacancies on the properties of Ni2MnGa alloys.Interstitial atom-doped Ni2MnGa alloys tend to occupy the O-I octahedral gap.Three interstitial atom doped alloys of B,C and N can increase the phase transition temperature and reduce the thermal hysteresis.According to Jahn-Teller effect,the reason why phase transition is easier to occur can be explained.In the study of vacancies,the occupation of vacancies under different components was obtained through structural calculations,and the magnetic state of the optimal structure was determined.In Ni2MnGa alloy,Ga vacancies stabilize martensite,and Ni vacancies stabilize austenite.Mn vacancies have different effects on structural transformation due to different vacancies.The vacancies directly occupy Mn positions to stabilize martensite,and when vacancies indirectly occupy Ni positions,they stabilize austenite.The vacancy-doped Ni2MnGa alloy is still a ductile material,and the direct occupation of Ga vacancies and Mn vacancies can improve the toughness of the material.Vacancies are not conducive to the magnetic properties of Ni2MnGa alloy.To sum up,the crystal structure,magnetic-structure transition,mechanical properties and electronic structure of Ni2MnGa alloy doped with different point defects are systematically calculated.The physical mechanism of Ni2MnGa alloy is revealed by the interaction of replacement atoms,interstitial atoms and vacancies.It is hoped that it can provide certain guidance for the design of NiMn-based ferromagnetic shape memory alloys.