First-principles Calculations Of Elastic Modulus,magnetic-structural Transformation And Electronic Structure Of Ni₂MnGa Ferromagnetic Shape Memory Alloys Doped By Different Electron Orbital Elements

Ni2MnGa alloy is a new type of ferromagnetic shape memory alloy with many excellent magnetoresponsive functions,and then bringing about potential applications in the fields of smart actuator and solid refrigeration.It is found that the sp element,3d element and interstitial atoms have an important influence on the magnetic properties,structural phase transition and mechanical properties of Ni2MnGa alloy.There is no report of s element doping Ni2MnGa alloy.At the same time,the physical mechanism of doping Ni2MnGa alloys with different orbital elements is still unclear,which hinders the performance optimization of Ni2MnGa alloys.Therefore,it is of great theoretical and practical significance to reveal the laws and mechanisms of different orbital elements affecting the magnetic-structure transition,electronic structure and elastic modulus of Ni2MnGa alloy.In this thesis,we systematically studied the features of crystal structure,magneticstructural transformation,elastic modulus,electronic structure and lattice vibration of the different contents of Mg,Cu,Si and B elements are substituted for Ni2MnGa1-xZx(x=0,0.25,0.5,0.75 and 1.0).With these calculations,the effects and underlying mechanism of variation of of s,3d,sp and interstitial doping were revealed.We found that the most stable crystal structure of most Ni2MnGal-xZx(x=0,0.25,0.5,0.75 and 1.0;Z=Mg,Cu,Si,and B)alloys is Cu2MnAl-type L21 structure.Only the most stable crystal structure of Ni2MnB possess Hg2CuTi-type structure.All Ni2MnGal-xZx(x=0,0.25,0.5,0.75,and 1.0;Z=Mg,Cu,Si,and B)alloys have negative formation energies and could be experimentally synthetic.For Ni2MnGal-xMgx(x<0.15 or x>0.9),Ni2MnGa1-xCux(x<0.15 or x>0.65),Ni2MnGa1-xSix(x=0?1)and Ni2MnGa1-xBx(x=0?0.55)are in accordance with BronHuang stability criterion and has a stable L21 austenitic structure.The results showed that NiMn-Ga magnetically induced shape memory alloy doped with sp element had better mechanical stability.What's more,it is found that the equilibrium lattice constant of Ni2MnGa alloy increase for substitutions of Ga by Mg.On the contrary,it is found that the equilibrium lattice constant of Ni2MnGa alloy reduce for all substitution of for Ga by Cu,Si and B The phase stability is related to the distribution of density of states(DOS)near the Fermi level.With the increase of doping content by Mg and Si,the austenite stability of Ni2MnGa1-xMgx and Ni2MnGa1-xSix alloys increases gradually,and the c/a of alloy martensite decreases gradually,which helps to reduce the heat hysteresis loss in alloy martensitic transformation.On the contrary,the energy difference between martensite and austenite of Ni2MnGa1-xCux and Ni2MnGa1-xBx increases with the increase of doping constant by Cu and B elements.This phenomenon is related to the distribution of electronic states near the Fermi level.The curvature difference between the two parts of the Fermi surface connected by the nested vector increases gradually with the increase of Si content,which is the cause of the increase of austenite stability of Ni2MnGa1-xSix(x=0?1)and the decrease of phase transition temperature in the experiment.The results show that the phase transition temperature of Ni2MnGa alloy is positively correlated with the valence concentration when it was doped by s elements and d elements,while the phase transition temperature of Ni2MnGa alloy doped by sp elements decreases with the increase of sp elements content.Ni2MnGa1-xMgx(x<0.15 or x>0.9)is predicted to be a possible new ferromagnetic shape memory alloys system.And Ni2MnCu is predicted to be a new ferromagnetic shape memory alloy of full 3d elements.We found that owing to the significantly weaker covalent bonding between Mg-Ni and Cu-Ni than that between Ga-Ni,the Ga substitution by Mg might be favorable for improving the ductility of Ni2MnGa alloys.In summary,the effects and mechanisms of the variation of s elements,3d elements,sp elements and interstitial doping on the features of crystal structure,magnetic-structural transformation,elastic modulus,electronic structure were studied.And predicts Ni2MnGa1xMgx(x<0.15 or x>0.9)is a new type of magnetically induced shape memory alloy system.Ni2MnCu is a new type of full 3d elements magnetron shape memory alloy.It is expected to provide some guidance and help for the development of Ni2MnGa and even NiMn-based ferromagnetic shape memory alloys.