First-principles Calculations Of Elastic Modulus,Magnetic-structural Transformation And Electronic Structure Of Ni₂Mn-based Ferromagnetic Shape Memory Alloys

Ni2MnX(X=sp element)Heusler alloy is a new type of ferromagnetic shape memory alloy with many excellent magnetoresponsive functional behaviors,and then bringing about potential applications in the fields of smart actuator and solid refrigeration.It is known that the sp element has important influences on magnetic properties,structural transformation and mechanical properties in Ni2Mn-based alloys.Therefore,it should be meaningful to reveal the effects of sp element on magnetic-structure transformation,electronic structure and elastic modulus.Moreover,very recently,people found the Ti doping can significantly improve magnetoresponsive and mechanical performances in Ni2Mn-based alloys.However,the underlying mechanism of Ti doping is still unclear,which seriously hinders performance optimization for Ni2Mn-based alloys.In this thesis,we systematically studied the features of crystal structure,magneticstructural transformation,elastic modulus,electronic structure and lattice vibration of Ni2MnX(X=Al,Ga,In,Si,Ge,Sn,Sb and Te)alloys and Ni2MnGa1-xTix(x=0,0.25,0.5,0.75,and 1.0),Ni2Mn1-xTixGa(x=0,0.25,0.5,0.75,and 1.0)and Ni2-xTixMnGa(x=0,0.25,0.5,1.0,1.5 and 2.0)alloys by means of first-principles calculations.With these calculations,the effects and underlying mechanism of variation of sp element and Ti doping were revealed.In addition,a high-throughput first-principles calculations software package ZVASP has been developed so as to design new ferromagnetic shape memory alloys.We found that the austenite phases of Ni2MnX(X=Al,Ga,In,Si,Ge,Sn,Sb and Te)alloys possess Cu2MnAl-type L21 structure.Among them,Ni2MnSb and Ni2MnTe alloys violate the Born stability criterion and then should be unstable.In Ni2MnX alloys,it exists d-d interaction between Ni-Mn atoms,p-d interaction between Ni-X elements and covalent interaction.The sp element dependence of elastic constant C11?C12?C44 and bulk modulus B is closely related to the covalent interaction in Ni2MnX alloy;The phase stability is related to the distribution of density of states(DOS)near the Fermi level,which is majorly decided by the p-d interaction between Ni-X atoms;It exists acoustic TA2 mode softening for Ni2MnX(X=Al,Ga and In)alloys,implying that these compounds might be unstable.What's more,it is found that the equilibrium lattice constant of Ni2MnGa alloy increase for all substitutions of Ga,Mn and Ni by Ti.With the increased Ti content,the stability of austenite in Ni2MnGa1-xTix alloys increases first and then decreases,whereas it monotonously enhances for Ni2Mn1-xTixGa and Ni2-xTixMnGa alloys.The stability of austenite was confirmed to be related to the distribution of electronic states near the Fermi level;Partial substitution of Ga by Ti(x>0.5)can improve the ductility of Ni2MnGa alloy,while the replacements of Mn and Ni by Ti have no significant effects.In order to find new ferromagnetic shape memory alloys efficiently,we coded a highthroughput first-principles calculations software package ZVASP.It includes five functional modules,i.e.,structural optimization(SO)module,structural transformation(TD)module,electronic structure(ES)module,elastic properties(EC)modules and crystal dynamics(LD)module.Using ZVASP,a series of possible ferromagnetic shape memory alloys,such as Ni2MnGa1-xZx(Z=Mg,V and Zn),were predicted.In summary,the effects and mechanisms of the variation of sp elements and Ti doping on the features of crystal structure,magnetic-structural transformation,elastic modulus,electronic structure and lattice vibration were studied.In addition,we developed a highthroughput first-principles calculations software package to predict new ferromagnetic shape memory alloys.This thesis is expected to provide some guidance for the development of Ni2Mn-based ferromagnetic shape memory alloys.