Investigation On Electronic Structure And Mechanical Property Of Ti-Ni Shape Memory Alloy Doped With Ca Or Mg Element

Based on the first principles calculation method of density functional theory(DFT),this dissertation focuses on the electronic structures and mechanical properties of Ti-Ni alloys doped with Ca or Mg chemical elements.The intrinsic relationship between comprehensive properties and electronic structures of Ti2Ni,TiNi and TiNi3 systems is analyzed systematically by calculating the enthalpy of formation,phonon spectrum,electronic structure,elastic properties,Debye temperature,thermal conductivity and Crystal Orbital Hamiltonian Population(COHP).The electronic structures and mechanical properties of Ti2Ni and TiNi alloys doped by Ca or Mg are further investigated,and the influence mechanism of doping on the phase transformation process is analyzed by taking TiNi doped with Ca or Mg as an example.Some calculated results show that:(1)Undoped TiNi,Ti2Ni and TiNi3 alloys are thermodynamically stable,and the order of thermodynamic stability is TiNi3>TiNi>Ti2Ni.The order of ductility is Ti2Ni>TiNi>TiNi3.Among the three alloys,TiNi has the highest degree of elastic anisotropy,TiNi3 has the highest Debye temperature,TiNi3 alloy has the strongest interatomic bonding ability.The mechanical and thermal properties of TiNi,Ti2Ni and TiNi3 alloys are mainly caused by the different contributions of Ti-3d and Ni-3d orbital electrons to the total density of state(TDOS).(2)The calculation of Ca-doped Ti2Ni system shows that the crystal system of the doping system changes from cubic to orthogonality,tetragonal,even to triclinic when the Ca doping concentration increases from 4.17 at%to 12.50 at%,resulting the symmetry decrease.The orbital hybridization between Ca-3d and Ti-3d weakens the original covalent bond,thereby,strengthens the metal bond among Ca,Ti and Ni atoms.The ductility of Ti64(Cax+Ni32-x)system is superior to(Cax+Ti64-x)Ni32 system.The hardness and Debye temperature of(Cax+Ti64-x)Ni32 system are higher than that of Ti64(Cax+Ni32-x)system.The Mg-doped Ti2Ni system has a similar doping effect.(3)The Ca or Mg doping tends to replace Ti sites in TiNi alloy.The addition of Ca or Mg element reduces the bonding strength between Ni and Ti atoms.The phase transition temperature has been decreased although which reduces the stability of the doped system structure.The elastic constant C44 and C’ of doped TiNi alloy with B2 phase increases in a varying degrees,the anisotropy factor A is still very small.The martensitic transformation path of TiNi alloy doped with Ca or Mg is B2→B19’,and the transformation temperature Ms decreases after doping.