First-principles Study On B2 NiTi With Point Defects Under Pressure

There are limited plasticity and strength for NiTi alloy,which leads to the restriction in the field of high performance structure,especially in aerospace.It is found that alloying is an effective method to improve the properties of materials,but there will be a large number of defects with alloying,including vacancy,anti-position,substitutional doping and so on.These structural defects will have a great influence on the properties of materials,such as yield strength,fracture strength and plasticity,so it is very important to study the point defects of materials.For the reason that the research on the point defects of NiTi alloy is mostly concentrated on the substitutional doping,this paper focus on the influence of vacancy and anti-position on the structure and properties of the B2 NiTi,and the first-principles plane-wave pseudopotential method based on the density functional theory is adopted.It is hoped that our study may provide theoretical guidance for the later experimental research of the later people,and the following conclusions are obtained:Compared to the LDA approximation method,the GGA approximation results agree better with the experimental values and other theoretical values,indicating that the GGA approximation is more accurate and reliable.Meanwhile,the error of lattice parameters of NiTi alloy at 0GPa is less than 0.5%,thus the setting of calculation parameters is feasible.Further analysis shows that the lattice distortion formed by the anti-position Ti_Nii leads to the larger lattice parameters and the volume expansion,and other point defects lead to the smaller lattice parameters and the volume contraction.The calculation shows that the formation heat and cohesive energy of NiTi alloys at 0GPa are both negative,which proves that the five structures are stable.The absolute values of formation heat and cohesive energy decrease as the order of Ni₈Ti₈>Ni₉Ti₇>Ni₇Ti₉>Ni₈Ti₇>Ni₇Ti₈,which indicates that the anti-position is more easily formed than the vacancy in NiTi alloy.Pressure calculation shows that the system has strong compressibility,and the point defects may weaken the anti-volume deformation of Ni Ti alloy.The elastic constants of NiTi alloys are consistent with the criterion of mechanical stability under pressure,the P-C curves change smoothly during the process of pressure loaded,indicating that there is no phase transition.VRH calculation shows that the volume modulus B,shear modulus G,Young's modulus E,Poisson's ratio and B/G value of NiTi alloy will increase under pressure,indicating that the pressure may improve anti-volume deformation,shear deformation,stiffness and plasticity of NiTi alloy.The volume modulus B of Ni Ti with point defects increase under pressure,which indicates that the pressure could also improve the anti-volume deformation of NiTi with point defects,the point defects may weaken the anti-volume deformation of NiTi alloy.The shear modulus G,young's modulus E and hardness H of NiTi alloy with point defects will change to different degrees,in general the point defects may weaken the shear deformation,stiffness and plasticity of NiTi alloy,and the point defect Ti_Nii has the minimal impact on the above three kinds of properties of Ni Ti alloy.It is found that the anti-volume deformation of NiTi alloys is stronger than the shear deformation under pressure.The calculation of Poisson ratio?and B/G show that NiTi alloy with point defects are the ductile material,and the pressure may improve the ductility of Ni Ti alloy.In addition,the pressure could also change the anisotropy of NiTi alloy with point defects,the anti-position defect Ni_Ti,the vacancy defect V_Nii and the V_Tii will increase the anisotropy of the NiTi alloy.The analysis of DOS shows that all the five NiTi alloys exhibit covalency and metallicity.Meanwhile,the values of E_F decrease under pressure,indicating that Ni Ti alloys tend to be stable without phase transformation.Population analysis shows that pressure may weaken the metallicity and enhance covalency of NiTi alloys.The thermodynamic analysis shows that the bulk modulus B and the Debye temperature?_D of NiTi alloys increase with pressure,and decrease with temperature,while the heat capacity C and thermal expansion coefficient?decrease with pressure,and increase with temperature.