First-principles Study Of ?-Fe With Rare Earth Element Doping

Iron and steel material is the most widely used structural material because of its excellent performance and low cost,and how to improve its performance has been one of the hot research spots recently.As we all know,adding a small amount of rare earth elements can greatly improve the steel's performance,but at present its specific mechanism at atomic scale is still misunderstood.In this thesis,we carried out the first principles calculations based on density functional theory to study the effects of 17kinds of rare earth elements doping on the structure,stability and mechanical properties of?-Fe,in addition,the structures and properties of Fe-Ce intermetallics that were commonly found in RE steels were also investigated.The calculated results of solid solution enthalpies show that all kinds of rare earth atoms cannot spontaneously solute in?-Fe at 0 K,of which the solid solution ability of Sc is relatively strongest,whereas Pr atom is the worst.According to the calculation results of binding energies,the Fe-Ce solid solution has the best stability,where the stabilities of Fe-Er and Fe-Yb were relatively poor,indicating that the Fe-Ce bond is the strongest among all Fe-RE chemical bonds,and Fe-Er and Fe-Yb bonds are relatively weak.The effects of rare earth element doping on the mechanical properties of?-Fe can be evaluated from two aspects of elasticity and plasticity.Firstly,the single crystal independent elastic constants of the solid solution were calculated according to the stress-strain method.Based on this,the polycrystalline elastic modulus of the solid solution can be obtained according to the V-R-H approximation.The calculation results show that the doping of rare earth elements will reduce the elastic constants and elastic modulus of?-Fe,among which Ce element has the greatest reduction.The calculated Pugh criterion B/G,Poisson's ratio?,and Cauchy pressure C'all indicated that only the Fe-Ce solid solution is brittle,and the others are all plastic.At the same time,the doping will increase its elastic anisotropy,among all the rare earth elements,the doping of Ce element has the least increasement in the elastic anisotropy.The plastic deformability of a crystal can be characterized by generalized stacking fault energy.First,the generalized stacking fault energies of{110}<111>,{112}<111>and{123}<111>slip systems of?-Fe were calculated.The{110}<111>slip system has the lowest unstable stacking fault energy?_us,which means the energy barrier of plastic deformation is the least in this slip system.After rare earth elements doped,the generalized stacking fault energy curves of each slip systems decreased to some extent,indicating that the doping of RE elements can promote the activation of each slip system.The calculated formation energies and binding energies of the Fe-Ce intermetallics show that only CeFe₂ can be generated spontaneously at 0 K,and it has the strongest stability,the stability of the three compounds is also analyzed according to the electronic density of states.The calculation results of elastic properties show that all three compounds are stable mechanically,and both of them belong to plastic materials.Among them,CeFe₂ has the best toughness and the lowest elastic anisotropy.Finally,based on the quasi-harmonic approximation,the thermodynamic properties of the most stable phase CeFe₂ were calculated.The phonon spectrum and corresponding phonon density of were obtained.Based on this,the Helmholtz free energy,bulk modulus,expansion coefficient and Heat capacity varies with temperature were further calculated.