Theoritical Investigations Of Phase Stability,Elastic And Thermodynamic Properties Of γ’ Phase In Co-Ta Based Superalloys

Ni-base superalloys are widely used in Aerospace field because of their excellent high temperature mechanical property,oxidation resistance and gas corrosion resistance.The high temperature comprehensive properties of Ni-base superalloys has inseparable relation with their special γ+γ’ two-phase structures.Researchers have improved the structural stability of metastable phase Co3Al by adding W element in Co-Al binary system and discovered γ’-Co3(Al,W)phase in Co-Al-W ternary system.This orderly precipitated phase γ’-Co3(Al,W)is similar to that of γ’-Ni3Al in Ni-base superalloys,and the structural stability and elastic properties of γ’ phase are critical to high temperature mechanical properties of Co-base superalloys.This discovery provides a guideline for the development of multi-component Co base superalloys.Under these circumstances,it is necessary to study the effects of alloying elements on the structural stability of metastable phase in Co-based superalloys.In order to guide the design and development of multi-component Co-based superalloys,based on the density functional theory,this study systemically investigated the effects of alloying elements Al,Ti,V,Nb,Mo and W on the structural stability,elastic property,electronic structure and thermodynamic property of metastable phase γ’-Co3Ta in Co-Ta binary system from the first-principles calculations.The main contents of this thesis are as follows:(1)The effects of alloying elements X(X:Al,Ti,V,Nb,Mo and W)on the structural stability of L12 and D019 ordered Co3Ta are reported by means of the first-principle calculation.The results show that Co3Ta has L12 structure in the ground state.The alloying elements prefer to occupy Co sites rather than Ta sites of Co3Ta phase in both L12 and D019 structures.The addition of Mo element could effectively improve the structural stability of L12 ordered Co3Ta compounds.(2)Combining with stress-strain methods and quasi-harmonic approximations,the influence of alloying elements X(X:Al,Ti,V,Nb,Mo and W)on the elastic and thermodynamic properties of metastable γ’-Co3Ta phases are discussed.The results indicate that the Co-Ta-X(X:Al,Ti,V,Nb,Mo and W)compounds in both L12 and D019 structures are mechanically stable at 0 K.The equilibrium volumes of the compounds increase and the shear moduli decrease with the alloying elements occuping Co sites.The equilibrium volumes and the bulk moduli are reduced in different degree by the alloying element occupying Ta sites.It is indicated that the volume thermal expansion coefficients of the compounds are significantly increased by alloying elements additions and the obvious decrease in the bulk modulus is attributed to the weakening of bond strength between atoms at high temperatures.(3)Based on the density functional theory,the concentration effects of alloying elements X(X:Al,Ti,V,Nb,Mo and W)on the structural stability and elastic properties of ordered Co3(Al,X)compounds are investigated.The results show that the alloying concentration within 6.25%could contribute to the formation of L12 phase and the structural stability of L12 Co3(Al,X)compounds are significantly improved by the alloying concentration of 6.25%.In addition,the bulk modulus decreases first and then increases with increasing Al and Ti concentration,and reaches the minimum at 9.375%concentration.It is found that the alloying concentration effect on the shear modulus shows a certain similarity to the Young ’s modulus.(4)According to the distributions of electron charge density and electronic density of states,the concentration effects of alloying elements X(X:Al,Ti,V,Nb,Mo and W)on the electronic structure of L12 ordered Co3(Al,X)in atomic scale are studied.The results indicate that the covalent bond between atoms are enhanced after adding W element.The shear modulus and Young’s modulus increase with the increase of W concentration.On the contrary,the strength of the binding force between atoms is weakened after adding Al element.The shear modulus and Young’s modulus decrease drastically with the increase of Al concentration.