Microscopy And First-principles Study Of Precipitates In Nickel-base Single Crystal Superalloys

Nickel-base single crystal superalloys have become the key materials of jet aero-engine and gas turbine blades,owing to its excellent creep and fatigue properties.In order to improve the high temperature mechanical properties of Nickel-base single crystal superalloys,a large amount of refractory elements have been added into superalloys.With the increase of the content of refractory elements,the tendency of TCP phase precipitating from the matrix increases.The formation of TCP phase not only consumes the refractory elements in superalloys,but also weakens the solid solution strengthening effect.And the formation and growth of TCP phase will be accelerated under the action of external stress,which greatly reduces the high temperature mechanical properties of superalloys.Therefore,the study of the formation mechanism and mechanical properties of TCP phase is of great significance to control the precipitation of TCP phase.At the same time,in order to further improve the mechanical properties of superalloys,it is necessary to develop new strengthening phases.From the first to sixth generation superalloys,the content of Ta element is relatively high because it can improve the oxidation resistance and corrosion resistance of superalloys.The results show that the addition of Ta element can promote the formation of γ"-Ni3Ta and significantly improve the mechanical properties of superalloys.In view of this,with the aid of electron microscopic analysis and first-principles calculation method based on density functional theory,the distribution behaviors of refractory elements Re and Ru in TCP phase,the orientation relationships between TCP phase and matrix,the internal defects and fine structures of TCP phase,and the mechanical properties of both TCP phase and γ"-Ni3Ta phase in Nickel-base single crystal superalloys were studied.The main conclusions are as follows:(1)The first-principles calculation results show that when Re replaces the Co site of C14,C15 and C36 type Laves-NbCo2 phase,the defect formation energy is lower.Compared with the density of states of the Nb site replaced by Re,when the Co site is replaced by Re,the density of states at the Fermi level decreases obviously and approaches the valley position of the hybrid peak,which is beneficial to the stability of the system.It shows that Re tends to occupy the Co site in Laves phase and promote the formation of Laves phase.Re and Ru replace five non-equivalent positions in μ-Co7W6 phase respectively.When Re replace Co and W sites,μ phase can exist stably,and Re tends to occupy Co site,indicating that Re promote the formation of μ phase.μphase is energetically stable when Ru replace Co sites.On the contrary,the system is unstable when Ru replace W sites.(2)Compared with the standard diffraction patterns of TCP phase,the orientation relationships between μ phase,P phase and R phase and the matrix were obtained,respectively.The orientation relationships between μ phase and matrix are[110]μ//[112]γ,(001)μ//(110)γ and(110)μ//(111)γ.The orientation relationships between P phase and matrix are[001]P//[112]γ,(100)P//(110)γ and(010)P//(111)γ.The orientation relationships between R phase and matrix are[111]R//[110]γ,(011)R//(111)γ and(112)R//(001)γ.In order to obtain other orientation relationship between TCP phase and the matrix,the matrix expressions of the orientation relationships between μ phase,P phase,R phase and matrix were estabilished,respectively.(3)Electron diffraction analysis shows that there are three kinds of defect structures in μ phase:stacking faults,twins and endogenous P phase.The orientation relationship between μ phase and P phase is[221]μ//[011]P and(012)μ//(100)P.The High-angle Annular Dark Field image shows that the twinning planes in μ phase are(012)plane and(102)plane,and the generation of(102)twin accompany with the detwinning of(012)twin.There are two different stacking faults in the direction of(012)twin.The formation process of twin structure:the perfect structure change to stacking fault by means of atom shear and shuffle,the stacking fault change to the twin structure by means of atom shear.According to the stacking sequence of structure units on(010)plane of μ phase,stacking faults are formed by pulling out Zr4Al3 and MgCu2 layers,and twins are formed by atoms in stacking fault moving one atom distance along the[010]direction.The twins in μ phase will transform to P phase through structural relaxation.There are composition fluctuations in μ phase,indicating that Re and Cr elements will promote the formation of P phase.(4)The formation enthalpies of C14,C15 and C36 type Laves-NbCo2 phases under different external pressure are negative,indicating that NbCo2 Laves phases are stable.The calculated elastic properties reveal the ductility of Laves phases.The sound velocity results in different directions prove that Laves phases presents elastic anisotropy.The change of Debye temperature shows that the bonding ability of Laves phases are enhanced,thus,the stability of crystal structure is enhanced with the applied external pressure.(5)The calculated binding energy shows that C07W6 has the strongest stability among μ--Co7X6(X=W,Mo and Nb)phases.The calculated elastic moduli(B,G and E),BIG and Poisson’s ratio(v)show that the μ phase exhibits ductility.The decrease of the density of states at the Fermi level agrees well with the energy results.The sound velocity of Co7W6 is anisotropic in different directions.With the increase of external pressure,both the bonding effect and the hardness of Co7W6 increase.The results show that Laves phase and μ phase are ductile.(6)The formation enthalpy and elastic constants show that γ"-Ni3Ta has thermodynamic and mechanical stability.With the increase of pressure,the deformation resistance and stiffness of y"-Ni3Ta increase.The calculations of B/G,Poisson’s ratio(v)and Cauchy pressure reveal the ductile nature of γ"-Ni3Ta.The calculated elastic anisotropy of γ"-Ni3Ta indicates that the anisotropy increase with the increase of pressure.The shear anisotropy index is higher than compress anisotropy.At high pressure,the broadening of the pseudogap near the Fermi level indicates the enhancement of the covalent of γ"-Ni3Ta.The calculation results show that γ"-Ni3Ta is a promising strengthening phase.