Mechanical Property Studies On The Major Phases In Ni-based Superalloys Based On First-principles

Ni-based superalloys are widely used in elevated-temperature components of aero-engines due to the excellent high-temperature mechanical properties.In order to enhance the mechanical properties of Ni-based superalloys,it is necessary to add suitable alloying elements according to the application conditions for solid solution strengthening and precipitation strengthening.As microalloying elements,carbon and boron are added to almost all Ni-based superalloys.Therefore,carbides and borides,such as M₂₃C₆-type carbides and M3B2-type,M5B3-type,and M2B-type borides,are often precipitated at the grain boundaries of superalloys.Carbides and borides can prevent grain boundary slips and inhibit the connection and expansion of grain boundary vacancies.Therefore,the creep life of Ni-based superalloys is improved,and its durable plasticity and toughness are also promoted.However,at present,the studies on the microcosmic mechanism of the formation of such excellent properties for Ni-based superalloys is still insufficient.The first-principles-based method can provide some basic explanations of mechanical properties prediction and the formation mechanisms of such properties at an atomic scale.The purpose of this paper is to study the mechanical properties of the major phases of Ni-based superalloys（Ni matrix phase,γ’ enhanced phase Ni3Al,grain boundary enhanced phases carbides and borides）by the first-principles and the results are of great theoretical significance and practical applying value for considering microstructure strength studies and the multiscale fatigue crack initiation mechanism.The main contents of this thesis are as follows:（1）By employing the generalized stacking fault energy model of Vitek and the ideal strength model,the effects of Co,Cr,Fe,Ta,Mo,and W on the twinnability and ideal strength of Ni are studied.The relation between generalized stacking fault energy and twinnability is constructed by combining different twinnability criterias.The present studies give the ranking sequence of twinning abilities as:Ni-Mo>Ni-Fe ≈Ni-Ta>Ni-W>Ni-Co>Ni>Ni-Cr.In addition,among the studied alloying elements,the improvement of the ideal strength of Ni by Co is the largest,while the decrease of the ideal strength by Ta is the largest.（2）Using the generalized stacking fault energy model of Vitek and the ideal strength model,the effects of alloying elements,such as Re,Ru,Ta,Mo,and W,on the generalized stacking fault energy,twinnability and ideal strength of N13AI are systematically studied.The results show that all the studied alloying elements can improve the energy barrier of Ni3Al forming complex stacking faults,anti-phase boundaries and super intrinsic stacking faults.Among these alloying elements,Re and W have the greatest influence and Ru has the lowest influence.The addition of alloying elements reduces the twinnability of Ni3Al,and the complex stacking faults tend to form the anti-phase boundaries.The ranking strength of alloying Ni3Al is:Ni3Al-Re>N13Al-W>Ni3Al-Mo>Ni3Al-Ta>Ni3Al-Ru>Ni3Al.（3）The generalized stacking fault energies of Cr-rich M₂₃C₆ carbides（namely Cr23C6）in the two main slip systems（{111}<112>and {111}<101>）and the critical cleavage stress on the planes of（100）,（110）,and（111）are determined theoretically.The results show that the generalized stacking fault energy curves of Cr23C6 slipping systems present the characteristics of double-saddle points compared to the materials with typical face-centered cubic structures.In addition,the ideal brittle fracture model and ideal strength model are combined to determine the minimum critical cleavage stress of Cr23C6,and it is the minimum at（11 0）plane,followed by（111）plane and（100）plane.（4）The stability,hi high-temperature mechanical properties and electronic structures of major borides M2B,M5B3,and M3B2（M=Cr,Mo,W）in Ni-based superalloys are calculated.The results show that among these M-B binary compounds,the shear modulus,Young’s modulus,and Vicker hardness of Cr-B compound are significantly the largest,and these mechanical properties increased with the increase of the atomic content of B.The tetragonal and rhombic structures of M2B-type borides are all thermodynamically and mechanically stable,and almost have the same shear modulus,bulk modulus,Young’s modulus,and Vickers hardness.Except for W5B3 and W3B2,almost all studied borides are isotropic.