| Modern industrial development have made new demands for nickel-base alloys. For example,(1) Generation IV reactors require nickel-base alloys to overcome the weaknesses of poor radiation resistance,(2) it is necessary for the nickel-base alloys as alternative materials of cobalt-base hardfacing alloys with excellent wear resistance in the reactor to enhance wear resistance and corrosion resistance without passive film, and (3) the development of high temperature applications force the high-temperature strength of nickel-base alloys to further enhance. For the further development of high temperature applications to go beyond the heat limit of nickel-base alloys, NiAl intermetallic compounds are high-temperature materials with higher heat potential compare with nickel-base alloy.In this dissertation, based on the above-mentioned demands of industrial development, the influence of alloying elements on structures and properties of nickel-base alloys and NiAl intermetallics are investigated theoretically using first-principles calculations, exploring the formation rule of the microstructure and the relationship between the microstructure and the performance and providing a theoretical basis to design or obtain a high performance alloy. The main contents include the vacancy diffusion, dislocation slip, stacking fault, electrode potential, the antiphase boundary and so on.First of all, by the supercell method and the different models, based on a series of energy calculations, the vacancy formation energies and migration barriers, the intrinsic stacking fault energy, the electrode potential in the y phase of the nickel-base alloys are obtained.(1) It is found that as the vacancy concentration in y phase is increased, the vacancy migration barrier is reduced and the frequency of the vacancy diffusion is increases, which lead to the increase in atomic diffusion rate. Irradiation creates a large number of vacancies in the nickel-base alloys, leading to the vacancy-mediated diffusion enhanced, thus the measures stabilizing the organization structure by inhibiting atomic diffusion in low-temperature lose and in the oversaturated solid solution (y phase) the precipitates is precipitated or coarsened by the nucleation and growth, resulting in the performance deterioration of the nickel-base alloy. The vacancy formation energy and migration barrier are the key parameters for the mechanism with vacancy concentration increased by radiation-diffusion activated precipitates precipitated and coarsened-performance degradated. It is indicated that Os, Re, W and Ir improve the vacancy formation energy, while Au, Hf, Pt and Ta reduce the vacancy formation energy, and all5d elements improve the vacancy migration energy barrier. From Hf, as the d-electron number is increased, the vacancy formation energy continuously increases until reaches its maximum at transition metal Re. As the d-electron number is further increased, the vacancy formation energy then starts to decrease. The trend of the vacancy migration energy barriers is contrary to the vacancy formation energy. The calculation of the electronic structure indicate that this changing trend is closely related to the bonding strength between the alloying elements and Ni atoms. This study shows that alloying elements can efficiently inhibit radiation-induced performance degradation related to radiation-induced vacancy formation and diffusion.(2) The calculation of the stacking fault of the y-phase indicate that Re and Au are the most effective to reduce the stacking fault energy of nickel-base alloys, following by Hf, Ta, W, Os, Pt, and Ir. This is decided by the bonding strength between the alloying elements and Ni atoms in the stacking fault region, namely, the bonding strength is stronger, the lower the stacking fault energy. This study shows that alloying elements can efficiently decrease the stacking fault, inhibiting the formation of the cell-type structure in the contact surface layer and enhancing wear resistance.(3) The calculation of the electrochemical stability of non-passivated nickel-base alloys{111} surface indicate that Cr, Hf, Ta, W, Re and Os can make the surface Ni atoms more stable on the{111} surface of the corresponding alloy compared to pure Ni{111} surface, while Sc, Ti, V, Mn, Fe, Co, Cu, Zn, Ir, Pt and Au can decrease the electrochemical stability of the surface Ni atoms. Water adsorption destabilizes the electrochemical stability of the surface Ni atoms. The water-surface bonding strength is followed by loss of bonding within the surface. This study shows that alloying elements can increase the electrode potential shift of the surface Ni atoms, increasing the electrochemical stability of the surface, improving the corrosion resistance without passive film and preventing corrosion wear. The above studies can provide theoretical guidance for the researches on the nickel-base alloys with anti-radiation and as a replacement for cobalt-based hardfacing alloys.Secondly, the first principle studies of alloying elements on the antiphase boundary of Ni3Al indicate that all5d elements can improve the<110> and<100> antiphase boundary energy. The antiphase boundary anisotropy of pure Ni3AI meets the cross-slip conditions. Only when the coverage of the alloying elements (Hf-Au) in the slip plane is less than the critical value, the anisotropy ratio for the APB energy of the corresponding alloy can meet the cross-slip condition and the yield stress anomaly can occur. A new method calculating the property of the alloy with multi-element is put forward, increasing practicability of the model. This study can provide theoretical guidance for the researches further enhancing the high-temperature strength of nickel-base alloys. Finally,<111> antiphase boundary energy, the Peierls stress, the unstable stacking fault energy, the cleavage energy and the plasticity criterion of NiAl are calculated. It is found that Cr is beneficial to activate the<111> dislocation of the NiAl. However the role of Cr on the deformation properties in the<111> direction is very weak, thereby a slight deviation will make Cr to lose the plasticity role, which clarify the real role of the Cr and explain the contradictory between the different experimental results. In comparison with Cr, Au, Fe and Mn are more beneficial to activate the<111> dislocation of the NiAl and if preferentially occupying Al sites, Pt, Re, Os, Ir and Co can be more beneficial. In addition, the Ni antisite at Al site is also more beneficial. This study provide a new idea improving the plasticity of the polycrystalline NiAl and can give guidance for experimental research. |
