Density functional theory calculations in conjunction with the climbing images nudged elastic band method are conducted to study the diffusion phenomena of the Ni-based single crystal superalloys.We focus our attention on the diffusion processes of the Ni and Al atoms in the γ and γ ’ phases along the direction perpendicular to the interface.The diffusion mechanisms and the expressions of the diffusion coefficients are presented.The vacancy formation energies,the migration energies,and the activation energies for the diffusing Ni and Al atoms are estimated,and these quantities display the expected and clear transition zones in the vicinity of the interface of about 3–7（002） layers.The local density-of-states profiles of atoms in each（002） layer in the γ and γ ’ phases and the partial density-of-states curves of Re and some of its nearest-neighbor atoms are also presented to explore the electronic effect of the diffusion behavior.
An effective multiscale simulation which concurrently couples the quantum-mechanical and molecular-mechanical calculations based on the position continuity of atoms is presented.By an iterative procedure,the structure of the dislocation core in face-centered cubic metal is obtained by first-principles calculation and the long-range stress is released by molecular dynamics relaxation.Compared to earlier multiscale methods,the present work couples the long-range strain to the local displacements of the dislocation core in a simpler way with the same accuracy.
Using DMol and the discrete variational method within the framework of the density functional theory, we study the alloying effects of Nb, Ti, and V in the [100] （010） edge dislocation core of NiAl. We find that when Nb （Ti, V） is substituted for Al in the center-Al, the binding energy of the system reduces 3.00 eV （2.98 eV, 2.66 eV）. When Nb （Ti, V） is substituted for Ni in the center-Ni, the binding energy of the system reduces only 0.47 eV （0.16 eV, 0.09 eV）. This shows that Nb （Ti, V） exhibits a strong Al site preference, which agrees with the experimental and other theoretical results. The analyses of the charge distribution, the interatomic energy and the partial density of states show that some charge accumulations appear between the impurity atom and Ni atoms, and the strong bonding states are formed between impurity atom and neighbouring host atoms due mainly to the hybridization of 4d5s（3d4s） orbitals of impurity atoms and 3d4s4p orbitals of host Ni atoms. The impurity induces a strong pinning effect on the [100] （010） edge dislocation motion in NiAl, which is related to the mechanical properties of the NiAl alloy.
The effect of H impurity on the misfit dislocation in Ni-based single-crystal superalloy is investigated using the molecular dynamic simulation. It includes the site preferences of H impurity in single crystals Ni and Ni3Al, the interaction between H impurity and the misfit dislocation and the effect of H impurity on the moving misfit dislocation. The calculated energies and simulation results show that the misfit dislocation attracts H impurity which is located at the γ/γ′ interface and Ni3Al and H impurity on the glide plane can obstruct the glide of misfit dislocation, which is beneficial to improving the mechanical properties of Ni based superalloys.
The effect of Re on stacking fault （SF） nucleation under shear strain in Ni is investigated using the climbing image nudged elastic band method with a Ni-A1-Re embedded-atom-method potential. A parameter （△Ebsf）, the activation energy of SF nucleation under shear strain, is introduced to evaluate the effect of Re on SF nucleation under shear strain. Calcu- lation results show that △Ebsf decreases with Re addition, which means that SF nucleation under shear strain in Ni may be enhanced by Re. The atomic structure observation shows that the decrease of △Ebsf may be due to the expansion of local structure around the Re atom when SF goes through the Re atom.
An Ni-AI-Co system embedded-atom-method potential is constructed for the γ（Ni）/γ＇（Ni3A1） superalloy based on experiments and first-principles calculations. The stacking fault energies （SFEs） of the Ni（Co, A1） random solid solutions are calculated as a function of the concentrations of Co and A1. The calculated SFEs decrease with increasing concentrations of Co and A1, which is consistent with the experimental results. The embedding energy term in the present potential has an important influence on the SFEs of the random solid solutions. The cross-slip processes of a screw dislocation in homogenous Ni（Co） solid solutions are simulated using the present potential and the nudged elastic band method. The cross-slip activation energies increase with increasing Co concentration, which implies that the creep resistance of γ（Ni） may be improved by the addition of Co.
The effects of boron and carbon on the structural, elastic, and electronic properties of both Ni solution and NiAl intermetallics are investigated using first-principles calculations. The results agree well with theoretical and experimental data from previous studies and are analyzed based on the density of states and charge density. It is found that both boron and carbon are inclined to occupy the Ni-rich interstices in NiAl, which gives rise to a cubic interstitial phase. In addition,the interstitial boron and carbon have different effects on the elastic moduli of Ni and NiAl. The calculation results for the G/B and Poisson’s ratios further demonstrate that interstitial boron and carbon can both reduce the brittleness of Ni, thereby increasing its ductility. Meanwhile, boron can also enhance the ductility of the NiAl while carbon hardly has an effect on its brittleness or ductility.
