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In Ni-based superalloys, it is usually found that borides can strengthen the grain boundaries, thereby resulting in an increase in mechanical strength and high-temperature creep properties. Due to their importance and prevalence in Ni-based superalloys, this study employs first-principles methods to investigate the crystallographic structure, anisotropic elastic response, and electronic properties of the major borides, such as M2B, M5B3 and M3B2 (M: Cr, Mo, W), respectively, which is necessary for the assessment of complex mechanical responses of Ni-based superalloys. The results demonstrate that the studied borides are all thermodynamically and mechanically stable. Among the MxBy binary borides analysed, CrxBy exhibits the largest shear modulus, Young's modulus, and Vicker hardness values, and these properties increase with the increase of B contents. The studied borides display nearly isotropic elastic properties except for W5B3 and W3B2. The electronic structure analysis of MxBy shows that the strong hybridisation between M-d and B-p orbitals leads to these borides exhibiting higher theoretical hardness, and the overlapping peaks of M-d and B-p orbitals move to a lower energy area with the increase of B contents, which leads to the increase of shear and Young's moduli of MxBy. Furthermore, for M3B2 borides, the Cr-B bonds and Cr–Cr bonds are much stronger than the W-B & Mo-B bonds, and W-W & Mo-Mo bonds, respectively, which leads to CrxBy yielding the largest values of elastic moduli. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • The GSFE of Cr 23 C 6 were calculated in prominent slip systems, for the first time. • Different to simple fcc structures, Cr 23 C 6 shows a double bell-shaped GSFE curve. • GSFEs of two slip systems revealed that {1 1 1}〈 1 1 2 ¯ 〉 is the most favorable system. • Electronic configuration alternation near slip planes govern the stacking behavior. Abstract In many high temperature-resistant Ni-based superalloys, M 23 C 6 carbides form during processing and remain as non-metallic inclusions during the materials use in application. M 23 C 6 type inclusions are principal sites for potential fatigue crack initiation, thus the accurate quantification of their energy barriers for deformation is necessary for in-depth understanding of the overarching mechanical behavior of the material system. In this study, the generalized stacking fault energies (GSFE) are theoretically determined for chromium-rich types of M 23 C 6 carbides (i.e. , Cr 23 C 6) in two prominent slip systems: {1 1 1}〈 1 1 2 ¯ 〉 and {1 1 1}〈 1 0 1 ¯ 〉. The results present GSFE curves with double saddle points in both slip systems. The double saddle points of the GSFE curves are explained via the electronic structure of atoms near the slip plane. At deformation quantified by half of a Burgers vector (i.e. , 0.5 b p), a pentacyclic charge density distribution appears representing a local minimum energy configuration, which results in a valley of the GSFE curve. Furthermore, the effective coordination number (n) and the distortion index (Δ d) of the coordination polyhedrons are employed to characterize the structural change. The analysis reveals that the values of the indexes n and Δ d at a displacement value of 0.5 b p are close to the values of a perfect Cr 23 C 6 crystal, while comparatively the distortion index is higher at the saddle points. These results provide new insights into the deformation pathways of M 23 C 6 carbides. [ABSTRACT FROM AUTHOR]

To clarify the different effects of alloying elements on the strength of Ni and Ni 3 Al, this study employs First-principles methods to investigate the effects of refractory elements Re, Ru, Ta, Mo and W on the generalized stacking fault energy and ideal strength of Ni and Ni 3 Al, respectively. The results reveal that the alloying elements Re, Ru, Ta, Mo, and W can help to improve the creep strength of Ni but reduce that of Ni 3 Al. Among these alloying elements, Re and Ru are more helpful for improving the strength of Ni, and for Ni 3 Al, Ru has the weakest enhancing effect on the strength of Ni 3 Al. Moreover, except for Ru, other alloying elements have more significant enhancing effects on the strength of Ni 3 Al. The electronic structure analysis shows that the d orbitals of alloying elements except Ru can form deep and wide pseudogap near the Fermi energy level, which causes the obvious enhancement of the strength of Ni 3 Al. Furthermore, the regular areas of charge distribution between atom Ru and its nearest neighbor atoms Ni are more vulnerable to be destroyed, resulting in Ni 3 Al–Ru owning a relatively lower ideal tensile strength and reaches its ideal tensile strength under a smaller strain. • Re, Ru, Ta, Mo and W can help to improve the creep strength of Ni, but reduce that of Ni 3 Al. • Re, W, Mo, and Ta have more significant enhancing effects on the mechanical strength of Ni 3 Al, compare with that of Ni. • The d orbitals of Ru cannot form deep and wide pseudogap near the Fermi energy level, as other elements do. [ABSTRACT FROM AUTHOR]