Search

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]

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]

In terms of first-principles method, the influences of Co, Cr, Fe, Ta, Mo and W atoms on the generalized stacking fault energy of Ni are investigated. The bridge between stacking fault energy and twinning is comprehensively constructed by various previous criterions. Our calculation results indicate that the addition of Mo has the maximum positive effect on the twinning of Ni, while Cr shows the minimum effect. The ranking sequence of twinning abilities can be drawn as: Ni-Mo > Ni-Fe ≈ Ni-Ta > Ni-W > Ni-Co > Ni > Ni-Cr. On the other hand, the addition of Fe or Ta significantly reduces the intrinsic stacking fault energy of Ni, which is beneficial to obtain the dislocation glide during deformations. Based on the analysis of charge density, the bonding characteristics near the fault plane are observed to reveal the electronic redistribution during the slipping. It is found that the intrinsic stacking fault configuration and double-layers twin configuration reconstitute stable bonds near the fault plane corresponding to two valley points of the GSFE curve, while these bonds will be disrupted under a series of displacements. • Generalized stacking fault energies (GSFEs) of Ni are calculated with the addition of solid-solution atoms. • Influences of the solid-solution elements on the twinning of Ni are qualitatively and quantitatively analyzed. • Ranking sequence of twinning abilities can be drawn as: Ni-Mo > Ni-Fe ≈ Ni-Ta > Ni-W > Ni-Co > Ni > Ni-Cr. • Intrinsic stacking fault configuration and double-layers twin configuration reconstitute stable bonds near fault plane. [ABSTRACT FROM AUTHOR]

The precise information on phase constitutions in the Co-Ta system is of practical significance for exploiting γ′-strengthened Co-Ta-based superalloys. The unidentified crystal structure of χCo 7 Ta 2 and the ambiguous description of phase relationship between χCo 7 Ta 2 and γ′Co 3 Ta result in the uncertainty of phase constitutions in the Co-rich region of Co-Ta. In this work, we present a systematical study of crystallographic data of Co 7 Ta 2 on the basis of first principles and an unbiased structure searching method. A layered-sandwich structure with C 2/ m monoclinic symmetry (Pearson symbol mC18) is identified for Co 7 Ta 2. The thermodynamic, mechanical and phonon calculations further verify the stability of C 2/ m. The quasi-harmonic thermodynamics exhibit that Co 3 Ta is stable and Co 7 Ta 2 should be metastable at ambient temperature. The entropy effect remarkably stabilizes Co 7 Ta 2 but destabilizes Co 3 Ta phase. The Co 7 Ta 2 would be the thermodynamically stable at T ≥ 1200 K. Further results of mechanical properties show that Co 7 Ta 2 yields an unexpected intrinsic ductility. Through the electronic configurations, it demonstrates that the ductility is associated with the bonding alteration in the specific layered-sandwich structure of Co 7 Ta 2 during deformation process. Image 1 • A layered-sandwich structure with C 2/ m monoclinic symmetry is predicted for Co 7 Ta 2. • Co 7 Ta 2 is metastable at 298 K, the entropy effect makes it be stable at T ≥ 1200 K. • Co 7 Ta 2 intermetallics yield an unexpected intrinsic ductility. • The origin of ductility is the specific bonding alteration under deformation. [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]