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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]

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]