Exploration of crystal structure and the origin of unexpected intrinsic ductility of χCo7Ta2

Abstract 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.