First-principles computations of mechanical properties of Ni2Cr and Ni2Mo

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Nickel-based alloys are being considered for use as the outer container of the waste package for the disposal of high-level nuclear waste. During fabrication processes and long-term storage, Ni-based alloy outer containers can undergo microstructural changes due to the formation of Ni2Cr, Ni2Mo, and Ni2(Cr, Mo), which are ordered orthorhombic (oI6) phases whose mechanical properties are unknown because of fabrication difficulties. To circumvent the experimental limitation, a first-principles quantum-mechanical code based on the full-potential linearized augmented plane-wave (FLAPW) method was used to compute the elastic constants and the theoretical stress-strain curves of Ni2Cr and Ni2Mo. The theoretical mechanical properties were then correlated with charge-density distributions of the stressed oI6 unit cell to identify the critical conditions at the onset of fracture. Using first-principles results as material input, the unstable stacking energy and the Peierls-Nabarro (P-N) barrier energy were computed and used to estimate the tensile ductility and fracture toughness of Ni2Cr and Ni2Mo. The influences of the elastic anisotropy and slip vector on dislocation mobility in Ni2Cr and Ni2Mo are identified and contrasted to those in MoSi2 with a tetragonal (tI6) crystal structure.