Intrinsic embrittlement of MoSi2 and alloying effect on ductility: Studied by first-principles

The electronic structure, surface energy (γs), unstable stacking fault energy (γus), and the elastic parameters of MoSi2 and (Mo,M)Si2 (M=Nb, Tc, Ta, W) are calculated using first-principles method. The influence of alloying elements on ductility is assessed by combining a “disembrittlement parameter” D=γs/γus and elastic properties. Ideal MoSi2 crystal investigation shows that the intrinsic brittlement is caused by strong directional covalent Mo4d–Si3p bonding. MoSi2 (0 0 1) Si-|-Si plane, which has lower surface energy is preferred cleavage plane. W alloying embrittles MoSi2. Nb, Tc, and Ta alloying are found to be beneficial to the enhancement of ductility. The softening effect can be interpreted by the replacement of Mo4d–Si3p bonding by weaker and less directional bonding.