Site preference of Ti and Nb in L1₂-ordered Co-Al-W phase and their effect on the properties of the alloy: First-principles study

In this work, the equilibrium structure, electronic and elastic properties of L1₂-ordered Co-Al-W and Co-Al-W-X (X = Ti and Nb) phase were calculated, using first-principles calculations. Among six nonequivalent sites (Al₁, Al₂, Co₃, Co₄, W₅, W₆), Ti and Nb prefer to occupy the W₆ site, since the formation enthalpy of the system is lowest when Ti and Nb occupy the W₆ site. Both Ti and Nb most affect the density of states of Al atoms. Compared with the Al₂ site, which is the sub-preference site of Ti and Nb, the density of states of Al atoms is higher with the addition of Ti and Nb in the W₆ site, which means that the latter system is more stable. According to the bulk modulus B, shear modulus G, Young's modulus E, hardness H_V and Poisson's ratio σ, for Co₃(Al, W) alloy, the addition of Ti and Nb in the W₆ site decreases its hardness but increases its ductility. This work confirms that Ti and Nb can stabilize the Co₃(Al, W) alloy and have a positive effect in solving the relatively poor ductility of this alloy, which has important implications for the development of cobalt-based alloys.