Electronic structure and mechanical properties of crystalline precipitate phases M23C6 (M=Cr, W, Mo, Fe) in Ni-based superalloys

The presence of precipitate phases M23C6 (M=Cr, Fe, Mo, or W) plays a significant role in the mechanical strength of Ni-based superalloys. The crystal structure of these complex carbides consists of 116 atoms in the cubic cell of an fcc lattice. Here we report the investigation of the electronic structure, interatomic bonding and mechanical properties for four binary phases of M23C6, and M23N6, and six ternary phases of M1(23-x) M2(x)C(6). Based on the results of our comprehensive calculations and the evaluation of a quantum mechanical metric called the total bond order density (TBOD), we conclude that Fe tends to make the intermetallic structure less stable whereas W makes the crystal more compressible. We trace the root of these observations to the size of the metal ions and the complex interatomic bonding between C atoms and the metal d electrons. In addition, we correlate the TBOD with calculated mechanical properties leading to confirmation that Cr23C6 is the most stable precipitate phase with superior overall physical properties. On the other hand, replacing C with N in these crystals does not result in discernable differences in their properties. Our detailed investigation sheds much light on the use of carbides precipitates as a stabilization strategy for Ni-based superalloys.