Experimental and DFT studies on the initial precipitation behavior of M₆C in Ni-based superalloy on atomic scale

Experiment and first-principles calculation were performed to investigate the segregation behavior of Mo to the Ni/M₂₃C₆ interface during the initial precipitation of grain boundary (GB) M₆C at the interface as well as the micro-mechanism of the precipitation. Besides, the effect of the precipitation on mechanical properties of the alloy was discussed by first-principles calculations. The results indicate that intergranular fractures, which closely relate to GB carbides, dominate the creep rupture of the alloy. During creep, Mo atoms segregate to the Ni/M₂₃C₆ interface to lower the energy of the system. It promotes the nucleation of M₆C, which exhibits certain orientation relationship with M₂₃C₆ and matrix, at the interface. The segregated Mo will replace the two layers of M atoms (metal atoms in M₂₃C₆) near the Ni/M₂₃C₆ interface in an alternating way. Then, the replaced two layers of atoms will transform into corresponding M₆C structures through local rearrangement of atoms at the interface, because of the similarity in atomic configuration between the (100)M₆C and (100)M₂₃C₆ planes. The site preference of Mo is influenced by the number of the chemical bonds and the symmetry of the charge distribution around the Mo atom. Moreover, the next-nearest neighbor atoms of Mo can also influence the site preference of the Mo by affecting the charge distribution around the Mo atom. The tendency of the bonding strength of the three interfaces is: Mo₄Co₂C/Cr₂₃C₆ > Ni/Mo₄Co₂C > Ni/Cr₂₃C₆. Therefore, the precipitation of M₆C at the Ni/M₂₃C₆ interface is beneficial for the mechanical properties of the alloy. The demonstrated concept enables a deeper understanding of superalloys and shed light on the rational design of higher property superalloys for various potential applications.