Analysis of carbide transformation in MC-M₂₃C₆ and its effect on mechanical properties of Ni-based superalloy

An investigation based on electron microscopy and crystallographic analysis was employed to gain insights into the carbide transformation in MC-M₂₃C₆ complex carbide of Haynes282; nanoindentation test and first-principles calculations were performed to study its effect on mechanical properties of the alloy. The results show that MC(M=Ti, Mo)-M₂₃C₆(M=Cr, Mo) complex carbide, which exhibits as bull's eye structure, shows an orientation relationship of (131̅)MC//(4̅20)M₂₃C₆, [11̅2̅]MC//[123]M₂₃C₆. Besides, intermediate phase M₂C forms at partial MC/M₂₃C₆ interface. During creep, Cr diffuses from M₂₃C₆ to MC and substitutes Ti under the driving force of concentration gradient. The substitution induces thermal expansion anisotropy in polyhedral clusters of MC, which will trigger the flipping of diagonals and transform MC into M₂₃C₆ through the transformation of coordination polyhedron of C atoms from octahedron to twisted cube. The hybridizations, especially the low-energy hybridizations, between C-p, Ni-d, Cr-d and Ti-d orbitals of the atoms located at M₂₃C₆/Ni and M₂₃C₆/MC interfaces make great contribution to the plentiful chemical bonds formed at the interfaces. These chemical bonds result in the high bonding strength of the two interfaces and thus the beneficial effects of the MC-M₂₃C₆ carbides on mechanical properties of the alloy. Moreover, nanoindentation test indicates that MC-M₂₃C₆ carbides do a better job of strengthening than individual MC or M₂₃C₆ carbide. Therefore, the transformation from MC to M₂₃C₆ is detrimental to the mechanical properties of the alloy. The demonstrated concept shed light on the rational design of higher property superalloys for various potential applications.