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

Abstract

An investigation based on electron microscopy and crystallographic analysis was employed to gain insights into the carbide transformation in MC-M 23 C 6 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)-M 23 C 6 (M=Cr, Mo) complex carbide, which exhibits as bull’s eye structure, shows an orientation relationship of ( 13 1 ̅ ) MC// ( 4 ̅ 20 ) M 23 C 6 , [ 1 1 ̅ 2 ̅ ] MC// [ 123 ] M 23 C 6 . Besides, intermediate phase M 2 C forms at partial MC/M 23 C 6 interface. During creep, Cr diffuses from M 23 C 6 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 M 23 C 6 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 M 23 C 6 /Ni and M 23 C 6 /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-M 23 C 6 carbides on mechanical properties of the alloy. Moreover, nanoindentation test indicates that MC-M 23 C 6 carbides do a better job of strengthening than individual MC or M 23 C 6 carbide. Therefore, the transformation from MC to M 23 C 6 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. • MC would in-situ transform into M 23 C 6 as the substitution of Ti in MC by Cr atoms. • Mechanism of the carbide transformation was studied by crystallographic analysis. • Transformation from MC to M 23 C 6 is detrimental to the mechanical properties of the alloy. • It proposed a promising method to improve the mechanical properties of Ni-based superalloy.