Evolution of MC carbides in Ni-Mo-Cr-Ti based alloy for applications in molten salt reactors

The nucleation, growth, and degeneration behaviors of MC carbides in the Ni-12Mo-7Cr-2Ti-0.06C alloy during the long-term thermal exposure at 650 °C – 850 °C have been investigated. Two kinds of MC carbides, namely the stacking-fault-precipitate (SFP) type ones and the flake-like ones, have been observed in the exposed specimens. The SFP-type MC carbides possess a semi-coherent interface of (111)MC // (111)γ with the matrix by the cube-on-cube orientation relationship (OR), which show the excellent thermal stability at 650 °C and the rapid coarsening and degeneration at the higher temperature. The flake-like MC carbides possess a semi-coherent interface of (111¯)MC // (002)γ with the matrix by the rotated OR, which play a dominant role at the initial stage of exposure at 850 °C. All MC carbides contain Mo, Ti and a small number of Cr and Ni. According to the first principle calculation results, the doping of Mo can facilitate the nucleation of MC carbides by decreasing the chemical interface energy. Based on crystallographic analyses, the evolution mechanisms of MC carbides were discussed. At the initial stage, the dislocations form around the primary MC carbides and the growing secondary ones. The Ti segregation can lower the stacking fault energy at dislocations and promote the dissociation of dislocation to form the clean stacking faults (SFs). Then, the expansion of SFs and the formation of MC carbides go on simultaneously as described by the Silcock-Tunstall model. The dense distribution of MC carbides on SFs aggravates the rapid coarsening and degeneration. The flake-like MC carbides tend to deviate from the strict rotated OR to eliminate the lattice mismatching during the coarsening process. In this premise, the predicted well-matched interface should be along (111¯)γ and (111)γ planes and possess the stepped morphology with a deviation angle of ∼9° from the (111¯)γ planes, which well agrees with the experiment observations.