Investigation on the Stability of Raft Structure in Single Crystal Superalloy

The thermal stability of CMSX-4 nickel-based single crystal superalloy with the raft structure is investigated by analyzing and comparing the macro-oxidation and in-situ oxidation experiments. After the macroscopic oxidation, it could be found that two kinds of oxidation morphologies appeared on the superalloy surface: region A (dendrite core) and region B (interdendrite). In regions A, the oxidation layer is relatively thick and composed of two oxide layers with the respective main components NiO and Co2O3. The interdendritic region B clustered Mo, Cr, Ti, Ta, Al and other elements has only one very thin oxide layer, the main components are Al2O3, Cr-O and TiO. The oxide layer is relatively dense and can isolate air and prevent further oxidation of the alloy. The further in-situ oxidation experiments of CMSX-4 superalloy showed that the thickness change is due to the migration of elements during the oxidation process, among which Ni element migrates from γ' phase to γ phase, Ta, Ti, Re, Hf, W, Co and Cr migrate from γ phase to γ' phase and the remaining Mo and Al elements did not show obvious migration characteristics. Due to the migration and oxidation of elements, the γ phase becomes thicker and the γ' phase is relatively thinner, thus the stress concentration easily occurs between the two phases, resulting in small voids aggregation, the cracks initiation and the final failure fracture.