Cracking Mechanism of Expansion Seal Sheetin Engine Nozzle

The expansion seal sheet in the nozzle of an engine was made of intermetallic compound superalloy JG4246A. It was found to have cracked during service. In order to find out the failure cause, macro and micro observation, material analysis, and over-heating microstructure simulation test were carried out. Results indicate that the cracking mode of the seal sheet is fatigue crack primarily induced by thermal stress. According to the results of the microstructure simulation test, the highest temperature the failure position had experienced exceeded the allowed service temperature, 1 200℃. Obvious redissolution of primary γ' phase caused the significant reduction of performance. In the condition of cold-and-hot alternation, there existed large thermal stress gradient at the failure position, which exceeded the material fatigue strength limit, leading to final fatigue crack. In the end, some measures were proposed to improve the casting technology and optimize the design.