Effect Of Carbon Additions On The As-cast Microstructure Of DD90 Ni-based Single Crystal Superalloy

Turbine blades(stator blades and rotor blades) of aircraft engines, industrial gas turbines and other systems are often operated in high-temperature environments for a prolonged time and thus are made of a Ni-based single crystal superalloy that has an excellent heat resistance.The minor boundary strengthening alloying elements such as carbon were added to the directional solidified single crystal superalloys, which lead to complex solidification processing, segregation of elements and precipitated phase in the solidified alloys.The effect of carbon addition on the microstructure and alloy elements segregation of a nickle based single crystal superalloy was investigated. The as-cast microstuctures and the microstuctures after heat treatment have been observed by light optical microscopy and scanning election microscopy. The segregation of alloying element has been observed by Electron Probe X-ray Micro-analyzer.The effect of carbon content on the solidification microstructure of single crystal superalloy DD90 was analyzed. The results show that, with the increase of C composition, the value of primary dendrite arm spacing increases, and the range of solidification temperature expands. With the increase of C composition, the content of carbide increases, the carbide morphologies change from blocky shape to Chinese script-type, then link together to skeleton–like. With the increase of C composition, the number of ?/??eutectic decreases,coarse primary ?? phase decreases. With the increase of C composition, the solidification segregation of metal elements in solid phase generally deducts. C addition promoted the degree of segregation of Al, while reduced the degree of segregation of Re, Mo, W.The solidification path, microsegregation of alloying elements in the interdendritic regions, solidification temperature ranges was investigated by using Scheil equation. The dendritic microstructure, carbides' chemical composition and solidification temperature range of the produced as-cast DD90 nickel-based single crystal superalloys with differentcarbon contents were characterized using optical and scanning electron microscopy equipped with energy dispersive X-ray spectrometer and TG–DSC analysis. The model and experiment confirmed the highest segregation behavior for Al and Ta. With the addition of carbon, the solidus temperature and liquidus temperature were becoming lower, while the solidification was becoming larger. And the intensity of the microsegregation of Al?Mo?Ta was becoming stronger.After heat treatment, the morphology of ?? precipitates became more cubic, the interface of ? phase and ?? phase became more smooth and round,?? phase coarsening,the sites of ?phase transition to ?? phase also very smooth, which could indicated that there existed to be transformation between ? phase and ?? phase during heat treat treatment processing.