The Effect Of Carbon On A Nickel Base Single Crystal Superalloy

The effect of carbon content on the microstructure and creep life of a nickel base single crystal superalloy was investigated. The as-cast microstructures and the microstructures 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 Microanalyzer. The fracture of the sample and the interrupted sample were observed and analyzed by Scanning Electron Microscopy and Transmission Electron Microscopy after creep tests.The results show that: With the increasing of carbon addition, the incipient melting temperature of alloys is decreased, the content of carbides is increased, and the carbide morphologies change from spot shape to net shape. The formation of carbide in interdendritic region lowers the size and the volume percentage ofγ'-γeutectic. The formation of carbides in interdendritic region also influence the size and the number of shrinkage porosity in alloy, the size and the number of shrinkage porosity in alloy firstly reduces and then increases. With the increasing of carbon addition, the segregation of W, Mo and Ta decreased gradually.Theγ' phase in dendrite core distribute evenly and with the size of 0.3μm after heat treatment. The size ofγ' phase in alloys is not influenced by carbon addition. while in interdendritic regions the microstructure are different. The alloy with little carbon and no carbon can be fully solutionized and the eutectic can be eliminated. With the carbon addition increasing, solid solution treatment temperature drop,γ-γ' eutectic microstructure, coarseγ' phase and carbide in interdendritic regions increase gradually. During the heat treatment and creep tests, some MC carbide transforms into the M₆C carbide, and secondary carbides M₂₃C₆ and M₆C precipitate from y matrix. The stress rupture life of the alloy with 0.01wt% carbon is the longest one, and with the increasing of carbon content the stress ruptures lives of the alloy decreases. The precipitation of M₂₃C₆ and M₆C carbides and the reduction of shrinkage led by the carbon addition are benefiting for the improvement the creep properties, when the alloy adds 0.01wt% carbon. The eutectic, largerγ' and MC carbide are the detrimental factors for creep properties, when more carbon content is added.