Effect of long-term aging on microstructural stability and tensile deformation of a Fe-Ni-based superalloy

The microstructural evolution, tensile behavior and deformation mechanism of a Fe-Ni-based superalloy for advanced ultra-supercritical coal-fired power plant during long-term isothermal exposure at 700-800 degrees C for 500-3000 h were investigated. The coarsening rate constant of gamma ' phase in the alloy exposured at 700 degrees C, 750 degrees C and 800 degrees C were about 47.543 nm3/h, 277.963 nm3/h and 2201.937 nm3/h, respectively. The activation energy for gamma ' growing is around 250.24 kJ/mol. The results showed Al, Ti and Ni elements primarily gathered into gamma ' precipitates, while Cr and Fe mainly partitioned into gamma matrix in Fe-Ni base alloy. After long-term aging, there was no precipitation of harmful phases such as the eta and sigma phases, but the decomposition of MC + gamma -> M23C6 + gamma ' was observed. The microcracks were prone to initiate and propagate around pores formed by MC decomposition and lamellar M23C6 carbide, which might result in the premature fracture failure of the alloy. The yield strength first remained stable with a slight increase in the gamma ' phase diameter and then decreased after further thermal exposure. The variation of tensile strength with precipitate size could be rationalized by the transformation between shearing model and Orowan looping mechanism of gamma ' precipitates with increasing the gamma ' precipitate size.

Automated summary

The microstructural evolution, tensile behavior and deformation mechanism of a Fe-Ni-based superalloy for advanced ultra-supercritical coal-fired power plant during long-term isothermal exposure at 700-800 degrees C were investigated. The results showed that long-term thermal exposure led to the coarsening and decomposition of gamma ' phase, which may result in premature fracture failure of the alloy.