Effect of long time service exposure on microstructure and mechanical properties of gas turbine vanes made of IN939 alloy

• Microstructure of service-exposed turbine vanes made of IN939 was investigated.
• Mechanical properties of service-exposed alloy were also investigated.
• Formation of M23C6 films on GBs and degeneration of MCs are microstructural changes.
• Despite its thermodynamic favorability, σ phase is not observed in microstructure.
• Microstructural changes lead to a loss in tensile ductility and creep life.

In the present study, the effects of long-term service exposure have been investigated on microstructure and mechanical properties of gas turbine vanes made of IN939 superalloy. The major microstructural changes for the investigated service-exposed vanes include the formation of continuous grain boundary carbides and the transformation (degeneration) of MC carbides located at the grain boundaries. The brittle σ phase, which is predicted to be stable on the basis of thermodynamic calculations, is not observed in the microstructure of service-exposed vanes. The microstructural changes during service lead to a loss in room temperature ductility as well as in creep properties of the alloy.