Microstructural evolution in creep exposed IN617

Inconel alloys are currently being investigated for high temperature applications such as HP and IP valve chest and rotor forgings in advanced steam power plant operating at temperatures of 700°C and above. One of the preferred alloys for these components is IN617. This is a solid solution strengthened austenitic Ni-based alloy containing ~23% Cr, 12% Co, and 9% Mo with small additions of Ti and Al which can contribute some additional precipitation strengthening. In the solution treated condition, the microstructure consists of equiaxed austenite containing M23C6 at the grain boundaries and occasional TiN particles within the matrix. Owing to high temperature exposure and the creep deformation processes that occur in-service, evolution of the microstructure occurs in the form of precipitation, precipitate coarsening and recovery effects. This paper discusses microstructural evolution occurring in this alloy in samples that have been exposed to temperatures up to 700°C and for durations up to 45,000 hours using advanced FEGSEM, TEM and XRD techniques.