Influence of service-induced microstructural changes on the failure of a cobalt-based superalloy first stage nozzle

The present work was carried out to investigate the influence of serviced-induced microstructural changes on the failure of cobalt-base superalloy X-45 using in a first stage gas turbine nozzle. The obtained results show that the continuous film of carbides in the grain boundaries and the formation of needle like phases in the interior of grains are due to transformation of primary carbides M6C type to secondary carbides M23C6 type as a result of high-temperature operations. This dense and continuous net of the carbides reduces ductility and toughness of the alloy in comparison with the initial values. Therefore, failure was facilitated by cyclic stresses during start-up/shutdown of gas turbine.