Microstructure and property degradation in René 80 due to Ostwald ripening

Abstract

Used fourth-stage turbine blades from a heavy-duty gas turbine were considered for reuse in rotor balancing. The blades were made of René 80, a legacy investment-cast nickel-base superalloy. The same blade type had recently experienced creep failures that were partly attributed to degradation of the γ’ morphology. Among other material tests, fatigue crack growth rate (FCGR) tests were conducted on compact-tension (CT) specimens machined from a blade root to determine whether the blades exhibited the expected properties. The FCGR results were poorer than expected based on prior experience. A metallurgical investigation was commissioned to identify possible microstructural defects that could explain the lower-than-expected FCGR results. The investigation concluded that Ostwald ripening caused by multiple refurbishment operations had degraded the morphology of the coarse γ’ precipitates and contributed to accelerated fatigue crack growth.