Inhomogeneous Distribution and Coarsening of γ″ Precipitates in a Ni-Based Superalloy and Their Effect on Creep

Modifications to INCONEL® alloy 725 (IN725) with higher Nb or Ta and high Ti/Al ratio previously revealed the formation of γ″ precipitatesPrecipitates as the sole precipitatePrecipitates strengthening phase which was accompanied with significant increases in elevated temperature strength and resistance to creepCreep deformation. In this study, the γ″ alloys (named M725-Nb and M725-Ta) were chosen to observe the microstructural evolutionMicrostructural evolution of γ″ precipitatesPrecipitates during creepCreep. Following aging, γ″ was found to be the main precipitatePrecipitates strengthening phase with no detectable γ′ in both M725 alloys. After creepCreep, the grip/gage sections of the crept M725-Nb/Ta exhibited preferential coarseningCoarsening and inhomogeneous distributionInhomogeneous distribution of γ″ platelets, and sandwich-like structures, comprising of cubic γ′ precipitatesPrecipitates with small γ″ discs on each facet. This γ″ variant selection behavior taking place in the grip section with no stress applied was different from the typical stress-induced variant selection reported in previous literature. However, creepCreep tested specimens with prior high-temperatureHigh temperature exposure to promote varying γ″ characteristics prior to deformation demonstrated a similar creepCreep behavior when compared to aged specimens tested under identical conditions. This suggested that the coarseningCoarsening and inhomogeneous distributionInhomogeneous distribution of γ″ precipitatesPrecipitates was not the determinant factor for creepCreep life in bulk M725-Nb/Ta alloys. The failure of bulk M725 was linked with the development of cracks along grain boundariesGrain boundary (GBs), where the emergence of Nb-rich δ and/or Ta-rich η phases at the GB markedly depleted the γ″ strengthening phase in the surrounding GB regions whichGamma-double prime led to localized weakening.