Phase stability and phase transformation related to Nb/Ta additions in a Ni-based superalloy

Alloys based on Inconel 725, and with elevated levels of Nb and Ta, have been designed for application in environments requiring high strength and corrosion resistance at elevated temperatures. The present study further examines the phase stability and subsequent phase transformation in those alloys following aging and long-term exposure of up to 10,000 h at 700 °C. Changes in the microstructure were characterized using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Two alloy bases with different Ti/Al ratio were considered. Following aging, the Nb/Ta additions favor the development of γ′/γ″ co-precipitates in the low Ti/Al ratio alloy, whereas the high Ti/Al ratio counterpart only contained γ″ precipitates. Upon 10,000 h exposure, complex geometrical closed-packed (GCP) phases formed from the γ′/γ″ co-precipitates in a layer-by-layer manner, i.e., the plate-like δ phase precipitates became decorated by blocky α-Cr and enveloped by a wavy γ′ film. Increasing the concentration of Nb and/or Ta did not change the basic characteristic of this phase transformation; however, the precipitate number density increased in the grain interior. On the other hand, the Ta addition in the alloy with low Ti/Al ratio promoted the formation of Ta-rich η precipitates at the grain boundaries while Nb promoted δ precipitates. The underlying phase transformation behavior of the layer-by-layer structure is likely initiating from the γ″-δ phase transition that ejects Cr and Ti into the neighboring phases, thus resulting in local phase separation into α-Cr and γ′ thin film. This phase transformation process had a significant impact on the phase morphologies in the high Nb/Ta alloys exposed at 700 °C for 10,000 h, with experimental results contradicting those from computational prediction.