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 degrees 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 gamma '/gamma '' co-precipitates in the low Ti/Al ratio alloy, whereas the high Ti/Al ratio counterpart only contained gamma '' precipitates. Upon 10,000 h exposure, complex geometrical closed-packed (GCP) phases formed from the gamma '/gamma '' co-precipitates in a layer-by-layer manner, i.e., the plate-like delta phase precipitates became decorated by blocky alpha-Cr and enveloped by a wavy gamma ' 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 for-mation of Ta-rich eta precipitates at the grain boundaries while Nb promoted delta precipitates. The underlying phase transformation behavior of the layer-by-layer structure is likely initiating from the gamma ''-delta phase transition that ejects Cr and Ti into the neighboring phases, thus resulting in local phase separation into alpha-Cr and gamma ' thin film. This phase transformation process had a significant impact on the phase morphologies in the high Nb/Ta alloys exposed at 700 degrees C for 10,000 h, with experimental results contradicting those from computational prediction.(c) 2023 Elsevier B.V. All rights reserved.

Automated summary

Alloys based on Inconel 725 with elevated levels of Nb and Ta were studied for their phase stability and transformation after aging and long-term exposure at 700 degrees C for 10,000 hours. Scanning electron microscopy and transmission electron microscopy were used to characterize the microstructure changes. The results showed that the addition of Nb/Ta promoted the formation of gamma '/gamma'' co-precipitates in the low Ti/Al ratio alloy, while only gamma '' precipitates were present in the high Ti/Al ratio counterpart. After 10,000 hours exposure, complex geometrical closed-packed (GCP) phases formed from the gamma'/gamma'' co-precipitates in a layer-by-layer manner, with plate-like delta phase precipitates decorated by blocky alpha-Cr and enveloped by a wavy gamma ' film.