Precipitate evolution during aging and its individual role on high-temperature hot corrosion response in Alloy 617

Abstract

In this work, we report the results of a systematic study on the evolution of the precipitates during aging in a Ni-based superalloy (Alloy 617) and their effect on the high-temperature hot corrosion (HTHC). It was observed that the extent of Cr-rich precipitation increased with the aging time while the other microstructural parameters such as grain size, retained strain, and Σ3n (n ≤ 3) boundaries fraction did not change in comparison to the as-received (AR) condition. To evaluate the role of precipitation on hot corrosion, the AR and the aged specimens were subjected to the HTHC test involving complete immersion of the specimens in a 75 wt% Na2SO4 + 20 wt% NaCl + 5 wt% V2O5 salt mixture at 1273 K for 24 h. The characterization of surface and cross-section morphologies of the corroded layers through scanning electron microscopy revealed that the oxide film formed on the AR specimen is highly porous and loose. In contrast, the layer formed on the specimen aged for 100 h (AR-100 h) is significantly thinner and dense. Post-HTHC analysis aided with the thermodynamic and diffusion calculations revealed that the improved HTHC performance of the AR-100 h specimen is due to the rapid formation of an oxide layer owing to the enhanced supply of Cr as a consequence of carbide dissolution. Also, the undissolved carbides at the grain boundaries acted as a potential obstruction to the inward transport of corrosive species, resulting in a significantly lower percolation depth in the AR-100 h specimen (~66 ± 6 µm) than the AR condition (~305 ± 15 µm).