Effect of Aluminum Addition on Thermal Stability of High Entropy Alloys Using an in-Situ TEM Heating Approach

Elaf A. Anber

doi:10.17918/00000095

High entropy alloys (HEAs) have received considerable attention due to their promising thermal, mechanical, and corrosion-resistant properties which make them widely suited for aerospace and marine applications. While the thermal stability of Al-containing HEAs has been studied at various annealing temperatures, the results have focused on the late stages of precipitation, highlighting the need for the analysis of the intermediate stage precipitation. In-situ heating in the transmission electron microscope (TEM) complemented by ex-situ characterization of bulk annealed specimens, thermodynamic calculations, and precipitation simulations were utilized to study the thermal evolution, by concentrating on the role of heterogenous nucleation sites, of AlxCoCrFeNi from nucleation to late stage precipitate growth, tracking the early, intermediate, and final stages of intermediate phases. In-situ TEM revealed the formation of additional intermediate phase, Co-B2 at 550°Cin Al0.3CoCrFeNi, where hundreds of hours are predicted for this phase to be shown during ex-situ experiments. At higher annealing temperatures between 700-900°C, in-situ TEM shows the formation of Cr-rich precipitates as the first intermediate phase, followed by NiAl precipitates forming coprecipitates. In conjunction with the in-situ and ex-situ TEM studies, thermodynamic calculations and precipitation simulations were performed to predict the formation of these phases and are found to support the experimental results. Additionally, the study showed , for the first time, that the quaternary alloy is unstable at an intermediate temperature, where Cr-BCC precipitates formed after a 10 min exposure at 750 °C during in-situ TEM heating. Moreover, the effect of Al addition (0.1-0.5 mole fraction) on the thermal stability of the base chemistry alloy was studied at a temperature between 700 to 900 °C where more complex coprecipitates that consist of NiAl-B2 and Cr-rich BCC were formed when Al content exceeded 7 at. %. Interestingly, the increase in Al content accelerates the formation of B2 (NiAl) phase and enhances the overall density of the secondary phases and the content of Cr in Cr-rich BCC precipitates. In addition to phase stability, the study aimed to understand the role of Al addition on the oxidation resistance of refractory HEAs, tracking the evolution of complex oxides in Alx (0.0-0.75)TiTaHfZr at 900 C for 10 hr. We successfully showed that addition of Al plays a vital role in formation of the passivation layer and prevention of pesting phenomena in TiTaHfZrNb. Employing precession diffraction technique, along with EDS, two Al-rich oxides form: Al-Ti-O (TiO2 structure) and [alpha]-Al2O3. Furthermore, addition of Al induced formation of more complex oxide scale, in comparison with TiTaHfZr, where only TiO2 and Nb2TiO7 structures were presented in the base RHEAs. The present work provides a new insight into the microstructural and oxidation evolution of HEAs and reveals the importance of intermediate stages of thermal evolution, enabling a more predictive view of phase evaluation in this class of alloys. Keywords: High Entropy Alloys, In-situ TEM, Intermediate phases, Oxidation, Phase stability

Effect of Aluminum Addition on Thermal Stability of High Entropy Alloys Using an in-Situ TEM Heating Approach

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Effect of Aluminum Addition on Thermal Stability of High Entropy Alloys Using an in-Situ TEM Heating Approach

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