High Entropy Alloys (HEAs) are a class of alloys that have 5 or more principal components, associated with a configurational entropy (∆Scon f ig) ≥ 1.5R. The large ∆Scon f ig of these alloys promotes a single phase solid solution, facilitating high strength through solid solution strengthening, and high ductility due to the absence of detrimental intermetallics. Limited studies also indicate sluggish diffusion kinetics, suggesting HEAs may exhibit low creep rates at high temperatures. Unfortunately, HEAs that are ductile at room temperature suffer from low strength at elevated temperatures. Numerous studies have shown that precipitates or dispersoids are necessary to stop dislocation motion at high temperatures.
Therefore, this work developed an HEA with a high volume fraction of precipitates. Alloy design was guided by L12 structured Ni3(Al, X) precipitates (γ′), which are used in Ni-base superalloys and exhibit anomalous strengthening with increasing temperature. Governed by the literature review, CALPHAD calculations and experimental trials, Al6.25C1.0Co15Cr13Fe4.4Mo1.75Nb0.6Ni48Ti5V5 was developed.
This research attempts to address whether a higher ∆Scon f ig compared with Ni-base superalloys provides any benefit in strength at high temperatures of ≥ 700 °C. Small cylindrical samples were arc-melted and suction cast, followed by solutionizing and heat treatment. Because of the COVID-19 pandemic, the experiments were cut short and heat treatment remained unoptimized. Nevertheless, we show that the alloy has potential for high temperature application.In addition, an important understanding was obtained based on microstructural and mechanical characterization of the alloy. The γ′ solvus was approximately 1110 °C, which is about 100–150 °C below most superalloys, but in full agreement with Thermo-CalcR© predictions. The compressive yield strength of the alloy up to 750 °C was comparable or better than superalloys such as CM 247LC, IN-100, CMSX-10, and various HEAs studied to date. XRD confirmed the presence of a γ-γ′ microstructure with a room temperature lattice misfit of −0.33%. SEM and TEM microstructures suggested approximately 60 volume percent of precipitates. Chemical analysis conducted on the TEM showed that theγ′phase was enriched in Ti and V. Both these elements were responsible for the high strength of the alloy. The density of the alloy was 7.9 g/cm3, and on an Ashby plot, the 750 °C strength was superior to most superalloys.
|Commitee:||Hargather, Chelsea Zacherl, Burleigh, T. David|
|School:||New Mexico Institute of Mining and Technology|
|Department:||Materials and Metallurgical Engineering|
|School Location:||United States -- New Mexico|
|Source:||MAI 82/1(E), Masters Abstracts International|
|Subjects:||Materials science, Engineering|
|Keywords:||Entropy, HEA, Superalloy, Yield strength|
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