Enhanced Oxidation Resistance of AlCoCrFeNi High Entropy Alloy Obtained by Annealing

Abstract

High-Entropy Alloys (HEAs) have emerged as a promising class of materials for extreme
environments due to their unique compositional complexity and the resultant enhanced thermal
stability. Among them, AlCoCrFeNi has garnered significant attention for its superior oxidation
resistance at elevated temperatures, primarily owing to the formation of protective alpha
alumina (Al₂O₃) surface oxide scales. This study investigates the high-temperature oxidation
behavior of arc-melted AlCoCrFeNi HEAs, focusing on the effect of isothermal annealing at
1150°C on the enrichment of the Ni-Al-rich B2 phase and its subsequent influence on oxidation
resistance. The alloy was synthesized via vacuum arc melting and remelted several times to
obtain compositional homogeneity. The AlCoCrFeNi as-cast sample comprises of Ni-Al rich
B2 phase and Cr-Fe rich BCC phase. This as-cast sample was then subjected to isothermal
annealing categorically at 1150°C for 5h leading to the formation of some Fe,Co-rich FCC
phase along with maximization of the Ni,Al-rich B2 phase at the expense of the Cr,Fe-rich
BCC phase, subjected to systematic oxidation tests at 850°C, 1000°C, and 1150°C for 5 hours
each. Microstructural and phase characterization was conducted using SEM, TEM/STEM,
EDS, and XRD in the as-cast, annealed and oxidized state of the HEA sample.
Thermogravimetric (TGA) analysis was conducted to obtain weight gain characteristics,
parabolic rate constants, and the activation energy. Results indicate that post-annealing
significantly enhances the volume fraction of the Ni,Al-rich B2 phase, promoting the formation
of dense, adherent and protective α-Al₂O₃ surface oxides over the competing spinel oxide and
chromia oxide at 1150°C. A clear correlation was established between the B2 phase fraction
and oxidation kinetics, with higher B2 content leading to reduced mass gain and more stable
oxide scales especially at temperatures in the range of 1000-1200°C. This work contributes to
the understanding of phase evolution-oxidation relationships in HEAs and supports the
development of next-generation materials for cutting tools, aerospace, energy, and high
temperature industrial applications.

Popular Abstract

Have you ever seen how iron rusts when it’s left out in the rain? That’s called oxidation—
when metals react with air and slowly break down. Now imagine this happening inside
airplane engines or power plants, where temperatures get really, high. If the metal parts there
start to "rust" or get damaged, it can be dangerous. That’s why scientists are trying to create
special metals that can protect themselves even in extreme heat. The solution is to use
special materials such as high-entropy alloys (HEAs) that resist heat. These are novel kinds
of metals produced by almost equal mixing of five or more elements. Made of aluminium
(Al), cobalt (Co), chromium (Cr), iron (Fe), and nickel (Ni), one such alloy shows
considerable potential for survival in demanding surroundings.
In our thesis, we investigated how this specific alloy known as AlCoCrFeNi might withstand
oxidation, or the reaction between metals and oxygen like rusting but at very high
temperatures . Heated metals create flaky, fragile surface layers that finally peel off and
weaken the substance. AlCoCrFeNi is different, though., Its robust, solid exterior layer of
aluminium oxide (Al₂O₃) formed by the aluminium in it serves as armour to guard the inside
metal. This layer keeps the metal safe kind of like how sunscreen protects your skin from the
sun.
We found that heating the alloy a process known as annealing, allows us to raise the
concentration of a unique structure in the metal known as the B2 phase, rich in aluminium
and nickel. During high-temperature exposure, this B2 phase aids in the creation even better
protective layers.
We heated the alloy samples at various temperatures ranging from 850°C, 1000°C, and
1150°C then observed how much weight they gained from oxygen sticking to them. We
discovered, using modern microscopes and thermal experiments, that the alloy resisted
oxidation better especially at the higher temperatures ,the more B2 phase it possessed.
This research helps us understand how to make tougher, stronger, longer lasting materials for
use in jet turbines, cutting tools, and energy systems are designed by scientists and engineers
using this work. Because of some clever chemistry and heat treatment, these smart alloys can
stay performing under pressure, heat, and stress instead of rusting or breaking down.

• author: Paul, Stany and Vijayan, Vishal
• supervisor: Rahul Bhattacharya
• organization: Production and Materials Engineering
• course: MMTM05 20251
• year: 2025
• type: H2 - Master's Degree (Two Years)
• subject: Technology and Engineering
• keywords: High-Entropy Alloys, Isothermal continuous oxidation, Alpha Alumina, Ni, Al-rich B2 phase, Spinodal decomposition
• report number: CODEN:LUTMDN/(TMMV-5375)/1-67/2025
• language: English
• id: 9198355
• date added to LUP: 2025-06-13 12:42:56
• date last changed: 2025-06-13 12:42:56