High-entropy Alloys: A New Frontier for Wear-resistant Materials in Aerospace Engineering

Joseph Hunt^*
*Correspondence: Joseph Hunt, Department of Mechanical Engineering, Commonwealth Open University, United Kingdom, Email:

Introduction

High-entropy alloys, characterized by their complex composition of multiple principal elements, have emerged as a new class of materials with remarkable mechanical, thermal, and chemical properties. These alloys, typically composed of five or more metallic elements in near-equal or nearly equal atomic percentages, exhibit superior strength, hardness, and resistance to wear, making them highly suitable for demanding applications in aerospace engineering. This research article explores the advancements in high-entropy alloys as wear-resistant materials, highlighting their potential to address critical challenges in the aerospace industry, including high-temperature wear, oxidation, and fatigue. Key factors influencing the wear resistance of HEAs, such as microstructure, phase formation, and elemental composition, are discussed, along with their applications in turbine blades, engine components, and other critical aerospace parts. The article also examines the current challenges in the development and large-scale production of HEAs and proposes future directions for research to unlock their full potential in aerospace engineering. The aerospace industry faces a unique set of challenges when selecting materials for high-performance applications, especially for components subjected to extreme environments, such as high temperatures, wear, corrosion, and fatigue. Traditional materials like stainless steels, nickel-based superalloys, and titanium alloys have been widely used for aerospace applications, but they often face limitations in terms of wear resistance, thermal stability, and overall performance in aggressive operating conditions.

High-entropy alloys, a relatively new class of materials developed in the 2000s, offer a promising alternative. These alloys are composed of multiple principal elements-typically five or more-at relatively high concentrations, as opposed to traditional alloys, which are dominated by one primary element. The combinatorial nature of HEAs leads to a wide range of possible microstructures and phase formations, providing opportunities for tailoring material properties to meet the specific demands of aerospace components, particularly in terms of wear resistance, oxidation resistance, and strength. In this article, we review the latest advancements in high-entropy alloys, particularly focusing on their potential as wear-resistant materials in aerospace engineering. We examine the underlying mechanisms that govern their wear behavior and highlight recent progress in their application for turbine blades, engine components, and other critical aerospace parts.

High-entropy alloys are defined by their composition, typically consisting of five or more principal elements in equal or nearly equal atomic ratios. The most common elements used in HEAs include transition metals such as chromium, nickel, Cobalt, Iron (Fe), and Titanium (Ti), although other elements like aluminum, molybdenum, and copper are also used depending on the application. The unique feature of HEAs is that they do not have a dominant element; instead, the constituent elements form a solid solution with a simple crystal structure, such as face-centered cubic, body-centered cubic, or a mixture of both. This multi-element composition imparts several desirable properties, including enhanced mechanical strength, improved corrosion resistance, and a higher degree of phase stability under extreme conditions. The design of HEAs revolves around the concept of maximizing the configurational entropy, which favors the formation of simple solid solution phases over complex intermetallic compounds.

Description

High-entropy alloys exploit the high entropy of mixing by combining multiple elements, which stabilizes the solid solution phase and prevents the formation of brittle phases that are commonly found in conventional alloys. The competition between different phases in HEAs- such as FCC, BCC, and amorphous phases-determines the mechanical and thermal properties of the material. The selection of elements and their concentrations significantly influences the alloy's microstructure and, consequently, its wear resistance. The varying atomic sizes of the elements lead to lattice distortions, which can strengthen the material by hindering dislocation motion and improving the alloy’s resistance to deformation and wear. Wear resistance in highentropy alloys is influenced by several mechanisms, including hardness, microstructure, and phase stability under high-stress conditions.

HEAs often exhibit superior hardness and strength compared to traditional alloys, owing to the solid-solution strengthening effect. The presence of multiple alloying elements, each with different atomic sizes and electronic properties, increases the resistance to dislocation motion and slip, thus enhancing the material’s hardness and wear resistance. The strength of HEAs is also influenced by the crystal structure. FCC-based HEAs tend to have excellent ductility and toughness, while BCC-based HEAs generally offer better high-temperature strength and resistance to wear in abrasive conditions. In many cases, a combination of FCC and BCC phases has been observed, which can result in improved overall mechanical properties.

The microstructure of HEAs can be tailored to improve wear resistance. For instance, the formation of hard precipitates, solid solution phases, or even intermetallic compounds within the matrix can provide additional resistance to abrasive wear. The control of phase composition and heat treatment processes allows for the optimization of mechanical properties such as hardness, strength, and ductility, which are critical for wear resistance. In particular, HEAs with a high proportion of solid-solution phases and minimal brittle phases exhibit better wear performance, especially under high-stress conditions such as sliding, abrasion, and erosion [1-3].

Oxidation is a critical issue in high-temperature wear environments, particularly in aerospace applications where components are exposed to extreme temperatures. HEAs, due to their complex multi-element composition, often demonstrate superior oxidation resistance compared to traditional alloys. This is because the presence of multiple alloying elements can lead to the formation of protective oxide layers on the surface of the material, which reduce the rate of oxidation and protect the material from wear and degradation. Alloying elements such as chromium and aluminum are particularly effective in forming stable oxide layers, which can prevent further oxidation and enhance the material’s durability in high-temperature environments.

Turbine blades and engine components are exposed to high temperatures, corrosive environments, and significant wear during operation. HEAs have shown great potential for these applications due to their excellent high-temperature strength, wear resistance, and oxidation resistance. For example, research has demonstrated that HEAs with high concentrations of elements such as Cr, Ni, and Co exhibit improved resistance to both thermal and mechanical degradation in turbine environments. The ability to tailor the composition of HEAs also allows for optimization of properties such as creep resistance, fatigue strength, and corrosion resistance, which are critical for the long-term performance of aerospace engines.

HEAs have also been explored for use as wear-resistant coatings on various aerospace components, including landing gear, wings, and airframe parts. These coatings help protect the underlying material from abrasive wear, corrosion, and thermal damage. By selecting appropriate elements that form durable oxide layers, researchers have demonstrated significant improvements in the wear life of coated components under extreme operating conditions [4,5].

Thermal protection systems are vital for aerospace vehicles, particularly those re-entering the Earth’s atmosphere or operating in high-temperature environments. HEAs with high melting points and thermal stability are being investigated for use in TPS, as they can withstand extreme thermal gradients and prevent structural failure during high-speed flight or re-entry. The fabrication of HEAs, especially in complex geometries required for aerospace components, remains a significant challenge. Conventional methods such as casting, forging, and welding may not be well-suited for HEAs due to their high melting points and complex phase behavior. Research into new processing techniques, such as additive manufacturing and powder metallurgy, is crucial for improving the manufacturability of HEAs.

The use of multiple expensive alloying elements in HEAs can lead to higher production costs. While HEAs show great promise, the development of cost-effective and scalable production methods is necessary for their commercial viability in aerospace applications. Although HEAs exhibit excellent wear and oxidation resistance under laboratory conditions, their long-term performance in real-world aerospace environments must be further investigated. More extensive testing is required to understand their behavior under cyclic loading, high-temperature conditions, and prolonged exposure to oxidative environments.

Conclusion

High-entropy alloys represent a promising frontier for the development of advanced wear-resistant materials in aerospace engineering. Their unique composition, microstructure, and mechanical properties make them suitable candidates for a range of demanding applications, including turbine blades, engine components, and thermal protection systems. While significant progress has been made in understanding the wear resistance mechanisms of HEAs, further research is needed to optimize their properties, improve processing techniques, and overcome challenges related to cost and scalability. As these challenges are addressed, HEAs are likely to play an increasingly important role in the next generation of high-performance aerospace materials.

Acknowledgement

None.

Conflict of Interest

None.

References

1. Xing, Yiteng, Pierre Bernstein, Muralidhar Miryala and Jacques G. Noudem. "High critical current density of nanostructured MgB2 bulk superconductor densified by spark plasma sintering." Nanomaterials 12 (2022): 2583.

Google Scholar, Crossref, Indexed at

2. Shimakawa, Y., J. D. Jorgensen, D. G. Hinks and H. Shaked, et al. "Crystal structure of (Cu,C) Ba2Ca3Cu4O11+δ (Tc=117 K) by neutron-powder-diffraction analysis." Phys Rev B 50 (1994): 16008.

Google Scholar, Crossref, Indexed at

3. Zhang, Yue, Wenhao Liu, Xiyu Zhu and Haonan Zhao, et al. "Unprecedented high irreversibility line in the nontoxic cuprate superconductor (Cu,C) Ba2Ca3Cu4O11+δ." Sci Adv 4 (2018): eaau0192.

Google Scholar, Crossref, Indexed at

4. Dou, Shi Xue, Olga Shcherbakova, W. K. Yeoh and J. H. Kim, et al. "Mechanism of enhancement in electromagnetic properties of MgB2 by nano SiC doping." Phys Rev Lett 98 (2007): 097002.

Google Scholar, Crossref, Indexed at

5. Dadiel, Joseph Longji, Sugali Pavan Kumar Naik, Paweł Pęczkowski and Jun Sugiyama, et al. "Synthesis of dense MgB2 superconductor via in situ and ex situ spark plasma sintering method." Materials 14 (2021): 7395.

Google Scholar, Crossref, Indexed at