High Entropy Alloys: A New Wave in Metallurgy

Ryan Zhou
Thomas Jefferson High School for Science and Technology

This article was included in the 2019-2020 Teknos Science Journal

A glossy, space gray strip sits in the blue palm of my glove. Its distinctive metallic timbre echoes a sense of density and elasticity. The ribbon of foil feels both familiar and foreign—aluminum foil is commonplace in our lives, but this material, tantalum, is clearly not the same. Compared to aluminum, tantalum is over six times denser, has a melting point 2300ºC higher, and is more than 50,000 times less abundant on earth. Tantalum is commonly used in simple capacitors, surgical tools, high-temperature furnaces, and jet engines. However, all of these applications deal with engineered alloys, not the raw element itself. At the U.S. Naval Research Laboratory (NRL) Materials Science and Technology Division, I worked with field experts to conduct cutting edge research to discover novel materials, such as tantalum, that will define our future through their use in naval applications.

Materials have always been at the core of humanity’s successes. We are where we are today because humans had the physical capability to use tools and the intelligence to create and improve upon tool design with more efficient materials. When we think back on history, we categorize civilization into the Stone Age, Bronze Age, and Iron Age, which shows the impact of material breakthroughs on the development of human civilization. Today, metals and other materials dictate the technologies that advance the human race. Once humans developed furnaces capable of smelting pure metals, the field of metallurgy took off, becoming the next source for material breakthroughs and technological progress. 

The first of these breakthroughs was copper. Copper was first alloyed with small amounts of tin to form bronze, which was much stronger and more resistant to corrosion than copper was. These properties revolutionized the way humans lived by allowing them to build more durable tools and structures. Another historically groundbreaking alloy was steel, an alloy of iron and carbon which first was developed in Han China. Furthermore, the Bessemer process fueled the Industrial Revolution and Carnegie’s monopolization of the steel industry.  Over a millennia later, 19th-century metallurgists introduced multicomponent alloy steels such as stainless steel. These steels incorporated varying amounts of chromium and molybdenum to create its signature sheen. 

More recently, scientists have created superalloys: lightweight, high-performance alloys applicable in turbine engines. These superalloys are a combination of corrosion-resistant nickel and refractory elements (such as niobium and tantalum). Previously, the vast majority of research was dedicated to alloys with only one principal element, such as the copper in bronze, iron in steel, or nickel in nickel superalloys. However,  this old approach to designing new alloys by adding tiny percentages of a different element to a base element has become increasingly expensive to synthesize and difficult to justify.

In the early 2000s, Jien-Wei Yeh and Brian Cantor published independent papers regarding the first multi-principal element alloys [5]. Yeh coined the term high-entropy alloy (HEA) because of the solid state’s high configurational entropy (entropy due to the location of constituent particles). In contrast to conventional alloys and superalloys, HEAs often contain five or more principal elements in near equimolar ratios. The vast realm of possible HEAs provides many areas of research and characteristics to find, causing the field to grow dramatically each year [5]. This project, on aluminum-based HEAs, aims to provide insight on the refractory properties, oxidation and corrosion resistance, high toughness, and wear resistance of titanium and nickel alloys in a more efficient frame. The ability to combine and improve upon the properties of known elements and alloys make HEAs ideal for naval and aerospace technologies.

With over 70 stable elements in today’s periodic table, hundreds of millions of HEAs with three to six principal components are possible. Modeling has been used to successfully predict the properties of specific alloys and simplify the design process. Researchers have used the Calculation of Phage Diagrams (CALPHAD), a method to either encourage solid-solution strengthening or precipitation strengthening techniques [4]. In the case of precipitation strengthening, Li et al. [3] have used multiple computational approaches to model variable cooling rates, including a Monte Carlo method to fine-tune the nucleation of precipitates and therefore the overall microstructure and mechanical properties of the alloy. 

Besides modeling, researchers can analyze the microstructure and properties of HEAs using both quantitative and qualitative analytical methods. Using electron backscatter diffraction (EBSD) in scanning electron microscopy (SEM) allows for a fast 2D overview of grain boundaries, precipitates, and phase maps. X-ray diffraction (XRD) and energy-dispersive X-ray diffraction (EDS) can both find the crystal structures present in an alloy. EDS can also estimate bulk composition across given regions. Atomic-scale analyses, including transmission electron microscopy (TEM) and atom probe tomography (APT), allow for single-atom precision in 3D reconstructions and solute analyses capable of resolving nanoscale precipitates. J. D. Poplawsky (personal communication, Jan. 9, 2020) was able to conclude the homogenization of a HEA by quantifying the Pearson coefficient, µ, in a binomial frequency distribution of APT data. Through the use of these novel techniques, Diao et al. [1] were able to precipitate-strengthen FCC Al0.3CoCrFeNi.

Through cooperation with our school’s mentorship program and the Chemical Analysis senior research lab, I studied similar HEAs and conducted atomic-scale analyses at the NRL in Washington, D.C. In previous work, NRL researchers observed nanoscale precipitates in as-cast Al2.7CrMnTiV. APT resolved cuboids coherent with a crystal structure similar to strengtheners seen in nickel superalloys. The hard phase seen in these cuboids was also recognized by Park et al. [6] in Al0.5CoCrFeMnNi when annealed at lower temperatures. My project conducted annealing heat treatments of as-cast Al2.7CrMnTiV to achieve a solid-state solution according to thermodynamic modeling results. In similar methods to Diao et al. [1], I confirmed homogenization using APT, EDS, and XRD. Future research will include the thermal aging of this homogenous sample to precipitate out the hypothesized cuboid strengtheners. Through the use of EDS, XRD, techniques from Park et al. [6], and differential scanning calorimetry (DSC) data, researchers can predict the aging temperatures most likely to precipitate these cuboids.

For the past six months, I have immersed myself in the field of metallurgy, attempting to harness the properties of elemental metals to form alloys with applications ranging from the tiny silicon chips in today’s mobile electronics to the heat shield of a future interstellar spacecraft. With the help of today’s computational modeling, novel design techniques, and advanced analysis methods, these attempts are no longer the shots in the dark or strokes of luck that have characterized previous material breakthroughs. Humanity no longer needs to endure the metallurgy guessing game we have played for centuries; these technologies have revolutionized alloy design to be faster, stronger, and better. As these technologies improve, breakthroughs may simply become an everyday occurrence.

References

[1] Diao, H., Ma, D., Feng, R., Liu, T., Pu, C., Guo, W., . . . Liaw, P. K. (2019). Novel NiAl-strengthened high entropy alloys with balanced tensile strength and ductility. Materials Science and Engineering: A, 742, 636-647. https://doi.org/10.1016/j.msea.2018.11.055

[2] Knipling, K. E., Narayana, P. U., & Nguyen, L. T. (2017). Microstructures and properties of as-cast AlCrFeMnV, AlCrFeTiV, and AlCrMnTiV high entropy alloys. Microscopy and Microanalysis, 23, 702-703. https://doi.org/10.1017/S1431927617004172

[3] Li, J., Chen, H., Liu, S., Fang, Q., Liu, Y., Liang, L., . . . Liaw, P. K. (2019). Tuning the mechanical behavior of high-entropy alloys via controlling cooling rates. Materials Science and Engineering: A, 760, 359-365. https://doi.org/10.1016/j.msea.2019.06.017

[4] Miracle, D. B., & Senkov, O. N. (2017). A critical review of high entropy alloys and related concepts. Acta Materialia, 122, 448-511. https://doi.org/10.1016/j.actamat.2016.08.081

[5] Murty, B. S., Yeh, J. W., Ranganathan, S., & Bhattacharjee, P. P. (2019). High-entropy alloys (2nd ed.). https://doi.org/10.1016/C2017-0-03317-7

[6] Park, J. M., Moon, J., Bae, J. W., Jung, J., Lee, S., & Kim, H. S. (2018). Effect of annealing heat treatment on microstructural evolution and tensile behavior of Al0.5CoCrFeMnNi high-entropy alloy. Materials Science and Engineering: A, 728, 251-258. https://doi.org/10.1016/j.msea.2018.05.041