Mixing Metals to Make Mighty Materials: The Magic of Atomic Disorder and Entropy

Written by: Prince Sharma (MEM ’24 PhD, Lehigh University)

Imagine taking at least four different metals, combining them, and ending up with an alloy that is stronger and more stable than any of its constituent metals. Sounds like alchemy, right? Well, it’s cutting-edge materials science, and we are using this approach to synthesize new materials with better properties compared to the conventional ones.

But why bother mixing so many metals? Traditional alloys usually have one main metal with small amounts of others added. High-entropy alloys, on the other hand, contain multiple metals in roughly equal proportions. This creates a jumbled (“amorphous”) atomic structure that leads to interesting and useful properties.

My colleagues and I have recently made a breakthrough in developing a novel high-entropy alloy powder. The research, published in the “Journal of Magnesium and Alloys,” describes an innovative way to combine magnesium, titanium, copper, zinc, and iron into a single, stable alloy as shown in Illustration 1. Our goal was to create a magnesium-titanium alloy, which is notoriously difficult to achieve because these metals don’t naturally mix well. By adding copper, zinc, and iron to the alloy, we were able to force the elements into a stable, amorphous structure.

The secret to the success lies in a process called mechanical alloying. Essentially, we put all the metal powders into a high-energy ball mill. As the powders are violently sandwiched and crushed together, the individual metal particles are forced to mix at the atomic level. Usually, it takes a long time to break down the orderly crystal structures of metals, but by carefully choosing the mix of elements, we accelerated how quickly the metals transformed into an amorphous structure.

Illustration 1. Schematic highlighting the phases of creating our novel high-entropy alloy powder.

The resulting material combines some of the best properties of its constituent metals. It’s lightweight like magnesium, strong like titanium, resists corrosion better than either metal on its own, and is biocompatible. Plus, unlike many similar materials, it remains stable even when heated to high temperatures.

This type of alloy could have a wide range of applications. Its combination of low weight and high strength makes it ideal for use in aerospace or automotive industries, where reducing vehicle weight can lead to significant fuel savings. The material’s corrosion resistance and biocompatibility also make it a promising candidate for medical implants and marine engineering applications.

Flat yet Fierce: Atomic Disorder in 2D

Similarly, mixing different components in two-dimensional (2D) materials can control disorder in the atomic structure and can be leveraged for applications in thermal insulation. Unlike most metals, which tend to conduct heat, high-entropy alloys can be fantastic insulating materials because the variety of atoms within them heat up at different rates. Alloys with extremely low thermal conductivity can even be harnessed to generate electricity in a branch of materials science called thermoelectrics. 

We recently published a study in “Carbon” that examines how to control heat flow in 2D MXene, a material in which carbon and/or nitrogen are sandwiched between metal(s). As shown in Illustration 2, this 2D material is derived from 3D crystal structures (MAX phases), known for their unique combination of metallic and ceramic properties, by selectively removing the ‘A’ layer, offering remarkable flexibility for tuning physical properties such as thermal conductivity. We aimed to understand the impact of converting 3D MAX-phase into 2D MXene structure on thermal conductivity. Transport of phonons, or heat-carrying vibrations, was studied in each material, including a high-entropy version of MXene (HE-MXene), to see how disorder in the atomic structure helped to scatter the phonons.

Illustration 2. The white ‘A’ layers are removed from the 3D crystal (MAX) to create the 2D material (MXene) composed of the remaining titanium and carbon, then thermal conductivity is further reduced by adding three other metals to the alloy.

Like the high-entropy alloy powder, the HE-MXene exhibits a complex, disordered structure. Such disorder helps to scatter those heat-carrying vibrations (phonons), making HE-MXene a much more effective insulating material compared to the 3D MAX phase. This phonon scattering can be seen in Illustration 3. The disorder in HE-MXene significantly shortens the distance phonons can travel before being scattered, interrupting the transfer of heat. This ultra-low thermal conductivity is all thanks to the various metals in the alloy having vastly different heat capacities from one another.

Illustration 3 Phonons broadening in (a) MAX phase; (b) MXene and (c) HE-MXene. The brighter streaks in each panel indicate phonons being scattered.

Both efforts highlight how introducing multiple elements in roughly equal proportions can lead to unique and superior properties. In the case of the high-entropy alloy powder, this approach resulted in a stable, amorphous structure with potentially improved mechanical properties. For the HE-MXene, it led to ultra-low thermal conductivity. By manipulating material structure parameters, we can design materials with tailored properties for specific applications. This innovative approach opens new possibilities in thermal management, energy storage, and structural materials design.