High-entropy alloys: How chaos takes over in layered carbides as metal diversity increases

In the tug-of-war between order and chaos within multielemental carbides, entropy eventually claims victory over enthalpy by pushing the system toward complete disorder as the diversity of elements in the material increases, as revealed in a study published in Science.

Researchers synthesized 40 layered carbide phases with composition MAlX materials (M is a transition metal, Al is for aluminum, and X is either C or N), where the number of M was between 2 and 9.

Their goal was to uncover the trends in short-range ordering and compositional disorder in so-called high-entropy systems. They found that in carbides with fewer constituent elements, short-range order driven by enthalpy dominated. However, as the number of elements increased, entropy took control, randomizing the metal configurations.

The interest in "high-entropy systems" began with alloys, where a number of different metallic elements are combined in specific proportions. These alloys soon gained a lot of attention owing to their unexpected mechanical and thermal properties, which defied the "rule of mixture"—calculations that helped scientists predict how an alloy may behave based on its elemental composition.

The alloy systems showed increased mechanical strength and reduced heat transfer abilities beyond what was predicted. One possible explanation for this behavior is that incorporating multiple metallic elements into a single-phase material could harness entropy to stabilize mixtures that are otherwise enthalpically unfavorable. This gave rise to a new class of materials termed high-entropy alloys.

As the high-entropy label expanded beyond alloys to include ceramics, oxides, and carbides, it sparked debate over the extent to which enthalpy still influenced the stability of a single-phase system. This controversy isn't unfounded, as studies have observed short-range ordering in these high-entropy materials, which suggests enthalpic effects are very much at play.

To understand the true role of entropy versus enthalpy in determining atomic arrangements in high-entropy materials, the researchers of this study studied 40 different M4AlC3 (MAlX) layered carbide phases using transition metals belonging to groups 4, 5, and 6 of the periodic table.

Density Functional Theory (DFT) and computer modeling were used to gain insights into the competition between enthalpy and entropy. Their prediction indicated that the transition from order to disorder likely occurred when a seventh element is introduced.

These findings were supported by the experimental observation of order-to-disorder transition carried out with the help of X-ray diffraction and secondary ion mass spectrometry.

The MAlX materials were then transformed into 2D MXene sheets via wet chemical synthesis. Characterization of the sheets revealed that the order-to-disorder transition impacted the surface chemistry of the material. It was observed that the number of transition metals increased from four to nine, –O terminations rose while –OH and –F terminations declined.

The order-to-disorder transition also impacted the electronic properties of the 2D sheets, where the resistivity increases from 2 to 4 metals but showed a decreasing trend with an increase in the number of metals. The researchers attributed this effect to two factors: a smaller number of group 6–group 6 transition metal neighbors with a decrease in order and structures with smaller differences in the total number of valence electrons with an increase in disorder.

This study established that the shift from order to disorder occurs when configurational entropy outweighs enthalpic preferences.

Written for you by our author Sanjukta Mondal, edited by Sadie Harley, and fact-checked and reviewed by Robert Egan—this article is the result of careful human work. We rely on readers like you to keep independent science journalism alive. If this reporting matters to you, please consider a donation (especially monthly). You'll get an ad-free account as a thank-you.