Sustainable high-entropy materials?

Abstract

High-entropy materials (HEMs) show inspiring structural and functional properties due to their multi-elemental compositions. However, most HEMs are burdened by cost-, energy-, and carbon-intensive extraction, synthesis, and manufacturing protocols. Recycling and reusing HEMs are challenging because their design relies on high fractions of expensive and limited-supply elements in massive solid solutions. Therefore, we review the basic sustainability aspects of HEMs. Solutions include using feedstock with lower carbon and energy footprints, sustainable primary synthesis routes from minerals, attenuation of the equimolar alloying rule, and a preference for scrap and dumped waste for secondary and tertiary synthesis. The high solubility, compositional flexibility, and chemical robustness of HEMs offer pathways for using higher fractions of mixed and contaminated scrap and waste feedstocks, which are not admissible for synthesizing conventional materials. We also discuss thermodynamic and kinetic design strategies to reconcile good material properties with high impurity tolerance and variable compositions.

Fig. 1.. Energy, carbon, cost abundance considerations of HEMs and metals.

(A) CO₂ footprint associated with the energy required for the metal extraction of more than 30,000 different HEMs (blue dots). The values of pure metals (red dots) and larger material classes, such as steels and aluminum alloys (green circles), are also shown in (155). (B) Relationship between metal prices and the relative abundance of chemical elements in Earth’s upper continental crust on the basis of the abundance of silicon with 10⁶ atoms (170). Notably, not only the abundance but also the dispersion (richness) across a larger scale and within the ore minerals determines the CO₂ and energy footprints of metal extraction. PGM, platinum group metal.

Fig. 2.. Increasing interest in sustainable high entropy materials design.

(A) Increase in scientific literature output on HEMs, sustainable material design, and sustainable HEMs. (B) Comparison of some real-world key metrics considering the required raw materials for the CoCrFeNiMn (Cantor) alloy when fabricated via primary synthesis (high-purity metallic materials from minerals) and secondary synthesis (from scrap). The analysis shows the average values for the price and CO₂ footprint.

Fig. 3.. Future synthesis-related aspects of the metallurgical HEMs sector toward a more sustainable and circular economy.

Reused materials, end-of-life products, and dumped waste all contribute to the manufacturing chain to achieve a more responsible carbon, energy, and scarcity footprint of these materials.

Fig. 4.. Sustainability and cost considerations for HEMs.

Aspects such as sustainability, responsible use of elements, absolute price, and cost variations need to be considered when designing sustainable HEMs. Figure reproduced in modified form with permission from (42).

Fig. 5.. The concept of one-step synthesis of bulk HEM from oxides.

(A) Thermodynamic design guidelines considering the bulk reducibility and substitutional alloying capability. (B) Kinetic conception sketching the possible microstructure states while considering the temperature, time, and conversion rate. (C) Microstructure evolution during the synthesis of bulk Fe-36Ni invar alloys from Fe₂O₃ and NiO using H₂. Figure reproduced with permission from (98).

Fig. 6.. Sustainable design and manufacturing strategies.

Different measures have to be taken into account in the design, synthesis, and processing of mass-produced HEMs to reduce energy consumption and CO₂ emissions and ensure multifunctionality and longevity.