The design of complex engineering alloys has gained new momentum since the concept of multiple primary elements was proposed [1,2], where instead of one element being strengthened by additions of other elements (like in steels, superalloys, aluminum-alloys, etc.) multiple elements occur in near-equiatomic concentrations. This new field, often referred to as high-entropy alloy (HEA) or compositionally complex alloy (CCA) research, sheds new light on the reciprocal understanding of structures, mechanisms, and properties at higher chemical and microstructural complexity. Significant examples of this are the strength-ductility trade-off in the Cantor alloy at low temperatures or the unexpectedly increased ductility of Ti-containing HEAs due to oxygen doping [3]. But HEAs/CCAs are not only interesting from a structural point of view. Their functionality is also of utmost importance, like the intrinsic electrocatalytic activity of the Cantor alloy [4] or the memory effect of high-entropy shape memory alloys (HESMAs) [5,6]. In light of today's planetary changes [7], it is crucial to address sustainability issues early on. Only a few sustainability challenges and opportunities for complex alloys have been identified so far, e.g., focusing on materials selection [8], alloy design [9], and recycling-oriented metallurgy [10]. This motivates the present investigation, based on a systematic literature review, to analyze the research/technology field of HESMAs as complex systems and to provide a first synthesized look at three critical aspects: (1) It discusses targeted thermodynamic, structural, and functional HEA/CCA properties in the light of the growing need and awareness of sustainability in metallurgy. (2) It analyzes HESMAs, considering resource availability and overall energy consumption. (3) Finally, the interdependencies between desired material properties, sustainability aspects, and system complexity are appraised in a first approximation [11].

Literature: [1] Yeh et al., Adv. Eng. Mater. 6 (2004). [2] Cantor et al., Mater. Sci. Eng. A 375 (2004). [3] George et al., Nat. Rev. Mater. 4 (2019). [4] Löffler et al., Adv. Energy Mater. (2018). [5] Piorunek et al., Intermetallics 122 (2020). [6] Piorunek et al., JALCOM 857 (2021). [7] Richardson et al., Sci. Adv. 9 (2023). [8] Fu et al., Scr. Mater. 138 (2017). [9] Cann et al., Progr. Mater. Sci (2021). [10] Raabe et al., Nature 575 (2019). [11] Ladyman & Wiesner, Yale Univ. Press, London, 2020.

Dr.-Ing. David Piorunek