High Entropy Alloys2 have been identified as a possible replacement for superalloys in propulsion systems’ components due to their inherent high specific strength and oxidation resistance at high temperatures.
HEAs are a relatively new class of materials, and although since 2004 more than 600 HEA journal and conference papers have been published, the whole HEA world is still dealing with numerous unanswered questions. Therefore, in order to exploit these advancements on HEA, further work is needed. These challenges are addressed explicitly in ATLAS to identify the necessary key enablers:
• Challenge 1 Material property database: Assessment of existing database to find possible applications is required. It is believed that HEAs and/or hybrid systems could solve many bottlenecks encountered by conventional materials. Although the database is still limited, many suggestions of their potential applications are seen from the literature2.
• Challenge 2 Material development: More fundamental studies are required. Material science and solid-state physics are mainly based on conventional materials with one or two principal elements. What happens in HEAs would be interesting for a better understanding of materials. Different contributions to mixing entropy such as mutual interactions in all unlike atomic pairs, short-range order, lattice distortion, electrical and thermal conductivity, thermal expansion, vacancy concentration, diffusion coefficients, phase transformation, Young’s modulus, dislocation energy, stacking fault energy, grain boundary energy, slip, twinning, serration behaviour, strengthening, toughening, fracture, fatigue, creep, wear, corrosion, and oxidation and their mechanisms are needed to be well understood.
• Challenge 3 Hybrid solutions: More research on multi-layered materials, on Metal Matrix Composites (MMC) with HEAs matrices (HEA-MMCs) in combination with ceramic materials (including Ceramic Matric Composites – CMCs) is required. Such a combination would generate many composites, among which many opportunities could be found for critical applications not easily attained by traditional composites.
• Challenge 4 Near-net shape manufacturing and materials integration: One of the primary advantages of near net shape production is the way it encourages efficient use of the new HEA material. The goal is to assess currently available manufacturing and joining methods to produce the selected demonstrators, described in Section 1.3.3. This task aims to reduce the number of HEAs required to produce demonstrators.