The aerospace and fossil energy sectors have long relied on Ni-base superalloys. However, to improve efficiency and environmental impact, new materials offering low cost, light weight, and high strength are needed. Significant research has focused on developing ferritic superalloys with a disordered body-centered-cubic (BCC) matrix and ordered B2 and/or L21 precipitates. These alloys offer good creep resistance at high temperatures and relatively low densities. Despite this progress, these precipitation-strengthened ferritic alloys suffer from low high-temperature strength and are not lightweight enough for many applications. High-entropy alloys (HEAs), also known as multi-principal-element alloys (MPEAs), offer a revolutionary approach to alloy design, employing multiple principal components (≥5) instead of one or two key components. The presence of multiple elements with different atomic sizes creates severe local lattice distortion, impeding dislocation motion and enhancing strength. To further improve strength, the formation of coherent intermetallic precipitates within a medium- to high-entropy matrix has been explored. HEAs can exhibit low lattice misfit, reducing the nucleation barrier for precipitation and stabilizing precipitates with high number density. This suggests HEAs as a promising avenue for developing lightweight, low-cost, high-temperature materials. The ideal material would utilize inexpensive raw materials, possess low density and high melting temperature (Tm), exhibit good oxidation and creep resistance, and achieve high strength with acceptable ductility. Careful composition selection is crucial to balance these factors. Previous research identified potential L21 precipitation-strengthened lightweight HEAs (LWHEAs) in the Al-Cr-Fe-Mn-Ti system; however, the formation of the brittle C14 Laves phase proved detrimental. The vast compositional space presents a significant challenge for efficient screening of suitable compositions. Experimental trial-and-error is impractical due to its cost and time consumption. Computational high-throughput screening offers a more efficient alternative. This study uses a CALPHAD-based high-throughput computational tool to explore the Al-Cr-Fe-Mn-Ti system for new precipitation-strengthened HEAs. Experimental and theoretical studies on the discovered LWHEAs provide insights into developing high-performance HEAs using this method.

The literature review section extensively cites previous research on ferritic superalloys, highlighting their advantages (creep resistance, low density) and limitations (low high-temperature strength). It discusses the emergence of high-entropy alloys (HEAs) as a novel alloy design strategy and their unique properties stemming from severe lattice distortion and coherent precipitate formation. Several key papers are referenced, showcasing successful examples of HEA design for high-temperature applications and challenges encountered, such as the formation of detrimental phases. The review sets the stage for this study by emphasizing the need for efficient computational methods to accelerate the discovery of optimal HEA compositions, particularly in complex multicomponent systems.

This study employed a CALPHAD (Calculation of PHAse Diagrams)-based high-throughput computational (HTC) method to screen for optimal LWHEA compositions within the Al-Cr-Fe-Mn-Ti system. The HTC process involved three stages. First, solidification HTCs were performed on 3246 alloys, using a 5 atomic percent (at.%) step size for each component. Alloys meeting criteria of Tm > 1250 °C and a single BCC phase at Tm were selected (1168 alloys). Second, OD point HTCs were conducted at 0.8Tm, selecting alloys meeting criteria of f(BCC) + f(L21) = 1 and 0.05 < f(L21) < 0.5 (44 alloys). Third, OD point HTCs were performed at 0.5Tm, using the same criteria, finally identifying eight alloys with densities ≤ 6.5 g/cm³ (Area 3). These eight LWHEAs were then experimentally investigated to validate the CALPHAD predictions and determine their microstructures and mechanical properties. Characterisation techniques included synchrotron X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), atom probe tomography (APT), and compression testing at room and elevated temperatures. In situ neutron diffraction was used to study deformation mechanisms and order-disorder transitions. Metropolis Monte Carlo (MC) calculations, using nearest-neighbor interaction parameters derived from the Aflow database, were performed to model order-disorder transitions. Ab initio molecular dynamics (AIMD) simulations were conducted to study the atomic structure and bonding in the liquid state of Alloy 1. The AIMD employed the Vienna Ab Initio Simulation Package (VASP) with PAW potentials and the PBE exchange-correlation functional.

The HTC successfully identified eight LWHEAs from thousands of potential compositions. Experimental validation confirmed the presence of L21 and BCC phases in the as-cast state, with no other intermetallic phases. Two distinct L21 morphologies were observed: high-density nanoscale precipitates (Alloys 1 and 8) and ordered L21 antiphase domains (APDs) with a disordered BCC phase at the antiphase domain boundaries (APBs) (Alloys 2-7). APT analysis revealed compositional differences between the L21 and BCC phases, with L21 enriched in Al and Ti, and the BCC phase containing primarily Cr, Mn, and Fe. Room-temperature compression testing showed yield strengths ranging from 500 to 1642 MPa. High-temperature testing of Alloys 1 and 8 revealed superior yield strengths compared to other BCC alloys up to 700 °C, maintaining significant strength even at 700 °C. In situ neutron diffraction revealed load-transfer behavior in Alloy 1 (nanoscale precipitates), demonstrating precipitation strengthening, while Alloy 7 (APDs) showed synchronized response, indicating behavior as a single L21 phase. In situ neutron scattering and MC calculations showed that the order-disorder transition temperature (T0) of Alloy 1 (1607 °C) was much lower than Alloy 7 (1997 °C), consistent with the observed morphologies. AIMD simulations indicated strong Al-Fe and Cr-Fe pair correlations, contributing to the formation of L21 and BCC phases. The different L21 morphologies were linked to Al content: Al ≥ 25 at.% resulted in a single-phase L21, while Al < 25 at.% resulted in BCC + L21. A schematic diagram depicted atomic site occupancy and morphology evolution based on composition.

The study successfully demonstrates the integration of high-throughput computational screening, multiscale modeling, and experimental validation for the discovery of novel high-performance materials. The key findings address the research question by identifying specific LWHEA compositions exhibiting superior high-temperature strength. The results are significant because they highlight the effectiveness of the CALPHAD-based HTC method in efficiently exploring the vast compositional space of multicomponent alloys. The discovered LWHEAs outperform existing counterparts, demonstrating the potential of this design approach for developing advanced structural materials. The understanding of the relationship between chemical composition, order-disorder transitions, and microstructure is crucial for future alloy design. The identification of two distinct L21 morphologies provides valuable insights into strengthening mechanisms.

This research successfully used a CALPHAD-based high-throughput computational method to discover high-performance lightweight high-entropy alloys for elevated-temperature applications. The combination of high-throughput screening, multiscale modeling, and experimental validation proved to be highly efficient and effective in accelerating the discovery process. Future research should focus on further optimizing the compositions, exploring the addition of minor refractory elements to improve high-temperature performance, and expanding the approach to other multicomponent alloy systems.

The study focused on a specific composition range within the Al-Cr-Fe-Mn-Ti system. Extending the investigation to broader compositional spaces and other alloy systems is needed to fully explore the potential of this approach. While the MC simulations provided insights into order-disorder transitions, the nearest-neighbor interaction model is a simplification. More sophisticated models incorporating longer-range interactions and lattice distortions might offer greater accuracy. The experimental validation, while extensive, was conducted on a limited number of alloys. Larger-scale experiments could help to solidify these findings and provide a more comprehensive picture.