Solid Solution High-entropy Alloys Research Articles

High temperature deformation in fine grained high entropy alloys

There is considerable interest currently in developing and understanding microstructural evolution, stability and deformation in the new class of highly concentrated, multi-principal element solid solution high entropy alloys. This overview provides a brief description of potential high temperature deformation mechanisms in such fine grained alloys, discusses the significance of diffusion, and compares the available experimental results with appropriate theoretical models. It is shown that diffusion is not substantially slower in such alloys, and that superplastic deformation follows the standard model established for conventional polycrystals. The data also suggest that deformation at higher stresses occurs by dislocation glide creep, followed by a power-law breakdown regime. It is necessary to exercise caution in interpreting spherical nanoindentation creep data, although some data on a nanocrystalline HEA suggest that deformation at room temperature occurs by Coble diffusion creep. Based on the available data and understanding, a deformation mechanism map is developed highlighting the dominance over different regimes of grain size and stress of Coble diffusion creep, superplastic flow, dislocation creep and power-law breakdown.

Elemental segregation in solid-solution high-entropy alloys: Experiments and modeling

Recent experiments showed that elemental segregation occurs in some high entropy alloys (HEAs) although their X-ray diffraction patterns exhibit a nominal single-phase solid solution structure. Based on the 3-D atom probe tomography (APT) results obtained from the FeNiCoCrCu HEA, here we propose a simple thermodynamics model based on the notion that elemental segregation facilitates stabilizing the solid solution phase with a minimized Gibbs free energy. Our model enables distinguishing nominal single-phased solid-solution HEAs with and without elemental segregation. The predictions of our model are found in good agreement with the experimental data reported hitherto in the literature.

Exploration and Development of High Entropy Alloys for Structural Applications

We develop a strategy to design and evaluate high-entropy alloys (HEAs) for structural use in the transportation and energy industries. We give HEA goal properties for low (≤150 °C), medium (≤450 °C) and high (≥1,100 °C) use temperatures. A systematic design approach uses palettes of elements chosen to meet target properties of each HEA family and gives methods to build HEAs from these palettes. We show that intermetallic phases are consistent with HEA definitions, and the strategy developed here includes both single-phase, solid solution HEAs and HEAs with intentional addition of a 2nd phase for particulate hardening. A thermodynamic estimate of the effectiveness of configurational entropy to suppress or delay compound formation is given. A 3-stage approach is given to systematically screen and evaluate a vast number of HEAs by integrating high-throughput computations and experiments. CALPHAD methods are used to predict phase equilibria, and high-throughput experiments on materials libraries with controlled composition and microstructure gradients are suggested. Much of this evaluation can be done now, but key components (materials libraries with microstructure gradients and high-throughput tensile testing) are currently missing. Suggestions for future HEA efforts are given.