Functional Properties of High Entropy Alloys

The vibrational properties of several High Entropy Alloy (HEA) based metal hydrides are investigated by inelastic neutron scattering (INS). HEAs have recently emerged as a new type of materials with a wide range of intriguing properties and potential applications such as hydrogen storage. The special properties of HEAs are believed to originate from the disordered lattice and internal strain that is introduced from the differences in atomic radii. This makes HEA hydrides provide an intriguing situation for the local H coordination, of several different transition metals. INS spectra were collected on a series of HEA-based metal hydrides starting with TiVNbHx and subsequently adding Zr and Hf to increase the atomic size mismatch. A general feature of the spectra are the optical peaks centered around an energy loss of 150 meV that can be attributed to hydrogen vibrations in a tetrahedral environment. Upon the addition of Zr and Hf, a shoulder appears on the optical peak at lower energy transfers that after comparison with in silico calculated INS spectra is indicative of hydrogen also occupying octahedral sites in the structure.

Fundamental design strategies for advancing the development of high entropy alloys for thermo-mechanical application: A critical review

Introduction. The paper discusses the prospects for studying high-entropy alloys (HEA), metal materials with unique properties. The study of high-entropy alloys is an urgent area of research in connection with its properties, environmental sustainability, economic benefits and technological potential. HEA are of interest to researchers due to its stability, strength, corrosion resistance and other characteristics, which makes it promising for use in the aerospace industry, automotive, medicine and microelectronics. Thus, HEA research contributes to the development of new materials and technological progress, providing opportunities for creating innovative products and improving existing solutions. To effectively use the potential of high-entropy alloys, research is required in a number of areas. First, it is necessary to improve the production technology of such alloys and develop new methods for obtaining HEA with improved characteristics and reduced cost. Secondly, it is necessary to establish the basic principles of operation of high-entropy alloys and to study the mechanisms influencing its properties. It is also necessary to develop new alloys with specified properties and conduct experiments and computer simulations to optimize the characteristics of the alloys and determine the best compositions. The purpose of the work is to study developments in the field of high-entropy alloys and conduct a comparative analysis of published studies on improving the properties of high-entropy alloys. The research method is a review and analysis based on developments mainly for 2020-2024, which were carried out by domestic and foreign scientists. The paper discusses the prospects for the study of high-entropy alloys, materials with a wide range of applications in various industries. The paper presents the results of research, mainly for 2020-2024. The main properties of high-entropy alloys are described, such as high strength, corrosion resistance, fatigue properties, plasticity and deformability, thermal stability, electrical conductivity and magnetic properties, as well as the possibility of creating alloys with specified characteristics. The most common methods of changing the properties of alloys have been identified. The directions of further development of research in this area are considered. Results and discussion: a literature review shows that developments and research are carried out on all possible properties of HEA, but most of it is devoted to corrosion-resisting properties and thermal stability. Of the methods used in high-entropy alloys, the most common and universal can be considered the alloying of high-entropy alloys with other metals. Studies also confirm that alloying metals are selected depending on its characteristic properties. The number of scientific works also confirms the relevance of this topic and the need for its study. The authors noted that future studies on the fatigue properties of high-entropy alloys, as well as the properties of alloys under the influence of magnetic and electric fields are the most interesting.

In recent years, the ideal- properties (young’s modulus, yield strength, toughness) and advanced application potential of high-entropy alloys (HEAs) have attracted numerous researchers. However, due to their unique structure and multiple structural combinations, it is challenging to explore the impact of various factors on their mechanical performance solely through experiments. This study considers the concentrations of five high-entropy alloy atoms and working temperature as input parameters. Molecular dynamics (MD) simulations and machine learning (ML) algorithms are employed to predict the tensile properties of FeNiCrCoCu HEAs, including Young’s modulus (E) and toughness (uT). A dataset of 1000 HEAs is generated through MD simulations, and feature selection is conducted using principal component analysis and Spearman correlation analysis. XGBoost, RF, DT, LGBoost, and AdaBoost are utilized to predict the mechanical properties of HEAs, comparing the impact of the two feature selection methods on prediction outcomes. During ML model training, 10-fold cross-validation and grid search are employed to obtain the best models and parameters. Root mean squard error (RMSE), coefficient of determination (R 2 ), mean absolute error (MAE) and relative absolute error (RAE) are used as evaluation metrics. Results indicate that Spearman correlation analysis for feature selection outperforms principal component analysis, and XGBoost demonstrates superior predictive performance for the mechanical properties of HEAs compared to other models. Predictions for E are more accurate than those for uT, with R 2 exceeding 0.9 for four out of the five ML models. This work may provide a new feature selection method for studying the mechanical properties of HEAs through ML. In the future, this method can be applied to other research areas of HEAs compositions, providing theoretical support for experiments. It can then be further applied to critical fields such as biomedical and aerospace industries.

Physical Properties of High Entropy Alloys

In this study the effect of three different nickel concentration on the microstructure, hardness and corrosion properties of high entropy alloys (HEAs) from AlCrFeCoNi system as an alternative material for medical instruments fabrication was investigated. The analyzed HEAs were AlCrFeCoNix obtained by vacuum arc remelting from high purity raw materials and having nickel atomic ratio x = 1.0, 1.4 and 1.8. The microscopy examination revealed the dendritic morphology for the reference alloy (AlCrFeCoNi) and that the extent of the interdendritic areas increased with the concentration of nickel while Cr was more segregated in the interdendritic areas than in dendrites. Hardness values decreased as the percentage of nickel increased due to the dissolution of the precipitates in a nickel-rich matrix and consequently the formation of continuous solid solutions. The corrosion properties of the synthesized HEAs were evaluated using a potentiodynamic polarization method. The alloys were immersed in Simulated Body Fluid during one week and the corrosion parameters were recorded. The low corrosion rates, low corrosion currents and high polarization resistance attest the good stability of these HEAs in simulated biological environment indicating their possible use for surgical and dental instruments.

Atomistic simulation of local chemical order in NbTiZrMoV high entropy alloy based on a newly-developed interatomic potential

High entropy alloys (HEAs) are promising electromagnetic wave absorption (EMA) materials due to its designable crystal structure, variable electromagnetic properties, and excellent corrosion resistance. However, the impedance mismatch owing to the high electric and dielectric conductivity severely hinders the application of HEAs in the field of EMA. Herein, the lattice distortion of FeCoNiCu HEA is manipulated accurately by doping and annealing strategies to tailor the EMA properties. Significant lattice distortion is observed in the FeCoNiCuC0.37, which leads to a decrease in the electrical conductivity and the creation of abundant dipoles. Owing to the optimal impedance matching and boosted polarization loss, the FeCoNiCuC0.37 delivers a minimal reflection loss of −65.4 dB accompanied by an effective absorption bandwidth (EAB) of 6.81 GHz. After annealing at 200 °C, the EAB of the FeCoNiCuC0.37 is further increased to 7.99 GHz at 1.95 mm, which is better than that of most HEA‐based EMA absorbers reported so far. Moreover, it demonstrates excellent corrosion resistance owing to the more tortuous diffusion path of corrosive medium origin from lattice distortion. Thus, the study provides a new insight into designing high performance HEA‐based EMA materials with superior anti‐corrosion property by lattice distortion engineering.

The emergence of High Entropy Alloys (HEAs) in the world of materials has shifted the alloy design strategy based on a single principal element to the multi-principal elements where compositional space can cover almost the entire span of the higher dimensional phase diagrams. This approach can provide advanced materials with unique properties, including high strength with sufficient ductility and fracture toughness and excellent corrosion and wear resistance for a wide range of temperatures due to the concentrated alloying that cannot be obtained by traditional microalloying based on a single principal element. In addition, the alloy design approach provides new alloy systems in astronomical numbers with variety of microstructural attributes that can yield different properties, and hence conventional trial and error experimental methods for alloy development are redundant. With the help of high throughput experiments along with efficient computational tools, and artificial intelligence, mechanisms based mechanistic development of the multi-principal element alloys with tailored solid solution strengthening, stacking fault energy and microstructure is possible. The current review discusses the various design strategies based on multi-principal elements alloys in semblance with the desired mechanical properties dictated by the micro mechanisms associated with them to overcome the bottlenecks presented by the conventional approaches with possible breakthrough applications. The article will shed light on the current status as well as the future prospects of using these approaches to design novel HEAs.

The design approach and validation of single phase senary refractory high entropy alloys (HEAs) MoNbTaTiVW and HfNbTaTiVZr were presented in first part of this dissertation. The design approach was to combine phase diagram inspection of available binary and ternary systems and Calculation of Phase Diagrams (CALPHAD) prediction. Experiments using X-ray diffraction and scanning electron microscopy techniques verified single phase microstructure in body centered cubic lattice for both alloys. The observed elemental segregation agrees well with the solidification prediction using Scheil model. The lattice constant, density and microhardness were measured to be 0.3216 nm, 4.954 GPa and 11.70 g/cm3 for MoNbTaTiVW and 0.334 nm, 5.5 GPa and 9.36 g/cm3 for HfNbTaTiVZr. To elaborate the single-phase stability of HEAs, CrxMoNbTaVW was examined over a certain range of Cr content in the second part of this dissertation. The change in composition led to different BCC structures with different microstructures and physical properties. Microstructure characterizations were performed using X-ray diffraction and scanning electron microscopy. Chemical micro-segregation during solidification predicted using the Scheil model generally agrees with the experimental results. The lattice constant, density, and Vickers' micro-hardness of the high-entropy alloy samples in the as-cast state are measured and discussed. For CrxMoNbTaVW, x=2.0 case appears exceeding the upper limit of maintaining a single BCC phase HEA, determined by the XRD patterns. The elemental dependence of the mixing thermodynamic properties (entropy, enthalpy and Gibbs energy) in BCC phase in the senary system is analyzed. The calculated entropy of mixing and enthalpy of mixing for CrMoNbTaVW are 14.7 J/K/mol and −662.5 J/mol respectively. Phase predictions and characterizations on as-solidified septenary refractory high-entropy alloy, CrMoNbReTaVW, are presented in the third part of the dissertation. The simulated solidification process predicts a single body-centered-cubic (BCC) crystal structure with the tendency of compositional segregation. X-ray diffraction results confirm the “single-phase-like” BCC structure, while further experimental characterizations reveal the existence of multiple grains with significantly different compositions yet the same crystal structure and similar lattice. For better understanding of corrosion properties of high entropy alloys, the CALPHAD method was further used to simulate the Pourbaix diagram and the corrosion layer evolutions under equilibrium conditions for CoCrFeNi based HEAs in the last part of the dissertation. The oxidation layer pitting and forming potential were calculated and compared favorably with published experimental results on CoCrFeNi, CoCrFeNiCu and CoCrFeNiAl0.5 HEAs.

Research progress of high entropy alloy: Surface treatment improves friction and wear properties

Binary and ternary transition metal carbides are stable ceramic crystals with outstanding mechanical properties. In recent years, multi-component single-phase high entropy alloys (HEAs) enjoyed explosive growth due to many of their outstanding physical properties with its large and flexible composition space. Hence the composite between them can be advantageous in forming a new class of ceramic materials with combined superiority in their properties for many applications. On the basis of a systematic large-scale ab initio simulations using density functional theory that are specifically designed for their compositional variations, subtle differences in their structures, electronic and mechanical properties are revealed and discussed in detail. Fifteen supercell models with 512 atoms with equal composition of C and HEAs occupying the sub-lattices of the rock-slat structure were constructed. These models are fully optimized, and their properties carefully characterized, compared and contrasted. By applying the novel concept of total bond order density and its partial components, the partial bond order density, we revealed many subtle variations in their properties that have not been known before. This large database can play an important and valuable role in the design and synthesis of high entropy ceramic carbides.

High-entropy alloy: challenges and prospects

Additive manufacturing (AM) is a modern manufacturing technique that facilitates the production of components layer by layer from CAD files, with more recent developments in the field leading to the ability to create these components from metal. Laser powder bed fusion (LPBF) is one of the many techniques used to manufacture metallic components and has drawn significant attention for its ability to create parts with high degrees of complexity, exceptional strength-to-weight ratios and internal structures. However, parts produced by AM are documented to suffer from build defects such as porosity, which can negatively affect not only its mechanical properties but its corrosion resistance, particularly its resistance to pitting corrosion. Whilst the mechanical properties of components produced through metal AM have been well documented since the technology’s inception, there are significant knowledge gaps in understanding the corrosion behaviour of metals produced in this way. This thesis aims to expand upon the current understanding of this manufacturing method with a particular focus on its corrosion resistance. High Entropy Alloys (HEAs) are a class of advanced materials that differ from conventional alloys in composition. Traditional alloys usually consist of one or two principal elements with smaller amounts of additional elements to impart specific properties. In contrast, HEAs are characterised by the presence of multiple principal elements in roughly equal proportions. HEAs' complex and disordered structure can result in unique mechanical, thermal, and magnetic properties. HEAs have shown promise in exhibiting high strength, hardness, and corrosion resistance, making them attractive for various engineering applications. Studies of HEAs have been increasing over recent years; however, significant knowledge gaps are still associated with this classification of materials, especially concerning their corrosion resistance. This lack of knowledge is intensified when discussing the properties of these alloys when manufactured by AM methods. LPBF was used to produce parts in 316L with process induced porosity by manipulating the process parameters to investigate the effect density has on the corrosion resistance of AM parts. The corrosion resistance of these parts were compared to their wrought counterpart using potentiodynamic polarisation. It was observed that increasing the porosity in the AM parts resulted in poorer corrosion resistance, both by weaker performance across key metrics and a greater degree of unreliability. It was also found that the AM parts proved to have a greater corrosion resistance than the wrought material. However, the decreased consistency in this resistance is often cited as a barrier these components must overcome to supplace conventionally manufactured components.316L was also produced through induction casting as well as a schedule more representative of industry that consisted of a solution anneal at 1080 °C followed by water quenching followed by a cold rolling reduction by 70 %, and a final anneal at 900 °C. The microstructures and corrosion resistance of these were investigated using SEM-EDS, XRD and potentiodynamic polarisation, and whilst the corrosion resistance of the cold rolled sample had increased, it was less than expected due to the formation of detrimental chromium carbides. A Swansea University developed AlCrFeMnNi HEA was put through the same 3 manufacturing processes to investigate their effect on the microstructure and corrosion resistance. It was found that, unlike 316L, the HEA suffered less from pitting corrosion and more from a generalised corrosion attack. Very similar corrosion results were seen across the manufacturing methods; however, the cast sample was observed to have the most consistent display of corrosion resistance. Based on the pitting resistance equivalent number, which relates the amount of Cr and Mo by wt.% in a stainless steel to its corrosion resistance, it was theorised that the addition of Mo to this HEA could also increase its corrosion resistance. The results were inconclusive; however, better corrosion resistance was seen in the AM sample of the HEA with the addition than in the AM sample without.

High entropy alloys: Substituting for cobalt in cutting edge technology