High-entropy alloy: challenges and prospects

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.

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.

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.

Presently, HETAs are of enormous research significance in material engineering and sciences. In contrast to the traditional alloys with one and occasionally two base constituents, high entropy alloys consist of many essential elements, which are considered in excessive number compared to the conventional alloys. The arrival HETAs ignites some basic questions that challenge the hypothesis, mock-up, and preparation techniques of the traditional alloys. Here, the review of the latest studies on the prospect and essential subjects related to HETAs were provided. Besides, the novelty in the characteristics of high entropy alloys were also examined, which includes: high wear and corrosion resistance, outstanding mechanical behaviour at high temperature, superb service strength, and ductility, exceptional fracture toughness at extremely low or cryogenic temperatures, super-conductivity and super-paramagnetism. As a result of their richness of design, great structural and operational potential, they are now seen as promising materials for novel and advance applications, which should be further investigated.

High-entropy alloys (HEAs), although often presumed to be random solid solutions, have recently been shown to display nanometer-scale variations in the arrangements of their multiple chemical elements. Here, we study the effects of this compositional heterogeneity in HEAs on their mechanical properties using in situ compression testing in the transmission electron microscope (TEM), combined with molecular dynamics simulations. We report an anomalous size effect on the yield strength in HEAs, arising from such compositional heterogeneity. By progressively reducing the sample size, HEAs initially display the classical “smaller-is-stronger” phenomenon, similar to pure metals and conventional alloys. However, as the sample size is decreased below a critical characteristic length (~180 nm), influenced by the size-scale of compositional heterogeneity, a transition from homogeneous deformation to a heterogeneous distribution of planar slip is observed, coupled with an anomalous “smaller-is-weaker” size effect. Atomic-scale computational modeling shows these observations arise due to compositional fluctuations over a few nanometers. These results demonstrate the efficacy of influencing mechanical properties in HEAs through control of local compositional variations at the nanoscale.

The phase selection via machine learning in high entropy alloys

High entropy alloys (HEAs) are mostly investigated for their mechanical and structural properties from the beginning of their discovery. However, the interest in developing high entropy functional materials is growing in recent times, with various fields such as magnetic, magnetocaloric, hydrogen storage, corrosion resistance, thermoelectric, superconducting behaviors, irradiation, catalytic, and biomedical properties being explored. These properties can be correlated to the distinctive structural properties of HEAs and their core properties, like lattice distortion and entropy stabilized multicomponent solid solution formation. This is a timely review of the development of high entropy alloys with some of the aforementioned functional properties which are discussed and reviewed in this chapter. Comparisons of functional properties of high entropy alloys with conventional alloys are provided to help understand the current and future prospects of this field.

Physical Properties of High Entropy Alloys

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.

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.

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.

High-entropy alloys differ from conventional alloys by incorporating multiple principal elements in near-equiatomic ratios, enabling an immense number of potential alloys, of which several have demonstrated outstanding material properties. Laser powder bed fusion offers unmatched design freedom for fabricating intricate components, although the limited portfolio of printable alloys is holding it back. Employing high-entropy alloys in laser powder bed fusion is a promising avenue to address this issue, although the large number of potential alloy systems presents challenges as experimentally based trial-and-error strategies are impractical in terms of the vast amount of resources required. Molecular dynamics simulations—atomistic simulation techniques that employ highly accurate interatomic potentials to describe the interactive forces between constituent atoms—have recently surged in modeling the properties of high-entropy alloys. Notwithstanding, research employing molecular dynamics simulations to guide the design of high-entropy alloys for laser powder bed fusion remains scarce. As such, this Ph.D. thesis aims to leverage molecular dynamics simulations to develop novel high-entropy alloys for laser powder bed fusion. Firstly, the correlation between generalized stacking fault energy and the strengthening mechanisms of Cr–Fe–Co–Ni high-entropy alloys during uniaxial tensile deformation is established using molecular dynamics simulations. Increasing the Cr and Fe contents reduces the generalized stacking fault energy values, while an opposite trend is observed for Co and Ni. A linear relationship is identified between unstable stacking fault energy and yield stress, resulting in alloys with low Fe and high Co and Ni contents exhibiting higher unstable stacking fault energy and, consequently, higher yield stress. A high density of Lomer–Cottrell locks or deformation twins results in higher flow stress, with deformation twins playing a more critical role in enhancing flow stress than sessile dislocations. Additionally, high unstable stacking fault energy stabilizes defects and contributes to increased flow stress, whereas low unstable stacking fault energy allows plastic deformation to progress at lower stress, leading to bidirectional defect transformation. The results are in agreement with recent experiments, and they serve as guidelines for enhancing yield strength and efficiency of strengthening mechanisms in Cr–Fe–Co–Ni high-entropy alloys by altering their composition. Secondly, molecular dynamics simulations establish the correlation between chemical composition and short-range ordering in Cr–Co–Ni high-entropy alloys. The results demonstrate that atomic-scale clustering can be tailored by modifying the chemical composition, particularly by reducing the proportion of elements that do not participate in the formation of the clusters. Short-range ordering significantly influences dislocation evolution and tensile properties in the studied alloys. Reducing the tendency of Cr–Co clustering enhances strengthening by increasing local unstable stacking fault energy. In contrast, promoting Ni–Ni clustering enhances strain hardening and impedes dislocation motion. These findings highlight the crucial role of chemical composition in short-range ordering and provide significant insights into designing high-performance high-entropy alloys. Thirdly, a combination of molecular dynamics, calculation of phase diagrams, and Scheil–Gulliver simulations guides the design of Ni50–xFe25Co25Cux high-entropy alloys for laser powder bed fusion. The atomistic simulations indicate that the composition is prone to form atomic-scale Cu clusters as a potential strengthening mechanism. Moreover, the printability is predicted to be enhanced by increasing the Ni content at the expense of Cu. Based on the design guidelines, crack-free samples were printed in a high-throughput manner by employing in situ–mixed elemental powder to circumvent the need for costly gas-atomized powder. The as-printed samples demonstrated superior tensile properties compared to conventionally fabricated Fe–Co–Ni–Cu alloys, and the atomistic simulations suggest that the trend of increasing yield strength with higher Cu content stems from short-range ordering at the grain boundaries, which increases the frictional strengthening. This thesis presents the development of a comprehensive computational framework to accelerate the design of novel high-entropy alloys for laser powder bed fusion. By combining molecular dynamics and thermodynamic simulations, the laser powder bed fusion process of in situ–mixed elemental powders is optimized to design high-entropy alloys with enhanced mechanical properties and high printability. This research advances the field by providing strategies to map the vast compositional space and expand the portfolio of printable alloys, bringing laser powder bed fusion–fabricated high-entropy alloys closer to industrial adoption.

Sintering of High Entropy Alloys: Processing and Properties

Ultrahigh cryogenic strength and exceptional ductility in ultrafine-grained CoCrFeMnNi high-entropy alloy with fully recrystallized structure

High entropy alloys: Substituting for cobalt in cutting edge technology