Transition from High-Entropy to Conventional Alloys: Which Are Better?

High-entropy alloys (HEAs) are an emerging class of advanced structural alloys under extensive research; yet, the properties of the liquid states of these materials, which are relevant to their processing, have been far less explored. In this work, we utilize ab initio molecular dynamics simulations to investigate the melt properties of a representative HEA—the Cantor alloy—and its derivatives: CrMnFeCoNi, CrFeCoNi, and CrCoNi. The atomic dynamics of these melts at various temperatures are investigated, specifically to analyze their electronic and atomic structures, including charge transfer, pair distribution functions, and short-range order. Results are compared with existing information for the liquids of metallic glasses, which also typically contain multiple principal elements, but retain the amorphous state under moderate to fast cooling rates. The present results provide insights into the structural and bonding factors favoring solidification to single-phase solid solutions in HEAs.

Global understanding of deformation behavior in CoCrFeMnNi high entropy alloy under high-strain torsion deformation at a wide range of elevated temperatures

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.

Electrocoagulation (EC) is one of the most effective techniques in removing color and organic pollutants from wastewater. This study aims to use a new alloy system, so-called High Entropy Alloys (HEAs), which contains at least five principal elements with 5-35 at. %, as an electrode in the EC process. The well-studied equiatomic CrMnFeCoNi HEA (Cantor alloy) was used as an anode in the treatment of the synthetically prepared wastewater (congo red solution) with EC. In the EC study, COD and color removal were evaluated at different current densities (5-100 mA/cm2) for 15 min electrolysis period. The results showed that removal efficiency of above 80 % was obtained for both parameters of COD and color at the lowest current density of 5 mA/cm2. The optimum current density was determined to be 10 mA/cm2, and the COD and color removal efficiencies were found to be 84 and 99.4 %, respectively. It was shown that the Cantor alloy possesses an effective removal in the EC process.

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 principal objective of this work was to elucidate the effect of microstructural features on the intrinsic dislocation mechanisms in two FCC alloys. First alloy Al0.1CoCrFeNi was from a new class of material known as complex concentrated alloys, particularly high entropy alloys (HEA). The second was a conventional Al-Mg-Sc alloy in ultrafine-grained (UFG) condition. In the case of HEA, the lattice possess significant lattice strain due to the atomic size variation and cohesive energy differences. Moreover, both the lattice friction stress and the Peierls barrier height are significantly larger than the conventional FCC metals and alloys. The experimental evidences, so far, provide a distinctive identity to the nature and motion of dislocations in FCC HEA as compared to the conventional FCC metals and alloys. Hence, the thermally activated dislocation mechanisms and kinetics in HEA has been studied in detail. To achieve the aim of examining the dislocation kinetics, transient tests, both strain rate jump tests and stress relaxation tests, were conducted. Anomalous behavior in dislocation kinetics was observed. Surprisingly, a large rate sensitivity of the flow stress and low activation volume of dislocations were observed, which are unparalleled as compared to conventional CG FCC metals and alloys. The observed trend has been explained in terms of the lattice distortion and dislocation energy framework. As opposed to the constant dislocation line energy and Peierls potential energy (amplitude, ΔE) in conventional metals and alloys, both line energy and Peierls potential undergo continuous variation in the case of HEA. These energy fluctuations have greatly affected the dislocation mobility and can be distinctly noted from the activation volume of dislocations. The proposed hypothesis was tested by varying the grain size and also the test temperature. Activation volume of dislocations was a strong function of temperature and increased with temperature. And the reduction in grain size did not affect the dislocation mechanisms and kinetics. This further bolstered the hypothesis. The second part deals with deformation characteristics of Al-Mg-Sc alloy. The microstructure obtained from the severe plastic deformation (SPD) techniques differ in dislocation density, grain/cell size, and in the grain boundary character distribution. Therefore, it is vital to understand the deformation behavior of the UFG materials produced by various SPD techniques, as the microstructural features basically control the deformation mechanisms. In this study, a detailed analysis was made to understand the deformation mechanisms operative in various regimes of a stress-strain in UFG Al-Mg-Sc alloy produced via friction stir processing. The stress-strain curves exhibited serrations from the onset of yielding to the point of sample failure. The serration amplitude and frequency was higher in UFG material as compared to CG material. Furthermore, the microstructural features that result in the serrated flow were investigated along with the avalanche characteristics. The presence of both ultrafine grains and Al3Sc precipitates were the necessary conditions to reach the critical stress required to push the grain boundary into a critical state to set off an avalanche. The microstructural conditions that did not satisfy both the requirements did not exhibit deep serrations.

High-entropy alloys are a class of materials that contain five or more elements in near-equiatomic proportions1,2. Their unconventional compositions and chemical structures hold promise for achieving unprecedented combinations of mechanical properties3-8. Rational design of such alloys hinges on an understanding of the composition-structure-property relationships in a near-infinite compositional space9,10. Here we use atomic-resolution chemical mapping to reveal the element distribution of the widely studied face-centred cubic CrMnFeCoNi Cantor alloy2 and of a new face-centred cubic alloy, CrFeCoNiPd. In the Cantor alloy, the distribution of the five constituent elements is relatively random and uniform. By contrast, in the CrFeCoNiPd alloy, in which the palladium atoms have a markedly different atomic size and electronegativity from the other elements, the homogeneity decreases considerably; all five elements tend to show greater aggregation, with a wavelength of incipient concentration waves11,12 as small as 1 to 3nanometres. The resulting nanoscale alternating tensile and compressive strain fields lead to considerable resistance to dislocation glide. In situ transmission electron microscopy during straining experiments reveals massive dislocation cross-slip from the early stage of plastic deformation, resulting in strong dislocation interactions between multiple slip systems. These deformation mechanisms in the CrFeCoNiPd alloy, which differ markedly from those in the Cantor alloy and other face-centred cubic high-entropy alloys, are promoted by pronounced fluctuations in composition and an increase in stacking-fault energy, leading to higher yield strength without compromising strain hardening and tensile ductility. Mapping atomic-scale element distributions opens opportunities for understanding chemical structures and thus providing a basis for tuning composition and atomic configurations to obtain outstanding mechanical properties.

A comprehensive review on advances in high entropy alloys: Fabrication and surface modification methods, properties, applications, and future prospects

Displacement cascade and defect-driven simulations in V-Ti-Ta-Nb high-entropy alloy

High-entropy alloy: challenges and prospects

CONSPECTUS: The ultimate goal of solid state chemistry is to gain a clear correlation between atomic, defect, and electronic structure and intrinsic properties of solid state materials. Solid materials can generally be classified as amorphous, quasicrystalline, and crystalline based on their atomic arrangement, in which crystalline materials can be further divided into single crystals, microcrystals, and nanocrystals. Conventional solid state chemistry mainly focuses on studying single crystals and microcrystals, while recently nanocrystals have become a hot research topic in the field of solid state chemistry. As more and more nanocrystalline materials have been artificially fabricated, the solid state chemistry for studying those nanosolids has become a new subdiscipline: solid state nanochemistry. However, solid state nanochemistry, usually called "nanochemistry" for short, primarily studies the microstructures and macroscopic properties of a nanomaterial's aggregation states. Due to abundant microstructures in the aggregation states, it is only possible to build a simple but imprecise correlation between the microscopic morphology and the macroscopic properties of the nanostructures. Notably, atomically thin two-dimensional inorganic materials provide an ideal platform to establish clear structure-property relationships in the field of solid state nanochemistry, thanks to their homogeneous dispersion without the assistance of a capping ligand. In addition, their atomic structures including coordination number, bond length, and disorder degree of the examined atoms can be clearly disclosed by X-ray absorption fine structure spectroscopy. Also, their more exposed interior atoms would inevitably induce the formation of various defects, which would have a non-negligible effect on their physicochemical properties. Based on the obtained atomic and defect structural characteristics, density-functional calculations are performed to study their electronic structures. Then, after the properties of the individual ultrathin two-dimensional materials or their assembled highly oriented thin film-based nanodevices are measured, the explicit relationship between atomic, defect, and electronic structure and intrinsic properties could be established. In this Account, we focus on our recent advances in the field of solid state nanochemistry, including atomic structure characterization of ultrathin two-dimensional inorganic materials by X-ray absorption fine structure spectroscopy, characterization of their different types of structural defects by positron annihilation spectra and electron spin resonance, and investigation of their electronic structure by density-functional calculations. In addition, we summarize the close correlation between atomic, defect, and electronic structure variations and the optoelectronic, electrical, magnetic, and thermal properties of ultrathin two-dimensional materials. Finally, we also propose the major challenges and opportunities that face solid state nanochemistry. We believe that all the past achievements in ultrathin two-dimensional materials could bring new opportunities for solid state nanochemistry.

Solution hardening and softening in niobium-molybdenum and niobium-rhenium alloys

High entropy alloys (HEAs) consisting of five or more principle elements are known as an emerging novel material because of superior properties resulting from the alloy effects of mixing elements. Several HEAs systems have been developed by adjusting the compositions. Among them, an equiatomic Fe-Cr-Ni-Co-Mn alloy (also known as the Cantor alloy) is a single FCC structure and has good resistance against brittleness at low temperatures. Several HEAs studies have detailed the microstructure, processing, and related mechanical properties. However, the materials scales only limit in the laboratory. The knowledge of microstructure and corrosion resistance of the Fe-Cr-Ni-Co-Mn alloy for industry application scales is still absent nowadays. In this study, a 50-kg-bulk Fe-Cr-Ni-Co-Mn alloy was melted and casted. The alloy was homogenized, forged, and rolled. The as-cast bulk and deformed bulk alloys had different microstructures. The as-cast ingot with a diameter of approximately 120 mm was sliced using wire electrical discharge machining. The as-cut disk showed brown color on the rim of the disk and dark len-shaped in the middle. The XRD results showed different patterns from the distinct regions. The microstructure and the related corrosion properties of the specific was compared. The effect of segregation of the as-cast bulk was re-constructed with the related corrosion resistance. The in-situ corrosion observation system was also built and employed to study the corrosion behavior of the Fe-Cr-Ni-Co-Mn alloy in comparison to 316 stainless steel.Key Words: high-entropy alloys, Fe-Cr-Ni-Co-Mn alloys, corrosion, segregation, as-casted

Because of the high entropy effect, a simple solid solution structure can be obtained in high entropy alloys (HEAs), giving the HEAs greater overall properties than conventional alloys. Cladding layers of FeCoCrNiMnTix HEAs were prepared on Q355 steel by laser cladding technology. X-ray diffraction (XRD), metallographic microscope (OM), and scanning electron microscope (SEM) were used to examine the microstructure of the cladding layers. The microhardness and wear resistance were tested using a Vickers microhardness tester and wear testing machine, respectively. The FeCoCrNiMn HEAs consisted of a face-centered cubic (FCC) solid solution. The addition of Ti promoted the formation of body-centered cubic (BCC) and hexagonal solid solution. When Ti content was higher than x=0.8, the Laves phase formed. The addition of Ti increased the microhardness of the FeCoCrNiMnTix (x=0.5, 0.6, 0.8, 1.0) HEAs, which is due to the effect of solution strengthening, microstructure strengthening, and grain refinement. When Ti content was less than x=0.6, the alloy’s wear resistance was positively correlated with its hardness, and the Ti0.6 alloy had the best wear resistance. However, as Ti content exceeded x=0.6, the brittle and hard Laves phase formed, which resulted in reduced wear resistance.

A geometric model for intrinsic residual strain and phase stability in high entropy alloys