Valence Electron Concentration Research Articles

Theoretical study on the lattice distortion and electronic structure in the Al–Co–Cr–Fe–Ni multicomponent alloys

At present, the phase structure design for multicomponent alloys (MCAs) is mainly based on the valence electron concentration (VEC). In this work, based on the classification of valence electrons by the empirical electron theory of solid and molecule (EET), the VEC is further dissected and found that the covalent electron concentration (CEC) can be used as a criterion for determining the phase structure of MCAs. The Alx(CoCrFeNi)1-x (x = 0, 1/24, 1/12 and 1/8) alloys are designed on the basis of the CEC. The effect of Al content on the lattice distortion in the FCC phase of the alloys by combining density functional theory (DFT) and EET. The results show that the degree of lattice distortion in the FCC phase gradually increases with the increase of Al content and the interatomic bonding decreases before the phase structure transition. The alloy reaches the criticality of the phase structure transition when the Al content is 9.48 at.%.

Research on the microstructure and properties of metastable β type Ti-8V4Mo3Cr3Zr3Al alloy with high strength and toughness

In this paper, a new type of Ti-8V-4Mo-3Cr-3Zr-3Al metastable β-type titanium alloy is designed based on alloy design parameters such as valence electron concentration (VEC), Bo, and Md, and combines them with the empirical criterion of molybdenum equivalent fractionation. Optimize the microstructure of the alloy through processes such as cold rolling, annealing, and aging treatment to obtain good mechanical properties. The microstructure of cold rolling and post rolling heat treatment was observed and analyzed using optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD), and transmission electron microscopy (TEM), and the tensile properties of the alloy were tested. The characteristics of the alloy in terms of microstructure and properties were summarized and analyzed. The results show that the cold formability of the alloy after solid solution treatment is good, with a cold rolling reduction of over 85 %, and a large number of deformation twins generated during the cold rolling process. The yield strength after annealing and recrystallization is up to 1160 MPa, and elongation is 18.9 %. The final performance of the aged alloy is 1510 MPa for yield strength and 5 % for elongation.

Optimizing toughness in high entropy alloys using a genetic algorithm: A combined computational and experimental approach

A genetic algorithm (GA) was developed to search for high entropy alloys (HEAs) with good combinations of mechanical properties. The algorithm was designed find single-phase face-centered cubic (FCC) HEAs, already known for their ductility, with high Hall-Petch constants (K) and high critical resolved shear stresses (τY). The objective was to develop a methodology that allows the design of HEAs in a multi-objective environment, focusing on alloys that exhibit enhanced ductility and strength, achieved through an increase in its yield point without significant loss of its ultimate deformation via adjustments of K and τY values, resulting in an alloy with high toughness. The most promising alloy suggested by the genetic algorithm was an unconventional composition (Co32.73Cu15.11Fe0.72Hf0.72Mn35.97Mo3.96Ni10.43Sn0.36), with eight different elements in non-equiatomic ratios with maximized K and τY values. This alloy was then experimentally produced and characterized by scanning electron microscopy (SEM), synchrotron x-ray diffraction (XRD), transmission electron microscopy (TEM) and microhardness, with the objective of validating the predictions made by the GA. An advantage of the proposed method is the possibility of more systematically identifying and exploring new compositions in the complex composition space characteristic of these multicomponent alloys, as it directs its search towards domains that can be used to solve challenging problems involving multi-objective optimization. However, the thermodynamic parameters used for single-phase FCC prediction, namely the valence electron concentration (VEC) and the dimensionless thermodynamic parameter φ, exhibited limitations. These limitations were further explored in the present study, as evidenced by the fact that the microstructure of the selected alloy was not single-phase, which hindered the study of the mechanical properties, predicted exclusively for monophasic FCC alloys. Therefore, this work highlights the necessity for the development of novel thermodynamic equations for phase prediction, or even the integration of the genetic algorithm with other methodologies, such as CALPHAD calculations, to achieve enhanced results.

Tailoring ZrWNb-Based Refractory High Entropy Alloys: A Multidirectional Characterization Through Elemental, Physical, Thermodynamic, and Nuclear Radiation Attenuation Properties

This study explores the development and characterization of 18 novel ZrWNb[(X)(Y)] (where XY are Hf, Ta, Mo, V, Ti) refractory high entropy alloys (RHEAs) designed to enhance mechanical robustness, thermodynamic stability, and radiation shielding effectiveness. Through a rigorous analysis, we evaluated the elastic modulus, entropy, and enthalpy of mixing, atomic size difference, valence electron concentration, and the omega parameter to understand the alloys’ phase behavior and structural integrity. Our findings revealed a broad range of elastic modulus values, indicating diverse mechanical adaptability suitable for high-stress applications. The thermodynamic assessment highlighted several RHEAs with favorable phase stability, particularly ZrWNbTaHf and ZrWNbTiTaHf, which are poised for high-performance usage due to their balanced mixing entropy and enthalpy. This study also benchmarks the RHEAs against conventional alloys, with sample R1 exhibiting the lowest fast neutron effective removal cross section, underscoring its superior neutron shielding capabilities. Additionally, R1 demonstrated exceptional photon attenuation, as evidenced by its competitive half-value layer performance across an extensive energy spectrum. Collectively, these results position R1 as a standout candidate, offering a significant advancement in materials science for applications demanding stringent radiation shielding, such as in nuclear reactor environments and space exploration.

Optimizing strength-ductility synergy of the solid-solution Ti50–ZrVNbCr MEAs via coordinately adjusting VEC and lattice constant

In this study, a series of Ti50–ZrVNbCr alloys were designed under with the constant atomic radius mismatch. It was suggested that valence electron concentration (VEC) and lattice constant, which evolve oppositely in the series of the Ti50–ZrVNbCr alloys, jointly influence the mechanical properties. Based on modeling, the evolution of ductility and yield strength of the designed alloys was predicted. Ductility was predicted to be superior in the VEC range from 4.22 to 4.25, with the lattice constant correspondently ranging from 3.340 Å to 3.334 Å. The optimization of strength-ductility synergy was discussed in terms of coordinately adjusting VEC and lattice constant. The experimental results were quite consistent with the prediction results, showing that the alloy with the VEC value of 4.25 and the lattice constant of 3.334 Å, which consisted of 50 at.%Ti, 28.1 at.%Zr, 9.4 at.%V, 9.4 at.%Nb, and 3.1 at.%Cr, possessed the best ductility and superior strength-ductility synergy.

Exploring thermodynamic, physical and radiative interaction properties of quinary FeNiCoCr high entropy alloys (HEAs): a multi-directional characterization study

To qualify for nuclear applications, materials must meet specific criteria, including mechanical properties, high-temperature behavior, corrosion resistance, and high-temperature oxidation resistance. High Entropy Alloys (HEAs) are particularly suitable for these applications due to their unique properties. Consequently, we conducted a theoretical and simulation-based approaches to assess some critical properties including radiation shielding properties of some quinary FeNiCoCr HEAs. In this study, we focused on quinary FeNiCoCr HEAs, whose corrosion properties have been previously examined in the literature. We investigated the thermodynamic and radiation shielding properties of HEAs with sixteen different compositions. Our methodology included evaluating thermodynamic parameters such as Mixing Entropy (∆Smix) and Mixing Enthalpy (∆Hmix), as well as structural characteristics like Valence Electron Concentration (VEC) and Atomic Size Difference (δ). This allowed us to systematically deduce the phase behavior and stability of various HEAs. Through computational modeling, we assessed the radiation shielding capabilities of these alloys, particularly their effectiveness in attenuating gamma ray and fast neutrons. The results identified FeNiCoCrW as the alloy with the lowest fast neutron removal cross-section values, highlighting its potential for nuclear applications. Its high melting point and the synergistic interplay between its elemental composition and thermodynamic properties suggest broad applicability in extreme environments. Thus, FeNiCoCrW emerges as a promising HEA with multifunctional capabilities, warranting further exploration and potential integration into advanced engineering solutions.

Prediction of the optimal hydrogen storage in high entropy alloys

Hydrogen is the most abundant element in nature, characterized by its vast resources, recyclability, and environmentally friendly. Also, it can exist in various forms, such as gas, liquid, or solid, to meet different storage needs. In solid-state storage, the hydrogen storage alloys have received widespread attention due to hydrogen is stored in the form of hydrides. In recent years, high entropy alloys (HEAs), as a new type of novel alloy design concept have attracted a lot of attention in many fields including hydrogen storage due to their unique structures and excellent properties. Moreover, the freedom of element selection leads to various types of HEAs, further enhancing the potential for their application in hydrogen storage. This article focuses on the hydrogen storage performance of HEAs and provides a comprehensive overview of the hydrogen storage mechanism, thermodynamics, and kinetics. Additionally, we summarize and compare the hydrogen storage capacities of previously published HEAs and introduce two empirical parameters, valence electron concentration (VEC) and atomic size difference (δ), and their relationship with hydrogen storage capacity. By applying the K-means clustering algorithm, we conclude that the optimal hydrogen storage capacity of HEAs occurs when 4 <VEC < 6 and 5.3 % <δ < 7.5 %. This finding offers valuable guidance for the future design of efficient hydrogen storage HEAs.

Design, preparation and properties of biomedical Ti-Nb quaternary alloys with high strength and low modulus: insights from first-principles calculations

In this paper, we designed and developed biomedical β-type Ti-Nb quaternary alloy materials with high strength and low modulus. Building upon the d-electron alloy design method and the valence electron concentration e/a method, we use a Python program and thermodynamic theoretical parameters to determine a Ti-Nb quaternary alloy with the characteristics of a single-phase solid solution. Subsequently, we engage in theoretical and experimental investigations on the low modulus alloys identified through first-principles calculations. The results show that both Ti7Nb6Zr1Al2 and Ti11Nb2Ta2Al1 maintain a single β phase before and after cold rolling, which is related to the strong electronic density of states of s and p orbitals at low energy levels in the crystal structure of the two alloys. Ti11Nb2Ta2Al1 has a strong charge distribution on the (001) crystal plane, which causes a large amount of internal stress under large deformation conditions to promote grain crushing and weaken the intensity of the (001)[1-10] texture in the ODF diagram of 2 = 45°. The cold-rolled Ti11Nb2Ta2Al1 exhibits better corrosion resistance and biocompatibility than the cold-rolled Ti7Nb6Zr1Al2, and the good biocompatibility is related to the strong covalent bond between Al and Ta that inhibits the diffusion of Al ions. 90% cold-rolled Ti11Nb2Ta2Al1 stands out as an exceptionally promising orthopedic implant material due to its high elastic allowable strain (ReH/E), good corrosion resistance and biocompatibility.

Revealing the fundamental relationship between the properties and microstructures of biomedical TiZrTaNbMo high entropy alloy

The body-centered cubic high-entropy alloys are a promising candidate as implanting materials for biomedical areas, but their severe strength-ductility trade-off limits their application. Herein, we designed and prepared the biomedical TiZrTaNbMo HEAs with different atomic ratios by adjusting their elemental composition based on the theoretical calculation of negative mixing enthalpy and valence electron concentration. It was found that the (TiZr)47(TaNbMo)2 alloy has BCC and HCP mixing structure, and the (TiZr)42.5(TaNbMo)5 and the (TiZr)45(TaNbMo)3.33 have simple BCC structure. the yield strength (YS) and total tensile elongation of the (TiZr)42.5(TaNbMo)5, (TiZr)45(TaNbMo)3.33, (TiZr)47(TaNbMo)2 are 866 ± 28 MPa and 16.5 ± 1.1 %, 929 ± 13 MPa and 17.8 ± 1.2 %, 899 ± 6 MPa and 5.5 ± 0.1 %, respectively. The theoretical calculation shows that the high YS strength mainly comes from the solid solution strengthening effect, the high ductility can be explained by the negative mixing enthalpy induced by Mo element. In addition, the (TiZr)45(TaNbMo)3.33 alloy has better wear resistance and corrosion resistance than the commercially available Ti6Al4V. This study shed light on synthesizing the high entropy alloy with the desired comprehensive properties as biomedical implant materials.

Microstructure evolution and catalytic activity of AlCo1−xFeNiTiMox high entropy alloys fabricated by powder metallurgy route

This investigation presents the synthesis of equiatomic and non-equiatomic AlCo1−xFeNiTiMox (x = 0, 0.1, 0.25 and 1.0) high entropy alloys fabricated by mechanical alloying. Mo partially replaced Co. Classic thermodynamic calculations, such as mixing enthalpy (ΔHmix), configurational entropy (ΔSmix), the atomic size difference (δ), entropy to enthalpy ratio (Ω), electronegativity difference (△χ), and valence electron concentration (VEC) were used. Considering δ, Ω and VEC parameters, a BCC solid solution and an intermetallic phase can be predicted due to the partial replacement of Co by Mo. X-ray and electron diffraction of equiatomic HEA without Mo content revealed that after 35 h of milling, a Fe-type BCC lattice phase was formed in the alloy and two L21 phases, in addition to a minimal amount of FCC phase. As the Mo content increased, the Fe-type BCC phase was steadily replaced by the Mo-type BCC phase and the Fe-type FCC phase, and two L21 phases were also developed. When the 5 at% Mo-containing (x = 0.25) alloy was further milled for 80 h, the amount of phases remained almost the same; only the grain size was strongly reduced. The influence of the Mo addition on the properties of studied alloys was also confirmed in the decolourisation of Rhodamine B using a modified photo-Fenton process. The decolourisation efficiency within 20 min was 72% for AlCoFeNiTi and 87% for AlCo0.75FeNiTiMo0.25 using UV light with 365 nm wavelength.

Impact of Hf alloying on the functional properties of Ni-Mn-Ga high temperature shape memory alloys

The paper reports on the functional properties and microstructure of polycrystalline Ni-Mn-Ga-Hf high temperature shape memory alloys, in which different amounts of Hf up to 4 at.% are added in substitution of Mn and Ga, while the nominal Ni content is kept constant. The increase in the amount of Hf promotes a decrease in the valence electron concentration (e/a), tetragonality (c/a) of the martensitic unit cell, and martensitic transformation temperatures, as well as a significant decrease of the transformation hysteresis. The non-modulated tetragonal martensite, typical of Ni-Mn-Ga high temperature shape memory alloys, is formed in all the alloys studied here, though the alloy with 4 at.% of Hf also contains a small fraction of modulated 14M martensite. Hf addition improves the thermomechanical response of the alloys under compressive stress up to 300 MPa and leads to nearly closed cycles with minimal irrecoverable strain and low hysteresis (8 K) for the alloys with 3 and 4 at.% of Hf. These alloys also demonstrate excellent stability under repetitive thermal cycling and little change upon aging at 870 K. A second phase rich in Hf and Ni starts to precipitate at 1 at.% of Hf addition and its volume fraction experiences an abrupt and progressive increase for higher Hf contents. The structure of the second phase looks like the usual f.c.c. γ phase reported in other Ni-Mn-Ga-based alloys, but it has a double lattice parameter. Two structural models based on the A6 face-centered tetragonal unit cell with space group I4/mmm (No. 139), equivalent to the double f.c.c. lattice, are proposed for this new phase.

Tribological Behavior and In‐Vitro Biocompatibility of a Newly Designed TiZrNbMoTa High‐Entropy Alloy

A novel Ti35Zr15Nb25Mo15Ta10 high‐entropy alloy has been designed, fabricated, and characterized as a viable substitute for the conventional Ti‐6Al‐4V alloy in biomaterial applications. Alloy design is conducted based on various parameters including mixing enthalpy (ΔHmix), omega parameter (Ω), delta parameter (δ), valence electron concentration (VEC), and biocompatibility aspects of its constituent elements with CALPHAD approach. The fabricated Ti35Zr15Nb25Mo15Ta10 alloy demonstrated a body‐centered‐cubic (BCC) solid solution phase with no evidence of intermetallics. Nanoindentation investigations exhibited high hardness of ≈5.1 GPa and a young modulus of ≈140 GPa. The alloy proposed better surface properties and tribological behaviour in comparison with the Ti‐6Al‐4V alloy in dry and wet (under PBS solution) conditions. Furthermore, the newly designed alloy revealed promising behaviour after investigation of in‐vitro biocompatibility compared to the Ti‐6Al‐4V alloy. These initial benefits of the Ti35Zr15Nb25Mo15Ta10 alloy concerning mechanical properties and wear resistance over the conventional Ti‐6Al‐4V alloy present an avenue for exploring novel orthopedic implant alloys.

Achieving room temperature hydrogen storage reversibility in Nb-rich alloys of the Nb-Cr-Mn system

Recent developments in the design of body-centered cubic (BCC) multicomponent alloys via computational tools have demonstrated the possibility of obtaining alloys with excellent hydrogen storage behavior. In this work, we employ the CALPHAD (Calculation of Phase Diagrams) method to design Nb-rich alloys of the Nb-Cr-Mn system that present hydrogen storage reversibility at room temperature under moderate pressure conditions. We employ the valence electron concentration (VEC) factor as a compositional guide to select compositions with suitable thermodynamic properties. Using electric arc melting, we synthesize two alloys, namely Nb85Cr10Mn5 and Nb70Cr20Mn10, both forming predominant BCC solid solutions with VEC ∼ 5.2 and minor amounts of a eutectic microconstituent composed of BCC and Laves C14 phases. Both alloys are easily hydrogenated at room temperature without the need for an activation treatment. The Nb85Cr10Mn5 alloy reaches a storage capacity of 2.1 wt% of H (298 K; Peq∼ 20 bar) whereas the Nb70Cr20Mn10 alloy reaches a capacity of 1.4 wt% (298 K; Peq∼ 21 bar). Benefits in the storage kinetic performance are correlated with the BCC + C14 microstructure. Pressure-Composition-Temperature (PCT) diagrams show moderate values of equilibrium pressure for hydrogen storage reversibility at room temperature. Room temperature absorption/desorption cycling measurements demonstrated a reversible capacity of 1.2 wt% of H (Peq∼ 29 bar) for the Nb85Cr10Mn5 alloy and 0.8 wt% of H (Peq∼ 31 bar) for the Nb70Cr20Mn10 alloy after twenty cycles.

Nanoporous CoFe2O4 with inherited cation doping from matrix realizing ultra-stable alkaline hydrogen evolution for over 1000 hours

Spinel-type oxides have been acknowledged for their eminent catalytic ability towards alkaline hydrogen evolution reaction (HER) due to the special metal occupying structures. To further improve conductivity, inherited cation doped spinel has been rapidly synthesized via scalable alloying-dealloying strategy. Specially, the resultant Al-doped CoFe2O4 possesses typical bi-continuous pore-ligament structure, showing a huge surface area for electrocatalysis. It only demands HER overpotentials of 22.9 and 275 mV to achieve 10 and 100 mA cm-2, respectively. Furthermore, it also exhibits ultra-stable hydrogen evolution for 1000 h Under the industrial conditions (6 M KOH, 60 °C), Al-CoFe2O4 || RuO2 couple just needs 1.75 V to reach high current density of 500 mA cm-2 and could steadily produce hydrogen for over 50 h with merely 8 % voltage loss. Theoretical calculation confirms that Al substitution could indeed increase valence electron concentration. Based on this optimized configuration, H2O molecule is preferred to adsorb on Co site and conducive to dissociate into *OH-*H co-adsorption. And thus, the energy barrier of alkaline HER can be reduced to 0.98 eV on Al-doping CoFe2O4 model. To further clarify the active species derived from HER, operando X-ray diffraction and Fourier transform infrared are used to prove that the superficial CoFe2O4 has been transformed into more active CoOOH/FeOOH. This work offers a guide to facilely fabricate cost-effective HER catalysts that are suitable for industrial hydrogen production.

Construction of hard BCC phases FeCrVMnTix wear-resistant high-entropy alloy coatings enhanced by solid solution of Ti elements

To address the complex working conditions encountered by 1Cr13 ferritic/martensitic steel during operation, it is imperative to develop high-entropy alloys (HEAs) protective coatings with enhanced friction resistance. In this work, guided by the valence electron concentration criterion, we constructed a single-phase body-centered cubic (BCC) structure with high hardness. This was achieved through the introduction of titanium (Ti), which has a significantly different atomic radius compared to other elements. FeCrVMnTix (x = 0, 0.5, 1.0, 1.5, and 2.0) coatings were fabricated on 1Cr13 steel surfaces using laser cladding techniques, and the microstructure, mechanical properties, and wear-resistance properties of Ti-doped HEA coatings were investigated. The results reveal that the FeCrVMnTix coatings are comprised of a BCC solid solution. Increasing Ti content refined the dendritic structure of the coatings, enhancing the Vickers hardness from 4.35 GPa to 8.27 GPa and the Young's modulus from 232.6 GPa to 273.2 GPa. Additionally, the wear rate decreased from 3.07 × 10−5 mm3·N−1·m−1 to 4.94 × 10−6 mm3·N−1·m−1. Notably, the FeCrVMnTi2.0 coating demonstrated excellent wear resistance that can be attributed to the solid solution strengthening effect of Ti. These findings underscore the pivotal role of Ti in enhancing the wear resistance of the coatings.

Designing cobalt-free face-centered cubic high-entropy alloys: A strategy using d-orbital energy level

High-entropy alloys (HEAs) are promising materials for high-temperature structural applications such as nuclear reactors due to their outstanding mechanical properties and thermal stability. Instead of the trial-and-error method, it is efficient to design and prepare single-phase face-centered cubic (FCC) structured HEAs using semi-empirical phase formation rules. However, almost all of phase formation rules were proposed without taking into account the cobalt-free situation. The HEAs containing cobalt are unsuitable for nuclear applications because of the long-term activation of cobalt. Here, six parameters, d-orbital energy level, valance electron concentration, entropy of mixing, enthalpy of mixing, atom size differences, and parameter of the entropy of mixing (Ω) were calculated to determine the solid solution phase, especially the FCC phase formation rules in cobalt-free HEAs. HEAs of 4 components were arc melted to verify the newly developed phase formation rules. The nanomechanical properties of produced HEAs were evaluated using nanoindentation. Among the six parameters, the d-orbital energy level and valance electron concentration are the critical factors that determine the FCC phase stability in cobalt-free alloys. Interestingly, the d-orbital energy level can be alone used as a benchmark for developing mechanical properties.

Advances in Nickel-Containing High-Entropy Alloys: From Fundamentals to Additive Manufacturing.

High-entropy alloys (HEAs) are recognized as a class of advanced materials with outstanding mechanical properties and corrosion resistance. Among these, nickel-based HEAs stand out for their impressive strength, ductility, and oxidation resistance. This review delves into the latest advancements in nickel-containing HEAs, covering their fundamental principles, alloy design strategies, and additive manufacturing techniques. We start by introducing HEAs and their unique properties, emphasizing the crucial role of nickel. This review examines the complex relationships between alloy composition, valence electron concentration (VEC), and the resulting crystal structures. This provides insights into design principles for achieving desired microstructures and mechanical properties. Additive manufacturing (AM) techniques like selective laser melting (SLM), electron beam melting (EBM), and laser metal deposition (LMD) are highlighted as powerful methods for fabricating intricate HEA components. The review addresses the challenges of AM processes, such as porosity, fusion defects, and anisotropic mechanical properties, and discusses strategies to mitigate these issues through process optimization and improved powder quality. The mechanical behavior of AM-processed nickel-based HEAs is thoroughly analyzed, focusing on compressive strength, hardness, and ductility. This review underscores the importance of microstructural features, including grain size, phase composition, and deformation mechanisms, in determining the mechanical performance of these alloys. Additionally, the influence of post-processing techniques, such as heat treatment and hot isostatic pressing (HIP) on enhancing mechanical properties is explored. This review also examines the oxidation behavior of nickel-containing HEAs, particularly the formation of protective oxide scales and their dependence on aluminum content. The interplay between composition, VEC, and oxidation resistance is discussed, offering valuable insights for designing corrosion resistant HEAs. Finally, this review outlines the potential applications of nickel-based HEAs in industries such as aerospace, automotive, and energy, and identifies future research directions to address challenges and fully realize the potential of these advanced materials.

Development and Mechanical Characterization of a CoCr-Based Multiple-Principal-Element Alloy

The development and mechanical characterization of a CoCr-based multiple-principal-element alloy are presented and discussed. In this work, ab initio synthesis and mechanical characterization of the (CoCr)100-x(TiNbZr)x (x = 0, 48 60, 78 and 100 % at) alloy family is reported; these include the calculation of thermodynamic parameters such as mixing entropy (ΔSmix), mixing enthalpy (ΔHmix), valence electron concentration (VEC), Ω and δ factors. The alloys were melted in a vacuum arc furnace; rod-shaped ingots were produced by suction casting. Phase characterization was carried out using optical microscopy, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction. Mechanical characterization was done via compressive and hardness tests. Calculation of phase diagrams was performed using Thermo-Calc © software. Yang’s model for phase prediction predicted a BCC solid solution. Multicomponent simulations predicted a more complex structure, with Laves (C14 and C15), Theta (C16), BCC 1, 2 and 3, Mu, CoTi2, CoZr3 and TiZr2. Contrary to Yang’s model for phase prediction, the experimentally obtained phases agreed reasonability well with those obtained by the Thermo-Calc simulation. The suction cast process cooling rate suppressed the nucleation and growth of some equilibrium phases, i.e., Laves (C14) or BCC 3 for the (CoCr)100-x(TiNbZr)x (x = 0, 48 60, 78 and 100 % at) alloys. The hardness test results were strongly related to the intermetallic phase formation, showing an increase of 331% with the x = 78 alloy. The BCC 1 phase played an important role in the yield strength behavior, as the (CoCr)22(TiNbZr)78 alloy, with a considerable amount of this phase, showed the highest yield strength value.

Understanding stacking fault energy of NbMoTaW multi-principal element alloys by interpretable machine learning

Stacking fault energy (SFE) is a crucial property influencing the deformation mechanisms of multi-principal element alloys (MPEAs). However, experimentally measuring SFE and exploring composition space of MPEAs is challenging. This study explores the SFE in NbMoTaW by integrating machine learning force fields (ML-FF), molecular dynamics simulations (MD), neural networks, and symbolic regression (SR) methods. A SFE dataset containing 2000 different NbMoTaW compositions were generated by combining ML-FF and MD. Then the neural-network model was developed for SFE prediction with an accuracy of 0.981 and a mean absolute error of 0.020. Using SR, the valence electron concentration (VEC), average shear modulus (G), radii gamma (γ) were identified as the key descriptors to determine SFE with the relationship of SFE=0.307·VEC+0.352·(G+γ). This work demonstrated a significant impact of chemical composition on SFE and established an accurate mathematical expression for SFE prediction, enhancing the understanding and alloy design of MPEAs.

Mechanism of Gd doping on microstructure and mechanical properties of FeCrNiCuTi0.4 high entropy alloy

The study focused on investigating the impact of rare earth element Gd content on both the microstructure and mechanical properties of FeCrNiCuTi0.4Gdx (x = 0, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.08, 0.1, 0.12, and 0.15, in mole ratio) high-entropy alloys, as well as elucidating the underlying mechanisms involved. The microstructure analysis revealed that the alloy exhibited a diphase structure consisting of FCC and BCC phases at low Gd content, and precipitation of the Cu5Gd phase occurred alongside the FCC and BCC phases at x = 0.05. The phase evolution of the current alloy system was assessed using the Ω - δ, △χ, and VEC (valence electron concentration) criteria. The mechanical properties indicated that the Vickers hardness increased with the increase of Gd content. Additionally, the compressive strength, yield strength, and plastic strain exhibit an initial increase followed by a decrease, while the average friction coefficient and wear demonstrated an initial increase followed by a subsequent decrease. The fracture mechanism shifted from plastic fracture to ductile fracture, ultimately transitioning to dissociative fracture with ductile fracture as the predominant mode. Similarly, the wear mechanism progressed from abrasive wear to a combination of abrasive wear with oxidation wear. The strengthening mechanisms of the present alloys included solid solution strengthening, fine crystal strengthening, and second-phase strengthening.