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The emergence of multi‐principal element alloys (MPEAs) heralds a transformative shift in the design of high‐performance alloys. Their ingrained chemical complexities endow them with exceptional mechanical and functional properties, along with unparalleled microscopic plastic mechanisms, sparking widespread research interest within and beyond the metallurgy community. In this overview, a unique yet prevalent mechanistic process in the renowned FeMnCoCrNi‐based MPEAs is focused on: the dynamic bidirectional phase transformation involving the forward transformation from a face‐centered‐cubic (FCC) matrix into a hexagonal‐close‐packed (HCP) phase and the reverse HCP‐to‐FCC transformation. The light is shed on the fundamental physical mechanisms and atomistic pathways of this intriguing dual‐phase transformation. The paramount material parameter of intrinsic negative stacking fault energy in MPEAs and the crucial external factors c, furnishing thermodynamic, and kinetic impetus to trigger bidirectional transformation‐induced plasticity (B‐TRIP) mechanisms， are thorougly devled into. Furthermore, the profound significance of the distinct B‐TRIP behavior in shaping mechanical properties and creating specialized microstructures c to harness superior material characteristics is underscored. Additionally, critical insights are offered into key challenges and future striving directions for comprehensively advancing the B‐TRIP mechanism and the mechanistic design of next‐generation high‐performing MPEAs. [ABSTRACT FROM AUTHOR]

We investigated the corrosion properties and transpassive behavior of CrMnFeCoNi and CrCoNi multi‐principal element alloys (MPEAs) in a 0.1 M NaCl electrolyte at pH 12. By using SECM‐based tip substrate voltammetry (TSV) in combination with the chemical analysis of the electrolyte, we were able to differentiate between anodic metal dissolution and oxygen evolution in the transpassive range. Our investigations have shown that CrCoNi has a significantly higher corrosion resistance compared to CrMnFeCoNi. In the studied alkaline environment, a transpassive oxide film is formed on the surface of CrCoNi during secondary passivation. This transpassive oxide film appears to play a significant role in oxygen evolution, as the increase in TSV currents at the microelectrode coincides with the corresponding current density plateau of the voltametric current trace. The formation of the transpassive oxide film was not observed in previous studies conducted in acidic environments. Moreover, the alkaline electrolyte induced a positive hysteresis and mild pitting corrosion, in addition to intergranular corrosion, which was the sole corrosion process observed at acidic pH levels. These findings enhance the understanding of the processes governing the transpassivity of CrMnFeCoNi and CrCoNi MPEAs in alkaline environments and have potential implications for the development of application‐tailored corrosion‐resistant MPEAs. [ABSTRACT FROM AUTHOR]

The influence of vacuum arc melting and mechanical processing (wrought) versus additive manufacturing (AM) on the creep behavior of multi-principal element alloys (MPEA) is investigated in this study. Annealed wrought and hot isostatically pressed (HIP) AM CrCoNi were creep tested under constant tensile stress of 40 to 200 MPa at temperatures of 1023 to 1173 K. Stress exponents of 4.5 ± 0.2 and 5.9 ± 0.1 and activation energies ranging from 240 to 259 and 320 to 331 kJ/mol were found for wrought and AM CrCoNi, respectively. The results indicate that the AM material exhibits superior creep resistance and inferior creep ductility compared to the wrought alloy. This difference is attributed to the AM material having a higher percentage of Cr-rich oxides, a smaller total low angle grain boundary (LAGB) length on a percentage basis, and a greater total twin boundary (TB) length on a percentage basis. The AM and wrought materials have similar grain sizes; however, the smaller LAGB length and greater TB length in the AM material reduce slip transmission on the other side of the boundaries and contribute to strength. The dislocation structures of the AM and wrought materials consist of individual curved dislocations with dislocation multijunctions and jogs, which is similar to the arrangements previously observed in CrMnFeCoNi.

Abstract The emergence of multi‐principal element alloys (MPEAs) heralds a transformative shift in the design of high‐performance alloys. Their ingrained chemical complexities endow them with exceptional mechanical and functional properties, along with unparalleled microscopic plastic mechanisms, sparking widespread research interest within and beyond the metallurgy community. In this overview, a unique yet prevalent mechanistic process in the renowned FeMnCoCrNi‐based MPEAs is focused on: the dynamic bidirectional phase transformation involving the forward transformation from a face‐centered‐cubic (FCC) matrix into a hexagonal‐close‐packed (HCP) phase and the reverse HCP‐to‐FCC transformation. The light is shed on the fundamental physical mechanisms and atomistic pathways of this intriguing dual‐phase transformation. The paramount material parameter of intrinsic negative stacking fault energy in MPEAs and the crucial external factors c, furnishing thermodynamic, and kinetic impetus to trigger bidirectional transformation‐induced plasticity (B‐TRIP) mechanisms， are thorougly devled into. Furthermore, the profound significance of the distinct B‐TRIP behavior in shaping mechanical properties and creating specialized microstructures c to harness superior material characteristics is underscored. Additionally, critical insights are offered into key challenges and future striving directions for comprehensively advancing the B‐TRIP mechanism and the mechanistic design of next‐generation high‐performing MPEAs.

Abstract We investigated the corrosion properties and transpassive behavior of CrMnFeCoNi and CrCoNi multi‐principal element alloys (MPEAs) in a 0.1 M NaCl electrolyte at pH 12. By using SECM‐based tip substrate voltammetry (TSV) in combination with the chemical analysis of the electrolyte, we were able to differentiate between anodic metal dissolution and oxygen evolution in the transpassive range. Our investigations have shown that CrCoNi has a significantly higher corrosion resistance compared to CrMnFeCoNi. In the studied alkaline environment, a transpassive oxide film is formed on the surface of CrCoNi during secondary passivation. This transpassive oxide film appears to play a significant role in oxygen evolution, as the increase in TSV currents at the microelectrode coincides with the corresponding current density plateau of the voltametric current trace. The formation of the transpassive oxide film was not observed in previous studies conducted in acidic environments. Moreover, the alkaline electrolyte induced a positive hysteresis and mild pitting corrosion, in addition to intergranular corrosion, which was the sole corrosion process observed at acidic pH levels. These findings enhance the understanding of the processes governing the transpassivity of CrMnFeCoNi and CrCoNi MPEAs in alkaline environments and have potential implications for the development of application‐tailored corrosion‐resistant MPEAs.

Valence electron concentration (VEC) is widely accepted as an effective guideline for designing the mechanical properties of Ti-containing refractory high-entropy alloys (RHEAs). In the present work, a series of Ti–Zr–Nb–Ta and Ti–Zr–Nb–Mo RHEAs with body-centered-cubic (bcc) structure are carefully designed by tailoring their VEC through changing the alloying composition. The elastic properties and mechanical properties are systematically calculated by using a first-principles method. Comparison with available experimental data demonstrates that the employed approach accurately describes the VEC dependence of the elastic and mechanical properties of RHEAs. In general, the elastic stability, elastic properties, ideal shear strength, Vickers hardness, and yield strength increase, whereas Zener anisotropy decreases with increasing VEC. Among all the considered RHEAs, the most isotropic RHEA Ti30Zr30Nb20Mo20 has the best strength-ductility trade-off. Mo has a stronger solid solution strengthening effect than Ta. The higher strength associates with larger lattice distortion induced by increasing VEC. Both elastic stability and mechanical properties are related to the electronic density of states of the alloys. The present work sheds deep insight into the design of high-performance RHEAs through tailoring the VEC.

Understanding the sluggish kinetics is of great significance for improving the properties of multi-principal element alloys (MPEAs). In this paper, the crystal growth in undercooled CoNi, CoNiFe and CoNiPd alloys was studied by molecular dynamics (MD) to show atomistic insights into sluggish crystal growth kinetics. The added Fe and Pd lead to a decrease in the crystal growth velocity and more significant sluggish crystal growth kinetics was observed in CoNiPd. After minimizing the instantaneous potential energy of atoms adjacent to the Solid/Liquid (S/L) interface, it was found that the decreased crystal growth velocity as the number of principal elements could be accounted by the accompanying change of inherent structure. The exploration of bulk undercooled liquid showed that the diffusion kinetic in liquid does not play a critical role on the sluggish crystal growth kinetics. Besides, the investigation of atomic structure in front of the smooth S/L interface revealed that the sluggish crystal growth kinetics induced by properties of element was associated with the atomic spontaneous ordering degree.

This work demonstrates the capabilities and advantages of a novel sintering technique to fabricate bulk composition gradient materials. Pressure distribution calculations were used to compare several tooling geometries for use with current-activated, pressure-assisted densification or spark plasma sintering to densify a gradient along the long dimension of the specimen. The selected rectangular tooling design retains a low aspect ratio to ensure a uniform pressure distribution during consolidation by using a side loading configuration to form the gradient along the longest dimension. Composition gradients of NixCu1−x, MoxNb1−x, and MoNbTaWHfx (x from 0 to 1) were fabricated with the tooling. The microstructure, composition, and crystal structure were characterized along the gradient in the as-sintered condition and after annealing to partially homogenize the layers. The successful fabrication of a composition gradient in a difficult-to-process material like the refractory multi-principal element alloy system MoNbTaWHfx shows the utility of this approach for high-throughput screening of large material composition spaces. [ABSTRACT FROM AUTHOR]

Multi‐principal element alloys (MPEAs) are gaining interest in corrosion and electrocatalysis research due to their electrochemical stability across a broad pH range and the design flexibility they offer. Using the equimolar CrCoNi alloy, we observe significant metal dissolution in a corrosive electrolyte (0.1 M NaCl, pH 2) concurrently with the oxygen evolution reaction (OER) in the transpassive region, despite the absence of hysteresis in polarization curves or other obvious corrosion indicators. We present a characterization scheme to delineate the contribution of OER and alloy dissolution, using scanning electrochemical microscopy (SECM) for OER‐onset detection, and quantitative chemical analysis with inductively coupled‐mass spectrometry (ICP‐MS) and ultraviolet visible light (UV/Vis) spectrometry to elucidate metal dissolution processes. In situ electrochemical atomic force microscopy (EC‐AFM) revealed that the transpassive metal dissolution on CrCoNi is dominated by intergranular corrosion. These results have significant implications for the stability of MPEAs in corrosion systems, emphasizing the necessity of analytically determining metal ions released from MPEA electrodes into the electrolyte when evaluating Faradaic efficiencies of OER catalysts. The release of transition metal ions not only reduces the Faradaic efficiency of electrolyzers but may also cause poisoning and degradation of membranes in electrochemical reactors. [ABSTRACT FROM AUTHOR]

Previous research has shown that multi-principal element alloys (MPEAs) using chromium, molybdenum, niobium, tantalum, titanium, vanadium, and zirconium can form stable body-centered-cubic (BCC) structures across a large temperature region (25°C to 1000°C). This is the same crystal structure as γ-uranium (U), which has shown desirable thermal and irradiation behavior in previous alloy fuel research. It is hypothesized then that the MPEA alloying approach can be used to produce a stable BCC uranium-bearing alloy and to retain its stability throughout anticipated operating regimes of power-producing reactors. Candidate elements were assessed using Monte Carlo N-Particle (MCNP) analysis to determine uranium densities necessary to make the alloy an economically viable fuel compared to conventional fuel forms. Following neutronic considerations, materials property databases and empirical predictors were used to determine the compositions with a high potential to form a BCC solid solution alloy. The final four alloys were MoNbTaU2, MoNbTiU2, NbTaTiU2, and NbTaVU2, which were cast using arc melting of raw elemental foils and chunks. Characterization of the fabricated alloys included scanning electron microscopy, X-ray diffraction, energy dispersive X-ray spectroscopy, and transmission electron microscopy. The results showed a two-phase system with a U-rich matrix phase surrounding the refractory precipitates. The U phase was found to contain varying concentrations of the alloying elements and was a BCC γ-U phase. These results warrant further research to identify ideal compositions for use as an advanced alloy fuel. [ABSTRACT FROM AUTHOR]

Refractory multi-principal element alloys constitute a novel category of metallic materials, and they have good properties, particularly at elevated temperatures. Currently, casting and powder metallurgy stand as the two predominant manufacturing routes. This study focuses on the production of the W19.8Mo19.8Nb19.8Ti19.8Cr19.8Al1 (At. %) alloy via the mechanical alloying process. Analyses of morphology and particle size distribution at intervals of 12, 24, 36, and 48 hours of milling showed a gradual decrease in particle size according to increase in milling time. The XRD showed the presence of the BCC phase after 48 hours of milling, and the application of the Williamson-Hall methods indicated that after 12, 24, and 36 hours of milling, the crystallite size decreased and the microstrain increased gradually. However, after 48 hours of milling, there was an increase in these values, suggesting the reordering of the structure of this alloy. In addition, Fe contamination increases significantly for higher milling time.

Face-centered cubic (FCC) single-phase multi-principal element alloys (MPEAs) have demonstrated promising irradiation resistance. The generation and evolution of irradiation defects are integral aspects to comprehend, as they offer valuable insights into the radiation resistance of materials. In this work, we use molecular dynamics simulations to investigate the primary radiation damage of CrFeNi FCC MPEAs, while focusing on the impact of local chemical ordering (LCO) on the generation and evolution of defects. Our simulation results reveal that the LCO reduces the probability of defect production, consequently mitigating the maximum number of defects. Furthermore, the LCO increases defect migration barriers, thereby impeding the migration of interstitial atoms and leading to an increased number of residual defects subsequent to primary damage. However, the dispersed distribution of defects throughout space facilitates the uniform nucleation of dislocations in subsequent irradiation cascades. Additionally, the increased migration energy acts as a barrier to the movement of defect clusters, impeding their growth and thereby enhancing the radiation resistance of MPEAs. Our findings underscore the potential for enhancing the radiation resistance of MPEAs through the deliberate tailoring of LCOs. [ABSTRACT FROM AUTHOR]

Internal strain and its distribution within the crystal lattice play crucial roles in modulating dislocation activities, thereby affecting mechanical properties of materials. Through the synergistic application of integrated differential phase contrast, in situ transmission electron microscopy characterizations, and computational simulations, a method is unveiled for homogenizing dislocation pinning in NiCoCr multi‐principal element alloy (MPEA) through the introduction of a high concentration of oxygen atoms with high diffusion mobility. The doping of massive oxygen atoms creates a high density of strong local pinning points for dislocation motion. Notably, oxygen interstitials exhibit remarkable diffusion and mobility across different octahedral and tetrahedral sites within the distorted crystal lattice of NiCoCrO alloy, even at room temperature. The capability allows for the release of severe stress concentrations arising from dislocation entanglement and the establishment of new strong local pinning points at alternative locations in a uniform way, enabling the material with high strength and outstanding deformability. These findings suggest that interstitial atoms can exhibit significant mobility, even at ambient temperature, in complex MPEAs with spreading lattice distortion, opening new possibilities for dislocation engineering. [ABSTRACT FROM AUTHOR]

Developing new bio-metallic materials with improved performance has attracted intensive research focus over the last decades. In the present work, a new series of (TiZr)95-xHfxMo5 (x = 0, 10, 15, 20, 25, 31.67) multi-principal elements alloys (MPEAs) have been designed for potential implant materials. The mechanical properties, deformation characteristics, tribological properties, corrosion resistance, and in vitro biocompatibility of the MPEAs were characterized. All the TiZrHfMo alloys possess a high yield strength (913–––1083 MPa) and tensile strength (931–––1118 MPa) and a large microhardness (296–––333 Hv). The addition of Hf into the alloys induced severe lattice distortion, which increased the solid-solution strengthening effect. It also resulted in a finer grain size and led to excellent wear resistance. Meanwhile, the results showed that the dislocation glide mode transformed from planar slip to cross-slip and the transformation became apparent with the decrease of Hf. Also, the MPEAs showed a good corrosion resistance compared to CP-Ti, which was credited to the superior passivation films. The in vitro biocompatibility experiments displayed that the TiZrHfMo alloys exhibited high cell viability ratios, which were close to that of CP-Ti. The present work demonstrates the huge potential of TiZrHfMo MPEAs as implant material for biomedical applications.

In order to develop a multi-principal element alloy with outstanding soft magnetic properties and high thermal stability, a Fe35Co35Mn30−xAlx (x = 9, 12, 15, 18, 21, 24) alloy system was designed. The findings demonstrated that the optimal Al/Mn content ratio contributes to the formation of a BCC and B2 dual-phase structure, which leads to a significant increase in the saturation magnetic flux density (Bs) and a considerable decrease in the coercivity of the alloy. The as-cast Fe35Co35Mn15Al15 alloy with a co-lattice structure exhibits exceptional soft magnetic properties, with a resistivity of 324 μΩ·cm, a Bs of 1.46 T and a Hc of 167.2 A/m. Corresponding soft magnetic powder cores (SMPCs) were fabricated and the impact of heat treatment temperature on the properties of the SMPCs was examined. The SMPCs after annealing at 660 °C exhibits high stable permeability (μe) of 48 up to 10 MHz, a low core loss (Pcv) of 1208 mW/cm3 (at 1 MHz, 20 mT), and excellent direct current bias performance. In addition, these SMPCs possess excellent high temperature magnetic stability, and the variation of both μe and Pcv at a high frequency (1 MHz) is less than 2 % over a wide temperature range of 25–150 °C.

One of the key problems restricting the commercial application of controllable nuclear fusion reactors is plasma facing materials (PFMs). As the most promising PFMs, there are still many problems in the applications of tungsten and tungsten alloys. Due to their high strength at elevated temperatures, high melting point and good irradiation resistance, refractory multi-principal-element alloys are expected to meet the needs of PFMs. In the present study, (TiVTa)95X5(X=Cr, Zr, W) was designed and manufactured by using arc-melting. The effects of the addition of Cr, Zr and W on the microstructure and phase stability at 900 ℃ of TiVTa-based alloys were investigated by XRD, SEM and EDS. The results show that as-cast TiVTa-based alloys are simple solid solutions with BCC structure. After homogenization treatment at 1200 ℃, phase decomposition occurs in the (TiVTa)95Cr5 alloy, and a small amount of second phase C15_Laves appears in the matrix. At 900 ℃, TiVTa-based alloys are all decomposed into a BCC main phase and a C15_Laves second phase which mainly distributed along the grain boundaries, and the volume fraction of the second phase in TiVTa and (TiVTa)95W5 alloys is small whereas the addition of Cr and Zr intensifies the phase decomposition. After phase structure and elemental analysis, the lattice constant and elemental composition of the precipitated C15_Laves phases in (TiVTa)95X5(X=Cr, Zr, W) alloys are inconsistent with the possible Laves phase in the binary alloy system, and the difference is mainly attributed to the elemental composition of the Laves phase.

We propose an active learning guided density functional theory calculation framework for rapid screening of multi-principal element alloys (MPEAs) with superior mechanical properties. Using this framework, we fast construct the datasets of the bulk modulus (B), shear modulus (G), yield strength, and Pugh’s ratio of 12,698 noble metal MPEAs. These datasets were obtained with density functional theory prediction accuracy (R2 = 0.98 and 0.96 for B and G, respectively) based on active learning guided 120 DFT calculated data. Analysis of the dataset shows that Ni and Au would enhance the yield strength and the toughness of these noble MPEAs, respectively.

Noble metal-free nanoparticles (NPs) based on multi-principal element alloys (MPEAs) were synthesized using a one-step pulsed laser ablation in liquids (PLALs) method for the electrochemical reduction of CO2. Laser ablation was performed in pure water or poly-(diallyldimethylammonium chloride) (PDADMAC)-containing an aqueous solution of Al8Cr17Co17Cu8Fe17Ni33 MPEA targets. Transmission electron microscopy (TEM) measurements combined with energy dispersive X-ray (EDX) mapping were used to characterize the structure and composition of the laser-generated MPEA nanoparticles (MPEA-NPs). These results confirmed the presence of a characteristic elemental distribution of a core-shell phase structure as the predominant NP species. The electrocatalytic performance of the laser-generated MPEA-NPs was characterized by linear sweep voltammetry (LSV) demonstrating an enhanced electrocatalytic CO2 activity for PDADMAC-stabilized NPs. The findings of these investigations indicate that MPEAs have great potential to replace conventional, expensive noble metal electrocatalysts.

This work demonstrates the capabilities and advantages of a novel sintering technique to fabricate bulk composition gradient materials. Pressure distribution calculations were used to compare several tooling geometries for use with current-activated, pressure-assisted densification or spark plasma sintering to densify a gradient along the long dimension of the specimen. The selected rectangular tooling design retains a low aspect ratio to ensure a uniform pressure distribution during consolidation by using a side loading configuration to form the gradient along the longest dimension. Composition gradients of NixCu1−x, MoxNb1−x, and MoNbTaWHfx (x from 0 to 1) were fabricated with the tooling. The microstructure, composition, and crystal structure were characterized along the gradient in the as-sintered condition and after annealing to partially homogenize the layers. The successful fabrication of a composition gradient in a difficult-to-process material like the refractory multi-principal element alloy system MoNbTaWHfx shows the utility of this approach for high-throughput screening of large material composition spaces.

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We propose an active learning guided density functional theory calculation framework for rapid screening of multi-principal element alloys (MPEAs) with superior mechanical properties. Using this framework, we fast construct the datasets of the bulk modulus (B), shear modulus (G), yield strength, and Pugh's ratio of 12,698 noble metal MPEAs. These datasets were obtained with density functional theory prediction accuracy (R2 = 0.98 and 0.96 for B and G, respectively) based on active learning guided 120 DFT calculated data. Analysis of the dataset shows that Ni and Au would enhance the yield strength and the toughness of these noble MPEAs, respectively. An active learning guided density functional theory calculation framework, which reduces computation time by three orders of magnitude, was proposed for rapidly screening multi-principal element alloys with superior mechanical properties. [ABSTRACT FROM AUTHOR]

Multi-principal element alloys (MPEAs) are in the forefront of materials science since their compositions can be found in the undiscovered central parts of phase diagrams. These materials contain 3-6 elements with similar fractions, i.e., the constituents do not play the classical solvent and solute roles in the alloys. This class of materials includes high entropy alloys (HEAs). Novel MPEA compositions often have unique and superior properties compared to conventional materials. It has been shown that the features of MPEAs can be further improved by nanostructuring using severe plastic deformation (SPD) techniques. In this study, the evolution of the microstructure in MPEAs during SPD-processing (defect formation, grain refinement and phase transformation) is overviewed on the basis of the literature. The corresponding changes of the mechanical and physical properties, such as the strength, corrosion resistance and hydrogen diffusivity are discussed. In addition, the potential applications of SPD-processed MPEAs are presented. [ABSTRACT FROM AUTHOR]

Multi‐principal element alloys (MPEAs) have been subjected to extensive research due to their promising potential for numerous applications. Up to now, most of the existing research has been focused on unraveling the microstructural evolution and describing the exceptional performance of these alloys when exposed to demanding environments. Nevertheless, it is especially important to understand their processability so that these advanced engineering alloys can be considered for real‐life applications where conventional manufacturing processes, such as welding, are widely used. Herein, gas tungsten arc welding (GTAW) is used for similar welding of a recently developed precipitation‐hardened Cr29.7Co29.7Ni35.4Al4Ti1.2 MPEA. The microstructural evolution and resulting mechanical properties are characterized by combining optical and electron microscopy, synchrotron X‐ray diffraction, microhardness mapping, and tensile testing. The different microstructure features across the welded joint are correlated to the weld thermal cycle and resulting local mechanical properties. Overall, the Cr29.7Co29.7Ni35.4Al4Ti1.2 MPEA exhibits excellent weldability and mechanical properties, reaching a tensile strength of ≈750 MPa and a fracture strain of ≈33% during tensile tests, making this alloy viable for structural applications. The innovative aspect of this work includes the expansion of the current understanding on the physical metallurgy of MPEAs, as well as the examination of this particular MPEA's processability. [ABSTRACT FROM AUTHOR]

Designing new materials with desired properties is a complex and time-consuming process. One of the most challenging factors of the design process is the huge search space of possible materials. Machine learning methods such as [Formula: see text]-nearest neighbors, support vector machine (SVM) and artificial neural network (ANN) can contribute to this process by predicting materials properties accurately. Properties of multi-principal element alloys (MPEAs) highly depend on alloys’ phase. Thus, accurate prediction of the alloy’s phase is important to narrow down the search space. In this paper, we propose a solution of employing SVM method with hyperparameters tuning and the use of weighted values for prediction of the alloy’s phase. Using the dataset consisting of the experimental results of 118 MPEAs, our solution achieves a cross-validation accuracy of 90.2%. We confirm the superiority of this score over the performance of ANN statistically. On the other dataset containing 401 MPEAs, our SVM model is comparable to ANN and exhibits 70.6% cross-validation accuracy. We also found that additional variables, including average melting temperature and standard deviation of melting temperature, increase prediction accuracy by 3.34% in the best case.

Investigation into non-equiatomic high-entropy alloys has grown in recent years due to questions about the role of entropy stabilization in forming single-phase solid solutions. Non-equiatomic alloys have been shown to retain the outstanding mechanical properties exhibited by their equiatomic counterparts and even improve electrical, thermal, and magnetic properties, albeit with relaxed composition bounds. However, much remains to understand the processing–structure–property relationships in all classes of so-called high-entropy alloys (HEAs). Here, we are motivated by the natural phenomena of crystal growth and equilibrium conditions to introduce a method of HEA development where controlled processing conditions determine the most probable and stable composition. This is demonstrated by cooling an equiatomic CrMnFeCoNiCu alloy from the melt steadily over 3 days (cooling rate ~4 °C/h). The result is an alloy containing large Cr-rich precipitates and an almost Cr-free matrix exhibiting compositions within the MnFeCoNiCu system (with trace amounts of Cr). From this juncture, it is argued that the most stable composition is within the CrMnFeCoNiCu system rather than the CrMnFeCoNi system. With further optimization and evaluation, a unique non-equiatomic alloy, Mn17Fe21Co24Ni24Cu14, is derived. The alloy solidifies and recrystallizes into a single-phase face-centered cubic (FCC) polycrystal. In addition to possible applications where Invar is currently utilized, this alloy can be used in fundamental studies that contrast its behavior with its equiatomic counterpart and shed light on the development of HEAs. [ABSTRACT FROM AUTHOR]

Due to their compositional complexity and flexibility, multi-principal element alloys (MPEAs) have a wide range of design and application prospects. Many researchers focus on tuning chemical inhomogeneity to improve the overall performance of MPEAs. In this paper, we systematically review the chemical inhomogeneity at different length scales in MPEAs and their impact on the mechanical properties of the alloys, aiming to provide a comprehensive understanding of this topic. Specifically, we summarize chemical short-range order, elemental segregation and some larger-scale chemical inhomogeneity in MPEAs, and briefly discuss their effects on deformation mechanisms. In addition, the chemical inhomogeneity in some other materials is also discussed, providing some new ideas for the design and preparation of high-performance MPEAs. A comprehensive understanding of the effect of chemical inhomogeneity on the mechanical properties and deformation mechanisms of MPEAs should be beneficial for the development of novel alloys with desired macroscopic mechanical properties through rationally tailoring chemical inhomogeneity from atomic to macroscale in MPEAs. [ABSTRACT FROM AUTHOR]

Real-time neutron imaging was utilized to produce a movie-like series of radiographs for in-situ observation of the remixing of liquid state immiscibility that occurs in equiatomic CoCrCu with the addition of Ni. A previous neutron imaging study demonstrated that liquid state immiscibility can be observed in-situ for the equiatomic CoCrCu alloy. In this follow-up study, equiatomic buttons of CoCrCu were placed alongside small Ni buttons inside an alumina crucible in a high-temperature vacuum furnace. The mass of the Ni buttons was specifically selected such that when melted in the same crucible as the CoCrCu buttons, the overall composition would become equiatomic CoCrCuNi. Neutron imaging was simultaneously carried out to capture 10 radiographs in 20 °C steps from 1000 °C to 1500 °C and back down to 1000 °C. This, in turn, produced a movie-like series of radiographs that allow for the observation of the buttons melting, the transition from immiscible to miscible as Ni is alloyed into the CoCrCu system, and solidification. This novel imaging process showed the phase-separated liquids remixing into a single-phase liquid when Ni dissolves into the melt, which makes this technique crucial for understanding the liquid state behavior of these complex alloy systems. As metals are not transparent to X-ray imaging techniques at this scale, neutron imaging of melting and solidification allows for the observation of liquid state phase changes in real time. Thermodynamic calculations of the isopleth for CoCrCuNix were carried out to compare the observed results to the predictions resulting from the current Thermo-Calc TCHEA3 thermodynamic database. The calculations show a very good agreement with the experimental results, as the calculations indicate that the CoCrCuNix alloy solidifies from a single-phase liquid when x ≥ 0.275, which is close to the nominal concentration of the CoCrCuNi alloy (x = 0.25). The neutron imaging shows that the solidification of CoCrCuNi results from a single-phase liquid. This is evident as no changes in the neutron attenuation were observed during the solidification of the CoCrCuNi alloy.

Nanostructure design is an effective strategy for strength enhancement for metals and alloys. Nevertheless, stacking faults and nanotwins are not thermodynamically favoured. Here, it is demonstrated that high-density 9R phase with a columnar nanotwinned structure can be obtained through rational compositional design in a novel NiMnAlFeCr multi-principal element alloy. A combined density functional theory (DFT) study and molecular dynamics simulation (MD) was also performed to study the impact of alloy composition on the stability of 9R phase and its consequences on strength. The results showed that the energy barrier of formation of stacking fault can be obviously reduced by introduction of Fe or Cr in NiMn solid solution and its stability can be enhanced by simultaneously addition a certain range of Fe and Cr, which in favoring formation of high-density 9R phase.The experimental and simulation results demonstrate the substantial impact of 9R phase on the mechanical properties of the (Ni50Mn30Al20)100-x(FeCr)x alloys. This study provides a novel insight into nanotwinned structure as well as a promising strategy to design materials with high-strength.

Designing new materials with desired properties is a complex and time-consuming process. One of the most challenging factors of the design process is the huge search space of possible materials. Machine learning methods such as k -nearest neighbors, support vector machine (SVM) and artificial neural network (ANN) can contribute to this process by predicting materials properties accurately. Properties of multi-principal element alloys (MPEAs) highly depend on alloys' phase. Thus, accurate prediction of the alloy's phase is important to narrow down the search space. In this paper, we propose a solution of employing SVM method with hyperparameters tuning and the use of weighted values for prediction of the alloy's phase. Using the dataset consisting of the experimental results of 118 MPEAs, our solution achieves a cross-validation accuracy of 90.2%. We confirm the superiority of this score over the performance of ANN statistically. On the other dataset containing 401 MPEAs, our SVM model is comparable to ANN and exhibits 70.6% cross-validation accuracy. We also found that additional variables, including average melting temperature and standard deviation of melting temperature, increase prediction accuracy by 3.34% in the best case. [ABSTRACT FROM AUTHOR]

A fundamental understanding of corrosion mechanisms is the key to developing suitable corrosion protection approaches, and for the prediction of service life of metallic structures. However, conventional corrosion testing methods such as mass loss and electrochemical testing do not guarantee estimation of "true" corrosion rate and often mask the underlying mechanisms, due to either low sensitivity or a lack of element‐resolved information. Relatively recent work in corrosion science has led to the development of a new class of corrosion testing approaches, namely atomic spectroelectrochemistry; whereby direct insight of dissolution and corrosion mechanisms can be obtained during electrochemical testing. Atomic spectroelectrochemistry provides real‐time and element resolved dissolution rate of material via coupling electrochemical flow cell with inductively coupled plasma – atomic spectroscopy. This concise review discusses the basic working principle of atomic spectroelectrochemistry and its recent applications in corrosion science to understand the true underlying corrosion mechanisms of a range of metallic materials. [ABSTRACT FROM AUTHOR]

In this work, we operate on a small dataset available from the technical literature to predict the oxidation-induced mass change at 1000 °C of thousands of new alloy compositions using "Tree-based Pipeline Optimization Tool" , an automated machine learning (ML) method. The ML pipeline we develop is trained on the log10 of the mass change per unit area. This yields a mean absolute error of 0.34 on the test set's values, which span 3.5 decades. With additional insights from thermodynamic simulations, a set of seven alloys is selected, manufactured, and characterized. Of these, the oxidation behavior of five alloys is well-predicted by the ML-based model, while results for two alloys show orders of magnitude deviations from predictions. The results show that ML-based methods can be useful for predicting composition-dependent oxidation behavior, despite its many complexities. [ABSTRACT FROM AUTHOR]

Multi-principal element alloys (MPEAs) have garnered widespread recognition owing to their remarkable mechanical properties. Among alloying elements, the vanadium (V) element exhibits unique characteristics and has a high strengthening effect in the face-centered cubic Co-Ni-V MPEAs family owing to its large atomic radius. Here, first-principle calculations are utilized to examine the physical and mechanical characteristics of extensively researched VCoNi MPEAs. The objectives of this work are to give guidance on the design of MPEAs with desirable capabilities and fresh insight for understanding the role of lattice distortion described by atomic size difference on lattice parameters, binding energy, generalized stacking-fault energy, elastic properties, and electronic properties. A high degree of lattice distortion can lead to an increase in lattice constants, a more stable structure, and enhanced plasticity. These trends are attributed to a shortened pseudo-energy gap, large variations in charge density difference, and compact band overlap with increasing lattice distortion. The above results aim to serve as a point of reference for exploring the physical and mechanical attributes of MPEAs. • Influence of lattice distortion on mechanical and electronic properties of VCoNi MPEAs is revealed by first-principle calculations. • High lattice distortion increases lattice constant and bonding strength, but reduces binding energy in VCoNi MPEAs. • Lattice distortion causes large changes of stacking-fault energy and charge density difference. [ABSTRACT FROM AUTHOR]

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. • Exploring the stacking fault energy (SFE) in NbMoTaW by machine learning. • High accuracy machine learning model for SFE prediction with an accuracy of 0.981. • Determining SFE by valence electron concentration (VEC), average shear modulus (G), and radii gamma (γ) descriptors. • Mathematical expression of SFE = 0. 307 ·VEC + 0. 352 · G + γ between descriptors and SFE. [ABSTRACT FROM AUTHOR]

Multi-principal element alloys (MPEAs) have attracted growing attention owing to the vast compositional field and the comprehensive properties. In this work, Fe 7 (CoNi) 77 B 16 MPEA was undercooled by vacuum melting furnace with low (61 K, 95 K), medium (115 K, 148 K), and high undercoolings (200 K). The maximum growth velocity of the alloy was 0.21 m/s due to the sluggish kinetics effect. The regular eutectic transformed to anomalous eutectic and the primary dendrite refined from 9.1 μm to 3.5 μm with the increase of undercooling. Meanwhile, the phase selection behavior at high undercooling was observed between the stable (Fe, Co, Ni) phase and the metastable (Fe, Co, Ni) 23 B 6 phase. The hardness of primary phase and compressive strength of the undercooled MPEAs were increased to 952.19 HV and 2061.72 MPa, which is much higher than the as-cast alloy. The enhancement of the mechanical properties was attributed to microstructure evolution. The decrease in plasticity resulted from the increasing volume fraction of brittle intermetallic phases. This study helps to theoretically understand the solidification mechanism of the Fe-Co-Ni-B MPEAs and provides a promising way to enhance the mechanical properties of the novel MPEAs. • The growth velocity of primary dendrite is determined. • Regular eutectic transforms to anomalous eutectic. • The microstructure refines with the increase of undercooling. • Phase selection occurs at high undercooling. • Mechanical properties enhance due to non-equilibrium solidification. [ABSTRACT FROM AUTHOR]

• Shock response of CoCrNi-based MPEAs tuned with Fe/Mn elements was investigated. • Fe/Mn elements reduce the defect nucleation barrier and improve the plastic deformability. • Mn element significantly reduces the Hugoniot elastic limit (HEL) and spall strength. • Two modes of void nucleation induced by waveform profiles were revealed. Recent researches on concentrated solid solutions have emphasized the role of various solute interactions in determining anomalous dislocation core and plastic deformation. However, the influence path of element tuning under extreme conditions is still unclear. Here, we investigated shock-induced deformation and fracture in CoCrNi-based multi-principal element alloys (MPEAs) tuned by Fe/Mn elements using large-scale molecular dynamics simulations. It was found that Fe/Mn elements could reduce the defect nucleation barrier and improve the plastic deformability. When single-element tuning is applied, the Mn element significantly reduces the production of dislocations, favoring more phase transitions from FCC to BCC or amorphous phase. The results show that Mn significantly reduces the Hugoniot elastic limit (HEL) and spall strength, while the addition of Fe element to CoCrNiMn can alleviate this effect by reducing the degree of lattice distortion. Specially, we analyzed the relationship between void nucleation and shock wave propagation, and explained the single-negative-pressure-zone nucleation as well as complex double-negative-pressure-zone nucleation phenomena. Empirical equations for the spall strength of CoCrNi-based MPEAs adjusted by Fe/Mn elements were established. This work demonstrates a potential strategy for elemental tuning to tailor the mechanical properties of polymorphism in MPEAs. [Display omitted] [ABSTRACT FROM AUTHOR]

In order to develop Co-free CrFeNi based multi-principal element alloys (MPEAs) with good synergies between strength and plasticity, the effects of Si content on the microstructures and mechanical properties of CrFe 2 Ni 2 Ti 0.2 Si x (x = 0, 0.125, 0.25, 0.375, 0.5 and 0.625) MPEAs are studied in this work. The results show that the addition of Si induces a phase transformation from a single-phase face-centered cubic (FCC) structure to a dual-phase structure, comprising an FCC solid solution phase and G-phase. The outcomes from phase formation theory and phase diagram calculations (CALPHAD) also confirm that CrFe 2 Ni 2 Ti 0.2 Si x MPEAs are prone to form dual-phase structures. With increasing Si content, the yield strength and hardness of the alloys increase by 47.2 % and 57.3 %, respectively, while still maintaining a strain of 34.2 %, which is indicative of commendable overall mechanical properties. This is ascribed to the coherence between FCC matrix and G-phase, enabling an enhancement in strength concurrently with the retention of considerable plasticity. Moreover, grain boundary strengthening and precipitation strengthening play a dominant role in the strengthening of CrFe 2 Ni 2 Ti 0.2 Si x MPEAs. The present work offers new insights for the further development of high-performance non-metallic MPEAs, so as to achieve a combination of targeted microstructures and excellent properties. • Effect of Si on precipitation behavior and mechanical properties of CrFe 2 Ni 2 Ti 0.2 Si x are studied. • Si induces a phase transformation from a single FCC phase to an FCC + G-phase. • Si enhances the mechanical properties due to the coherence between G-phase and FCC matrix. • Si0.625 shows a better combination of an ultimate strength of 1379.2 MPa and a strain of 34.2 %. • Grain boundary strengthening and precipitation strengthening play a dominant role in the alloys. [ABSTRACT FROM AUTHOR]

The insufficient surface properties of multi-principal element alloys (MPEAs) hinder their potential biomedical applications. Here, a facile thermal oxidation plus nitridation treatment is conducted to fabricate a protective layer on a novel Ti-Zr-Nb-Ta-Mo MPEA. The total surface layer of NT5-ON consists of a top compound layer, including TiO 2 , TiN, TiON, ZrO 2 , and ZrN, and an underneath N-diffusion zone, leading to an exceptionally high wear performance. Also, electrochemical testing and in-vitro evaluations demonstrated enhanced corrosion resistance and cytocompatibility of NT5-ON. As a result, thermal oxidation plus nitridation treatment is an effective technique for designing high-performance hip implant materials. [ABSTRACT FROM AUTHOR]

• FeMnCoCrNi and Fe 40 Mn 40 Co 10 Cr 10 alloys were rolled at room temperature and 77 K. • Slip, twinning and γ → ε MT are dominating mechanisms in Fe 40 Mn 40 Co 10 Cr 10 alloy. • Fe 40 Mn 40 Co 10 Cr 10 alloy does not show a superior strengthening over the Cantor alloy. Deformation-induced microstructure and strength properties were studied in cold-rolled metastable Fe 40 Mn 40 Co 10 Cr 10 and stable FeMnCoCrNi (Cantor alloy) multi-principal element alloys. Contrary to prevailing beliefs, a joint development of dislocation slip, twinning and deformation-induced γ → ε martensitic transformation in Fe 40 Mn 40 Co 10 Cr 10 alloy does not provide a superior strengthening over the dislocation-dominated refinement in Cantor alloy during room-temperature rolling. Twinning-dominated deformation during cryogenic rolling of Cantor alloy provides higher strength compared to the metastable alloy. [ABSTRACT FROM AUTHOR]

The unsatisfactory wear resistance and lubrication performance of CoCrNi multi-principal elements alloys (MPEAs) have emerged as critical challenges, impeding their extensive utilization as advanced engineering materials, despite their desirable mechanical and physical properties. Incorporating graphite to obtain MPEAs-based self-lubricating composites is believed to improve the wear resistance. However, it is challenging to prepare composites with high strength, large plasticity, and excellent anti-friction performance simultaneously. The present study reports the fabrication of novel CoCrNi MPEA-based self-lubricating composites, consisting of in-situ graphite and high-hardness silicides/carbides as reinforcement, prepared by powder metallurgy and reactive sintering. The 20.6 vol% Gr@CoCrNi–SiC composite exhibits an optimal trade-off between strength and ductility, along with anti-friction and wear resistance which is superior to the reported composites with different solid lubricants. The wear mechanism is attributed to the coordinated deformation mechanism between the CoCrNi FCC matrix and silicides/carbides, and the stabile lubrication effect supported by the structural transformation from the original polycrystalline lattice structure to core-shell nanocomposite structure (similar to diamond-like carbon) of the in-situ graphite. This simple, economical and practical strategy contributes to the development of MPEA self-lubricating composites with excellent comprehensive properties. [Display omitted] [ABSTRACT FROM AUTHOR]