Entropy Alloys Research Articles

Atomic-scale insight on nanostructures and electrochemical properties of (NiCrCo)82Ti9Al9 high-entropy alloy coatings by density functional theory calculations

(NiCrCo)82Ti9Al9 high−entropy alloy was attached to act as coating material by combining of laser cladding and laser remelting. In this study, the electrochemical properties of obtained HEA coatings were comparatively investigated using an electrochemical workstation, and the corrosion mechanism was revealed by the density functional theory calculations. The results show that the laser cladded HEA coating is mainly consisted of face-centered and body-centered cubic structures, and the face-centered cubic structure of the laser cladded HEA coating is transferred to the amorphous structure by laser remelting, which provides the stronger anti−corrosion capability compared with the face-centered cubic structure of HEA coating.

Characteristic deformation microstructure evolution and deformation mechanisms in face-centered cubic high/medium entropy alloys

Face-centered cubic (FCC) high/medium entropy alloys (HEAs/MEAs), novel multi-principal element alloys, are known to exhibit exceptional mechanical properties at room temperature; however, the origin is still elusive. Here, we report the deformation microstructure evolutions in a tensile-deformed Co20Cr40Ni40 representative MEA and Co60Ni40 alloy, a conventional binary alloy for comparison. These FCC alloys have high/low friction stresses, fundamental resistance to dislocation glide in solid solutions, respectively, and share similar other material properties, including stacking fault energy. The Co20Cr40Ni40 MEA exhibited higher yield strength and work-hardening ability than in the Co60Ni40 alloy. Deformation microstructures in the Co60Ni40 alloy were marked by the presence of coarse dislocation cells (DCs) regardless of grain orientation and a few deformation twins (DTs) in grains with the tensile axis (TA) near <1 1 1>. In contrast, the MEA developed three distinct deformation microstructures depending on grain orientations: fine DCs in grains with the TA near <1 0 0>, planar dislocation structure (PDS) in grains with other orientations, and a high density of DTs along with PDS in grains oriented <1 1 1>. Three-dimensional electron tomography revealed that PDS in the MEA confined dislocations within specific {1 1 1} planes, indicating suppression of cross-slip of screw dislocations and dynamic recovery. In-situ X-ray diffraction during tensile deformation showed a higher dislocation density in the MEA than in the Co60Ni40 alloy. These findings demonstrate that FCC HEAs/MEAs with high friction stresses naturally develop unique deformation microstructures which is beneficial for realizing superior mechanical properties compared to conventional materials.

Pursuing ultrahigh strength–ductility CoCrNi-based medium-entropy alloy by low-temperature pre-aging

Developing high-performance alloys with gigapascal strength and excellent ductility is crucial for modern engineering applications. The concept of multi-component high/medium entropy alloys (H/MEAs) provides an innovative approach to designing such alloys. In this work, we developed the Co1.5CrNi1.5Al0.2Ti0.2 MEA, which exhibits outstanding mechanical properties at room temperature through low-temperature pre-aging followed by annealing treatment. Tensile testing reveals that the MEA possesses an ultrahigh yield strength of 20±0785 MPa, an ultimate tensile strength of 2365 ± 70 MPa, and exceptional ductility of 15.8% ±1.7%. The superior tensile properties are attributed to the formation of fully recrystallized heterogeneous structures (HGS) composed of ultrafine grain (UFG) and fine grain (FG) regions, along with discontinuous precipitation of coherent nano-size lamellar L12 precipitates. The mechanical incompatibility between the UFG region and the FG regions during deformation induces the accumulation of a large number of geometrically necessary dislocations at the interface, resulting in strain distribution and hetero-deformation-induced (HDI) stress accumulation, contributing significantly to HDI strengthening. HDI strengthening, precipitation strengthening, and grain boundary strengthening are the primary mechanisms responsible for the ultra-high yield strength of the MEA. During deformation, the dominant deformation mechanisms include dislocation slip, deformation-induced stacking faults, and Lomer–Cottrell locks, with minor deformation twinning. The synergistic interaction of these multiple deformation modes provides the MEA with excellent work hardening capability, delaying plastic instability and achieving an excellent combination of strength and ductility. This study provides an effective strategy for synergistically strengthening MEAs by combining HDI strengthening with traditional strengthening mechanisms. These findings pave the way for the development of advanced structural materials with high performance tailored for demanding applications in engineering.

Effect of hot-rolling process on the microstructure, mechanical and corrosion behaviors of dual-phase Co-based entropic alloys

Two compositions from dual-phase Co-based entropic alloys with high corrosion resistance were chosen, and the effect of hot-rolling process on the microstrcture, mechanical property and corrosion resistance was investigated in this work. The processing parameters were designed and optimized by CALPHAD calculations. For the case of hot-rolled alloys, fcc + hcp dual-phase structures were confirmed by different characterization techniques, including XRD, ECCI, and EBSD. After testing various mechanical properties and electrochemical corrosion behaviors at room temperature, the hot-rolled alloys exhibit an outstanding mechanical and corrosion resistance property. The hot-rolling process improves the electrochemical corrosion resistance slightly and promotes hardness as well as strength-ductility greatly. The experimental characterizations provide plentiful information about microstructure, crystallographic orientation, lattice misorientation, grain boundaries, and so on. More details for the strengthening and hardening mechanisms could be obtained from the EBSD analysis. The current work shed lights on the design of new alloy recipe as well as the synthesis of novel grade dual-phase entropic alloys with excellent combination of mechanical properties and corrosion resistance which could be applied in harsh environments.

EFFECT OF STRAIN PATH ON MICROSTRUCTURE AND MECHANICAL PROPERTIES IN COLD - ROLLED [FeNi]69Cr15Mn10Nb6 HIGH - ENTROPY ALLOY

[FeNi]69Cr15Mn10Nb6 high - entropy alloys (HEAs) were subjected to heavy cold-rolling process at ambient temperature through two distinct routes, namely unidirectional cold-rolling (UCR) and multistep cross cold-rolling (MSCCR) routes, to investigate the effect of the strain path on the microstructure and mechanical properties of the alloys. In the MSCCR route, the specimen rotated 90° around the normal direction axis between each cycle. The as-homogenized specimen revealed a two-phase microstructure consisting of FeNiNb - rich dendrites distributed in a nearly homogenous face-centered cubic (FCC) HEA matrix. Here, FeNiNb - rich dendrites were elongated,broken down, and stretched along the rolling direction during both processing routes. In the MSCCR - processed specimen, the dendrites exhibited a smaller average size with a more uniform distribution compared to that of the UCR specimen, which could be mainly ascribed to the elongation mechanisms in the normal and transverse directions during the MSCCR route. These microstructural features, introduced through MSCCR, appeared to be most prone to shear band formation and grain refinement. The MSCCR - processed specimen shows a microhardness of 445 HV, a tensile strength of 1245 MPa, and a remarkable elongation of 16 % at ambient temperature, outperforming the UCR-processed specimen with a microhardness of 418 HV, a tensile strength of 1180 MPa, and an elongation of 14 %. The superior mechanical properties of the MSCCR - processed specimen are attributed to its refined microstructure,increased dislocation density, and uniform distribution of FeNiNb - rich dendrites within the microstructure.

Predicting Yield Strength and Plastic Elongation in Body-Centered Cubic High-Entropy Alloys.

We employ machine learning (ML) to predict the yield stress and plastic strain of body-centered cubic (BCC) high-entropy alloys (HEAs) in the compression test. Our machine learning model leverages currently available databases of BCC and BCC+B2 entropy alloys, using feature engineering to capture electronic factors, atomic ordering from mixing enthalpy, and the D parameter related to stacking fault energy. The model achieves low Root Mean Square Errors (RMSE). Utilizing Random Forest Regression (RFR) and Genetic Algorithms for feature selection, our model excels in both predictive accuracy and interpretability. Rigorous 10-fold cross-validation ensures robust generalization. Our discussion delves into feature importance, highlighting key predictors and their impact on mechanical properties. This work provides an important step toward designing high-performance structural high-entropy alloys, providing a powerful tool for predicting mechanical properties and identifying new alloys with superior strength and ductility.

Enhanced Strength–Ductility Combination in Laser Welding of CrCoNi Medium-Entropy Alloy with Ultrasonic Assistance

The welded joints of high/medium entropy alloys (H/MEAs) have shown sound mechanical properties, indicating high promise for the industrial application of this new type of metal alloy. However, these joints possess either relatively low strength or low ductility. In this paper, we used ultrasonic-assisted laser welding to weld CrCoNi MEA with the nitrogen as shielding gas. The results showed that the tensile strength of the joint at room and cryogenic temperature is 686 MPa and 1071 MPa, respectively. The elongation at room and cryogenic temperature is 26.8% and 27.7%, respectively. The combination of the strength and ductility in our joints exceeds that of other welded H/MEA joints. We attributed this excellent combination to the refined dendrite, the solution of nitrogen into the matrix, and the low stacking fault energy of the CrCoNi MEA. The findings in this paper not only provide a novel way to weld H/MEAs with high strength and ductility, also are useful for additively manufacturing the high-performance component of H/MEAs.

Medium entropy alloy-induced strong size dependence in the strengthening and shear instability of nanolayered metallic composites

Medium/high entropy alloys (M/HEAs)-based nanolayered metallic composites have attracted intensive scientific interests due to their superior mechanical properties. However, the shear instability of the M/HEAs-based composites is rarely studied and the underlying mechanism remains uncovered. A combination of theoretical analysis and nano/microindentation tests was conducted to investigate the size-dependent strengthening and shear instability of Cu/CrCoNi composites. The results show that the shear banding-induced instability of the composites exhibit a clear transition from layer interfaces-mediated kinking to grain boundaries-accommodated one at a critical layer thickness of 25 nm, which apparently differs from those in traditional M/HEAs-free composites. The sudden change in the deformation mode originates from the MEA-induced size-dependent microstructure transformation, i.e., from horizontally aligned layer interfaces at large layer thicknesses to vertically aligned grain boundaries at small ones. The size-dependent strengthening and plasticity can be respectively captured by the confined layer slip model and the proposed theoretical model.

Architecture design and strengthening-toughening mechanisms in heterogeneous-structured medium/high entropy alloys

Architecture design and strengthening-toughening mechanisms in heterogeneous-structured medium/high entropy alloys

First-principles and cluster expansion study of the effect of magnetism on short-range order in Fe–Ni–Cr austenitic stainless steels

Short-range order (SRO), the regular and predictable arrangement of atoms over short distances, alters the mechanical properties of technologically relevant structural materials such as medium/high entropy alloys and austenitic stainless steels. In this study, we present a generalized spin cluster expansion (CE) model and show that magnetism is a primary factor influencing the level of SRO present in austenitic Fe–Ni–Cr alloys. The spin CE consists of a chemical cluster expansion combined with an Ising model for Fe–Ni–Cr austenitic alloys. It explicitly accounts for local magnetic exchange interactions, thereby capturing the effects of finite temperature magnetism on SRO. Model parameters are obtained by fitting to a first-principles data set comprising both chemically and magnetically diverse face-centered cubic configurations. The magnitude of the magnetic exchange interactions are found to be comparable to the chemical interactions. Compared to a conventional implicit magnetism CE built from only magnetic ground state configurations, the spin CE shows improved performance on several experimental benchmarks over a broad spectrum of compositions, particularly at higher temperatures due to the explicit treatment of magnetic disorder. We find that SRO is strongly influenced by alloy Cr content, since Cr atoms prefer to align antiferromagnetically with nearest neighbors but become magnetically frustrated with increasing Cr concentration. Using the spin CE, we predict that increasing the Cr concentration in typical austenitic stainless steels promotes the formation of SRO and increases order–disorder transition temperatures. This study underscores the significance of considering magnetic interactions explicitly when exploring the thermodynamic properties of complex transition metal alloys. It also highlights guidelines for customizing SRO through adjustments of alloy composition.

Enhancing the plasticity of HfZrTiCr x alloys by forming single phase solid solutions via reducing Cr content

ABSTRACT The solid solution design strategy bolsters the comprehensive mechanical properties of high/medium entropy alloys (H/MEAs), yet its utilisation within hexagonal close-packed (HCP) H/MEAs remains notably limited. Here, exploring the impact of reduced Cr content (x = 0.3, 0.1, and 0) from the HfZrTiCr0.5 prototype alloy, our study successfully obtained a novel HCP single phase MEA and investigated the relationship between microstructure and mechanical properties. Decreasing Cr content led to the diminishing presence of the Laves phase, culminating in a single HCP phase solid solution at x = 0.1. The Laves phase, arising from negative enthalpy between Cr and other elements and lower atomic radius of Cr, acted as crack initiation sites, resulting in brittle fracture (x = 0.5) alloy and limited plasticity strain of 3.5% (x = 0.3). Conversely, the HfZrTiCr0.1 alloy exhibited impressive combined yield strength (1360 MPa) and plasticity strain (14.5%), attributed to prominent solid solution strengthening and enhanced dislocation movement within the solid solution structure. These findings offer insights into manipulating Cr content to form HCP solid solution structures, potentially broadening solid solution alloy design strategies for superior mechanical properties in HCP H/MEAs.

Shear banding suppression in Cu/TiZrNb nanolayered composites through dual crystalline/amorphous interfaces

Medium/high entropy alloys (M/HEAs)-based nanolayered composites have attracted intense scientific interest due to the improved mechanical properties. However, the shear banding (or shear instability) behavior of the M/HEAs-based nanolayered composites is rarely studied due to the lack of theoretical model to quantify the shear banding-induced strain localization. Herein, the size-dependent strengthening and shear instability behavior of Cu/amorphous TiZrNb nanolayered composites with individual layer thickness (h) of 2–100 nm was systematically investigated by using a combination of nano/microindentations and a detailed theoretical analysis. The results show that the strength of the Cu/TiZrNb composites obeys the trend of “smaller is stronger”. The size-dependent strengthening behavior of the samples with h in the range of 10–100 nm can be well described by the confined layer slip model, while the continuous increase in strength of those with h = 2 and 5 nm is related to the gradually amorphization of the Cu layers. It is intriguingly found that the shear instability of the Cu/TiZrNb composites shows a first-decrease-and-then-increase size dependence, which is apparently different from that of existing face-centered cubic/amorphous nanolayered composites that exhibits a monotonic size dependence. The non-monotonic trend leads to a significantly suppressed shear instability in the sample with h = 50 nm, in which only slight kinks are observed in microindentation. The enhanced resistance to shear banding is attributed to the dual crystalline/amorphous interfaces generated by the layered structure and the amorphous TiZrNb constituent layers that contains crystalline phase at h > 10 nm.

Pursuing ultrastrong and ductile medium entropy alloys via architecting nanoprecipitates-enhanced hierarchical heterostructure

Nanoprecipitate-strengthened medium/high entropy alloys (M/HEAs) with ultrahigh strengths and unusual physical properties represent promising candidates for structural applications. However, further strengthening these materials usually relies on increasing the volume fraction of nanoprecipitates and/or introducing harder intermetallics that inevitably sacrifice the ductility. Here, we report a strategy to break this trade-off in a Co-Cr-Ni-based MEA by architecting a hierarchical heterostructure that integrates high-volume fraction (up to ∼30%) D022-γ’’ nanoprecipitates, shearable and non-shearable intermetallic particles, with a low stacking fault energy (SFE) matrix with heterogeneous grain structure. This hierarchical heterostructure can arouse hybrid strengthening mechanisms and lead to cross-scale and dynamic strain partitioning during plastic deformation, contributing to a superior strain-hardening capability and an excellent combination of the yield / ultimate tensile strength of 1323 MPa / 1690 MPa and 29% tensile elongation. This study provides a promising paradigm to design ultrastrong and ductile metallic materials via controlling multiscale microstructural heterogeneities.

Effect of He-ions irradiation on the microstructure and mechanical properties of TiTaNbZr refractory medium-entropy alloy film

In this work, TiTaNbZr refractory medium-entropy alloy (RMEA) films were irradiated by 60 keV helium (He) ions at the fluence of 5 × 1016 cm−2 - 2 × 1017 cm−2. After irradiation, a structural transformation of amorphous-BCC single phase, namely self-healing behavior, in TiTaNbZr RMEA films was revealed on account of the enhanced diffusion and redistribution of atoms, which could also lead to a decrease of defects. In this process, increased grain boundaries, acting as effective defect sinks, have inhibited the He bubbles within crystals. The short-range migration of atoms did not destroy the uniformity of elements dramatically. In addition, the correlation between the structural transformation and mechanical properties evolution in the RMEA films was also further clarified. No degradation of mechanical properties even for irradiation fluence as high as 2 × 1017 cm−2 due to the increased grain boundaries and self-healing mechanism. These results all indicate that the TiTaNbZr RMEA have outstanding irradiation resistance and potential applications in reactor structural materials. Meanwhile, the microstructural transformation and performance evolution of the TiTaNbZr RMEA explored in this study could provide more understanding of irradiation effect and a new perspective for the development of the alloys with self-healing ability in refractory high/medium entropy alloys (RHEAs/MEAs).

Enhanced strength-ductility synergy in high-entropy alloys via architecting three-level gradient hierarchical nanostructure

Gradient microstructural design represents a new strategy for enhancing strength-ductility synergy of high/medium entropy alloys (H/MEAs). However, most of the reported gradient H/MEAs have only one type of gradient microstructure, which has limited ability to realize the full potential of gradient structure. Here, we report a three-level gradient hierarchical nanostructure (GHN) in a Al0.3CoCrFeNi HEA by ultrasonic surface rolling processing (USRP) followed by aging treatment that can result in a six-fold enhancement in yield strength with almost no loss in ductility compared to the coarse structured counterpart. The proposed three-level GHN is characterized with gradient variations in grain size, nano-twin and dislocation density, B2 and σ precipitates content. Systematic experiments indicate that strong hetero-deformation-induced (HDI) hardening associated with the activation of deformation twinning, are responsible for this exceptional strength-ductility synergy. This three-level GHN engineering strategy in this work can also be feasibly applied to many other H/MEAs, and offers an available pathway to develop high strong and ductile metallic materials.

High-strength medium-entropy alloy designed by precipitation-strengthening mechanism via machine learning

Precipitation-hardened high/medium entropy alloys (PH-HEAs/MEAs) have attracted extensive interest due to their excellent mechanical properties at both room and high temperatures. Strengthening mechanism models can be used to guide the discovery and design of new alloys, however, the parameters of the models are often lacking, limiting their use for compositional screening from a large space. We proposed a strategy combining machine learning with precipitation strengthening mechanism for high-performance PH-HEAs/MEAs. Using this strategy, we successfully synthesized an alloy Al5Ti8Co40Fe7Ni40 with a yield strength approaching 1.15 GPa, 10% higher than the best value of the existing AlTiCoCrFeNi HEAs/MEAs strengthened by L12-type nanoprecipitates. The contribution of precipitation strengthening from the L12 phase is approximately 717 MPa. Our strategy that unites the strengthening mechanism and machine learning may inspire the rapid design and discovery of precipitation-hardened alloys.

Optimal corrosion resistance of cobalt-based dual-phase entropic alloys via compositional modulation of aluminum

A new category of Al-doped cobalt-based dual phase entropic alloys with high corrosion resistance properties was designed in the present work. CALPHAD (CALculation of PHAse Diagram) aided prediction was performed to facilitate the alloy design. Two groups of alloys have been extensively investigated by various techniques. One group was quenched in water from the homogenization temperature (i.e. 1200 °C) to room temperature. The other group was subsequently quenched in liquid nitrogen from room temperature. The microstructure of the proposed alloys was characterized by X-ray diffraction (XRD). Several kinds of electrochemical tests were performed to study the corrosion behaviors of the designed alloys in NaCl solution, including open circuit potential (OCP), polarization and cyclic polarization curves, electrochemical impedance spectroscopy (EIS), electrochemical noise (ECN), linear voltammetry curves and staircase voltammetry scan, and Mott–Schottky method. The obtained results show that Al has a minor influence on the corrosion property of this series alloy with a 0 to 2 at.% Al addition. All the designed alloys hold quite acceptable corrosion resistance. In addition, a mechanical property, i.e. micro-hardness, is also provided as a supplement information. This work sheds light on the correlation between computational thermodynamics and corrosion behaviors in cobalt-based dual-phase entropic alloys, as well as to provide a feasibility study on the important role of CALPHAD method for a rapid development of a new category of high performance alloys.

Effect of local chemical order on the irradiation-induced defect evolution in CrCoNi medium-entropy alloy

High- (and medium-) entropy alloys have emerged as potentially suitable structural materials for nuclear applications, particularly as they appear to show promising irradiation resistance. Recent studies have provided evidence of the presence of local chemical order (LCO) as a salient feature of these complex concentrated solid-solution alloys. However, the influence of such LCO on their irradiation response has remained uncertain thus far. In this work, we combine ion irradiation experiments with large-scale atomistic simulations to reveal that the presence of chemical short-range order, developed as an early stage of LCO, slows down the formation and evolution of point defects in the equiatomic medium-entropy alloy CrCoNi during irradiation. In particular, the irradiation-induced vacancies and interstitials exhibit a smaller difference in their mobility, arising from a stronger effect of LCO in localizing interstitial diffusion. This effect promotes their recombination as the LCO serves to tune the migration energy barriers of these point defects, thereby delaying the initiation of damage. These findings imply that local chemical ordering may provide a variable in the design space to enhance the resistance of multi-principal element alloys to irradiation damage.

OVERVIEW OF THE HIGH-ENTROPY ALLOYS CONCEPT

The article presents a review of the results of research on high-entropy alloys, describing the principles of their formation, the basic concepts and properties of high-entropy alloys. The existing categories of the entropic alloys are listed. The effects resulting from the formation of high-entropy alloys are described: high entropy, lattice distortion, sluggish diffusion and cocktail effects. It is noted that the traditional thermodynamic representations for multicomponent alloys require additions. It is also noted that the application of Hume-Rothery rules to predict the phase composition of high-entropy alloys shows some difficulties, since it is difficult to select a large number of elements having the same type of lattice and valence. The results of the analysis of a number of parameters and conditions which, according to the researchers' opinion, affect the structural state created by high-entropy alloys, taking into account which could allow to correctly predict the formation of structures in high-entropy. Analysis of the literature data has shown that at present there is no universal parameter that could allow the correct prediction of the formation of structures in multicomponent alloys systems. Methods for the preparation of powders of high entropy alloys are presented. The results of frequently used methods of obtaining coatings on the basis of high-entropy alloys, such as laser cladding, magnetron sputtering, electrochemical deposition and thermal spraying are reviewed. Disadvantages of obtaining coatings methods are described.

Microstructural evolution and phase stability in semi-Heusler NiMnSb and vanadium added equi-atomic NiMnSbV alloys

Phase and microstructure evolution in semi-Heusler NiMnSb and vanadium added quaternary equi-atomic NiMnSbV alloys has been studied in the as solidified state. In the as solidified NiMnSb alloy, cubic semi-Heusler phase and in the as solidified NiMnSbV alloy, Frank-Kasper type cubic SbV3 phase along with the cubic semi-Heusler NiMnSb phase are observed. Nucleation and growth of SbV3 phase takes place in the alloy through solute rejection mechanism, and at the interface of SbV3 and NiMnSb semi-Heusler phase, lamellar structure is observed. While SbV3 phase is stable upto 700 °C, the semi-Heusler NiMnSb phase is not stable beyond 400 °C and it transforms to hexagonal (Ni/Mn)Sb phase. Line defects are observed in the NiMnSb semi-Heusler phase. However, extensive twin like defects are observed in the cubic SbV3 phase. In the SbV3 phase edge sharing 12-fold coordinated icosahedral clusters and 14-fold coordinated clusters are present. Atomic size difference of Sb and V indicates that the clusters are distorted, which might be the viable cause behind the formation of such kind of compound deformation twins. The phase formation and stability in these two alloys has been studied through Miedema’s model. It is observed that apart from atomic size difference, enthalpy of mixing; the ternary and higher order self-interactions, electronic contribution to enthalpy also play an important role. Systematic substitution in the lattice of semi-Heusler NiMnSb phase based on enthalpy of mixing and atomic radius may not be the design strategy for development of semi-Heusler NiMnSb based single phase multicomponent/high entropy alloys.