High-entropy Alloy Composites Research Articles

Development of a Multi-phase AlCuTaVW High-Entropy Alloy Using Powder Metallurgy Route and its Mechanical Properties

Equiatomic AlCuTaVW high-entropy alloy (HEA) composition have resulted in a single-phase solid solution with a bcc crystal structure after 25 h of ball milling. Two fcc phases and an ordered B2 phase have evolved during spark plasma sintering at 1523 K. The morphology of sintered disc contains a continuous bright phase and a discontinuous dark phase. The dark phase is detected to be Al-rich. Microhardness of the sintered product is 13 ± 1 GPa, and it is very high compared to other HEAs, conventional ceramics and cermets. A fracture toughness of 8.36 MPa m1/2 is measured from the cracks generated along the edges of Vickers indentation.

Preparation of WC/CoCrFeNiAl0.2 high-entropy-alloy composites by high-gravity combustion synthesis

The WC/CoCrFeNiAl0.2 high-entropy alloy (HEA) composites were prepared through high-gravity combustion synthesis. The preparation method is presented below. First, using a designed suitable multiphase thermite system, the molten CoCrFeNiAl0.2 HEA was fabricated using low-cost metal oxides. The molten HEA was subsequently infiltrated into the WC layer to fabricate WC/CoCrFeNiAl0.2 composites in a high-gravity field. The porosity of the WC/CoCrFeNiAl0.2 composites was down-regulated, and their compressive yield strength was up-regulated when the high-gravity field was increased from 600g to 1500g because this infiltration process of a HEA melt into the WC layer is driven by centrifugal force. The WC particles in the composites exhibited a gradient distribution along the direction of the centrifugal force, which was attributed to the combined action of the high-gravity field and the temperature gradient field. The Vickers hardness of the sample was down-regulated from 9.53 to 7.41 GPa along the direction of the centrifugal force.

Atomic structure revolution and excellent performance improvement of composites induced by laser ultrafine-nano technology

Carbon nanotubes (CNTs) reinforced high-entropy alloy composites (HEACs) were fabricated on a TA2 titanium alloy by mean of a laser melting deposition (LMD) of the FeCoCrAlCu-(nano-SiB6)-(Ni/Ag-coated CNTs) mixed powders, noting that large quantities of the ultrafine nanocrystals (UNs) were formed, portion of them were able to obtain the enormous energy from laser beam, which were able to permeate into the original ordered atom arrangement due to the high diffusion rates and their ultrafine microstructure. In substance, partial UNs can destroy the balanced state of atoms, increasing significantly the free energy of the crystal boundaries. Such CNTs/UNs modified LMD composites showed the better corrosion/high-temperature oxidation resistance properties than those of the TA2 alloy. Identification of the synthetic UNs in such LMD HEACs, more importantly the UNs transition effect, contributes theoretical/experimental basis to upgrade the quality of the industrial light alloy.

Verification and Optimization of Single Phase Solid Solution Rule in High Entropy Alloys

High-entropy alloys (HEAs) with single face-centered cubic (FCC) structures always exhibit excellent ductility, and those with a single body-centered cubic (BCC) structure exhibit high strength. The mixture of FCC and BCC structures contributes to a synergy of strength and ductility. Thus, investigating the formation rules of different structures in HEAs is beneficial for obtaining the desired HEA composition. In this article, several HEAs were designed and prepared to verify the pure single-phase solid-solution rule, i.e., δ 8 for single FCC solid solution and δ 12.47 J mol−1 K−1, while for the other HEAs, the critical values of the rule will be extended.

Microstructure and Strengthening Mechanisms in an Fcc-Structured Single-Phase TiC-CoCrFeCuNiAl <sub>0.3</sub> HEACs with Deformation Twinning

A nonequiatomic CoCrFeCuNiAl0.3 high-entropy alloy composite (HEAC) reinforced with 5vol%TiC nanoparticles was produced by mechanical alloying and spark plasma sintering. X-ray and TEM analysis with microanalysis confirmed the predominance of a nanotwinned single face-centered cubic (FCC) solid solution with TiC nanoparticles. Further findings revealed that the formation of the twin structure in FCC HEAs by the powder metallurgy technology was dependent on the dislocation and the stratified structure formed in particles during MA, low stacking-fault energy γ SF, necessary pressure, and hard ness phase. TiC5vol%- CoCrFeCuNiAl0.3 HEAC alloy had excellent comprehensive mechanical properties, with yield strength, fracture strength, and plastic strain of 1582 MPa, 2185 MPa, and 23.60%, respectively. The dislocation glide and semi-twin expansion in high-entropy alloys provided excellent plasticity, while TiC particles and transgranular twins contributed to the material strength .

Assessment of utilization of ab-initio and Calphad calculations for a design of high-entropy alloy for metal forming

The goal of the current work was to check the possibility of the use of various modelling methods of multicomponent systems calculations, for a design of optimum composition of a new high entropy alloy. Three modelling methods were utilized and discussed in the light of production of an alloy with solid solution of BCC and FCC phases as well as Laves phase what should provide unique properties of this alloy – compared to already proposed high-entropy alloys. The amount of Laves phase should be however strictly controlled and limited due to their low ductility at room temperature. Based on the calculations, optimized composition of Fe19Mn38Ni30Nb7Al3Ti3 system was designed and produced using arc melting. After solution heat treatment the produced specimen was tested using XRD. Based on the comparison of the results, conclusions regarding the capability of presented modelling methods for a design of high entropy alloys for metal forming are drawn.

Microstructure evolution, texture and laser surface HEACs of Al-Mg-Si alloy for light automobile parts

Effect of the rare earth (RE) elements and Cu on the microstructure performance and texture of the hot-rolled Al-Mg-Si alloys during the solution and artificial aging was investigated in this study. The results showed that Cube{100}〈001〉 and CubeND{001}〈310〉 textures were suppressed in a certain extent with the Ti/Zr addition, leading the Copper{112}〈111〉 and the {110}〈011〉 textures to be formed; the texture strength of Cube{100}<001> increased with Ce/Er-added, also the F{111}〈112〉 texture was varied, the strength of {110}//ND decreased. The orientation density of the hot-rolled T6 Al-Mg-Si-Cu alloys increased with the Ce/Er addition, the nucleation mechanism of the re-crystallization was the Cube texture, also the nucleation mechanism of the Cube texture and PSN were weaken. Then, the mixed powders of FeCoCrAlCu (high-entropy alloy powder)-MoSi2-Mn-Sb were deposited on the produced hot-rolled Ce/Er modified Al-Mg-Si-Cu alloy substrate by mean of a laser melting deposition (LMD) technique to form the high-entropy alloy composites (HEACs) to improve the surface performance of alloy substrate. Identification of the synthetic ultrafine nanocrystals (UNs) in LMD HEACs contributes theoretical/experimental basis to improve the quality of the laser 3D print materials. The research on the texture of the hot-rolled Al-Mg-Si-Cu alloy also the Co3Mo2Si UNs in laser-treated HEACs can provide the essential theoretical/experimental basis to improve the quality of the light alloys for the automobile parts.

Development of a novel light weight Al35Cr14Mg6Ti35V10 high entropy alloy using mechanical alloying and spark plasma sintering

A new light-weight high entropy alloy (LWHEA) composition Al35Cr14Mg6Ti35V10 is designed using the thermodynamic parameters of stability with a density of 4.05 g cm−3. The alloy was prepared by mechanical alloying (MA) using a high-energy planetary ball milling and compacted by using spark plasma sintering (SPS). X-ray diffraction (XRD), scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS), and hardness testing were used for analysis of the ball-milled powders and bulk sample. After 30h of milling a single-phase BCC was obtained with some minor impurities of WC. After SPS, the final microstructure was having one HCP and two BCC phases with minor contamination of WC embedded into it. The hardness of a light-weight Al35Cr14Mg6Ti35V10 alloy was found to be 460 HV which is superior to other Al, Mg, and Ti-based conventional alloys.

Powder interface modification for synthesis of core-shell structured CoCrFeNiTi high entropy alloy composite

Interface design is a critical topic for development of multifunctional materials. In an effort to achieve novel microstructural materials, we synthesized CoCrFeNiTi-TiN high entropy alloy composite with core-shell microstructure using powder metallurgy process. Initially single phase CoCrFeNiTi0.5 powder with BCC crystal structure was obtained using mechanical alloying process. Afterwards alloyed powder was heat treated at 900 °C under partial pressure ratios (PN2/PAr) of 1.00, 0.67, 0.33 and 0.10. During the heat treatment the powders mainly consisted of FCC, CrN and TiN phases. At higher values of PN2/PAr, the CrN and TiN phase was were present throughout the powder. By decreasing the PN2/PAr value, nitride precipitation was confined at the powder interfaces. PN2/PAr = 0.10 was the optimum atmosphere conditions where a distinct core-shell structure was formed. At this condition, TiN made up the shell while rest of the elements formed the core. Finally, core-shell powder was consolidated using spark plasma sintering (SPS) to achieve a bulk with core-shell microstructure. This fabrication method reveals a promising avenue to achieve multifunctional high entropy alloys.

Microstructural development in nanostructured AlCoCrFeNi–ZrO2 high-entropy alloy composite prepared with mechanical alloying and spark plasma sintering methods

In this research, ZrO2-reinforced AlCoCrFeNi high-entropy alloy (HEA) composites were fabricated using mechanical alloying (MA) and spark plasma sintering (SPS) methods. It was found that after 30 h of milling a body-centered cubic (BCC) solid solution structure with an average particle size of 54 μm for HEA sample and 46 μm for HEA-10 wt% ZrO2 could be obtained. The addition of 10 wt% ZrO2 particles to Al–Co–Cr–Fe–Ni system resulted in finer particle size during the mechanical alloying process. Consolidation of MA-processed HEA powders with SPS method at 1000 °C was led to the formation of a combination of FCC and BCC phases due to the high heating rate of SPS process and limited time for the completion of BCC to FCC phase transformation. With the aid of Rietveld refinement analysis, it was found that the addition of zirconia particles has a restrictive effect on the grain growth of HEA phases during SPS. Interestingly, it was observed that while the weight percentage of FCC phase was almost similar in HEA and HEA-ZrO2 composites, after the addition of zirconia reinforcements the content of BCC phase was decreased. It is well known that the introduction of hard ceramic particles to metal matrices can enhance the interfaces in the composite samples and facilitate the outward diffusion of aluminum from the BCC phase. Thus, the content of BCC phase was decreased in the HEA-based composite samples compared to the pure HEA sample.

High-throughput synthesis of Mo-Nb-Ta-W high-entropy alloys via additive manufacturing

High-entropy alloys (HEAs) are a class of alloys that can exhibit promising properties including enhanced irradiation resistance, high-temperature strength, and corrosion resistance. However, they exist in a relatively unexplored region of quasi-limitless composition space. Thus, to enable the development of promising compositionally complex alloys, such as HEAs, high-throughput methods are needed. Such high-throughput capabilities are developed and presented in this work. In situ alloying through additive manufacturing was employed to produce arrays of different HEA compositions. Sample arrays were then characterized by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD), all while remaining on the build plate. The top surface of each sample was compositionally homogeneous, as determined by EDS, and each sample exhibited a single-phase, disordered crystal structure, as determined by XRD. CALculation of PHAse Diagrams (CALPHAD) modeling was used to determine the equilibrium phases of each HEA composition at lower temperatures, the results of which were compared with XRD results. Implications of high-throughput synthesis techniques and the coupling of high-throughput characterization and modeling techniques are discussed in the context of alloy development.

Mechanistic origin of high strength in refractory BCC high entropy alloys up to 1900K

The body centered cubic (BCC) high entropy alloys (HEAs) MoNbTaW and MoNbTaVW show exceptional strength retention up to 1900K. The mechanistic origin of the retained strength is unknown yet is crucial for finding the best alloys across the immense space of BCC HEA compositions. Experiments on Nb-Mo, Fe-Si and Ti-Zr-Nb alloys report decreased mobility of edge dislocations, motivating a theory of strengthening of edge dislocations in BCC alloys. The strength of BCC HEAs can be controlled by edge dislocations, especially at high temperatures, because the random field of solutes in these high-concentration alloys creates large energy barriers for thermally-activated edge glide. A parameter-free theory for this mechanism of edge motion in BCC alloys qualitatively and quantitatively captures the strength versus temperature for both MoNbTaW and MoNbTaVW alloys and other BCC HEAs. A reduced analytic version of the theory, embodying the edge-strengthening mechanism, then enables screening over > 600,000 compositions in the Mo-Nb-Ta-V-W family to identify promising new compositions with high retained strength and/or reduced mass density. Overall, the theory reveals an unexpected mechanism - edge dislocation motion in the random alloy - as responsible for high temperature strength in BCC alloys and paves the way for theory-guided design of new high-performance HEAs.

Synthesis and characterization of hydrogenated novel AlCrFeMnNiW high entropy alloy

A novel high entropy alloy (HEA) i.e. AlCrFeMnNiW is synthesized via high-energy planetary ball milling with an average crystallite size of 10.37 nm. The morphology study of hydrogenated and dehydrogenated HEA is carried out through Scanning Electron Microscope (SEM). The HEA is charged with hydrogen using inhouse Sievert's Apparatus which results to be maximum hydrogen storage capacity of 0.615 wt% at atmospheric pressure and temperature. The dehydrogenation of the sample is performed through thermogravimetry (TG) at different scanning rate. The crystalline structure (i.e. lattice parameters) and chemical composition of HEA is studied using X-Ray Diffraction (XRD) and Energy Dispersive X-Ray analysis (EDX) respectively. The unit cell volume of as-prepared alloy is estimated as 0.03131 nm3 whereas the average crystallite size as 10.37 nm. It is observed that the unit cell volume is increased by 0.67% and crystallite size decreased by 10.8% upon hydrogenation whereas it is then decreased by 0.2% and increased by 6.7% respectively upon dehydrogenation. Activation energy during hydrogen desorption is found to be −8.161 kJ/mol. The enthalpy and entropy of the mixing are estimated to be −2.645 kJ/mol and 1.793 R J/mol K.

Tungsten Matrix Composite Reinforced with CoCrFeMnNi High-Entropy Alloy: Impact of Processing Routes on Microstructure and Mechanical Properties

Tungsten heavy alloy composite was developed by using novel CoCrFeMnNi high-entropy alloy as the binder/reinforcement phase. Elemental tungsten (W) powder and mechanically alloyed CoCrFeMnNi high-entropy alloy were mixed gently in high energy ball mill and consolidated using different sintering process with varying heating rate (in trend of conventional sintering &lt; microwave sintering &lt; spark plasma sintering). Mechanically alloyed CoCrFeMnNi high-entropy alloy have shown a predominant face-centered cubic (fcc) phase with minor Cr-rich σ-phase. Consolidated tungsten heavy high-entropy alloys (WHHEA) composites reveal the presence of Cr–Mn-rich oxide phase in addition to W-grains and high-entropy alloys (HEA) phase. An increase in heating rate restricts the tungsten grain growth with reduces the volume fraction of the Cr–Mn-rich phase. Finally, spark plasma sintering with a higher heating rate and shorter sintering time has revealed higher compressive strength (~2041 MPa) than the other two competitors (microwave sintering: ~1962 MPa and conventional sintering: ~1758 MPa), which may be attributed to finer W-grains and reduced fraction of Cr–Mn rich oxide phase.

ICME Design of Corrosion Resistant High Entropy Alloys for Harsh Environments

The integrated computational materials engineering approach is inherently suited to explore the vast, multi-dimensional high entropy alloy (HEA) compositional and processing space, and has been adopted in this work, coupled with empiricism, to design highly corrosion resistant HEAs. Using the combination of empirical and computational approaches, three non-equimolar HEA compositions were identified for their predicted ability to form a single-phase structure and to exhibit high corrosion resistance. Phase diagrams and E-pH Pourbaix diagrams for HEAs were calculated by CALPHAD. One of the HEAs, Ni38Cr21Fe20Ru13Mo6W2, has been successfully synthesized on the lab-scale and homogenized at 1250°C for 120 hours. Exceedingly high corrosion resistance of the Ni-rich HEA was demonstrated by electrochemical testing, including potentiodynamic polarization and electrochemical impedance spectroscopy, even in harsh acidic solutions. The successful validation of the computational design efforts with pre-set property goals shows that ICME is suitable for corrosion resistant HEA design, including complex non-equimolar HEAs.

Enhanced mechanical properties and near unity yield asymmetry in equiatomic high entropy alloy particles reinforced magnesium composites

This work presents the use of equiatomic high entropy alloy (HEA) particles as unconventional reinforcement phase in Mg matrix. Mg-HEA composites were fabricated by powder metallurgy method incorporating microwave sintering and hot extrusion. Recrystallized fine grains (∼50% reduction) were observed in all the composites due to the capability of HEA particles to serve as nucleation sites and for grain boundary pinning following extrusion. Grain refinement coupled with intrinsic high hardness of HEA particles led to an improvement in hardness of 47%, 51% and 54% in Mg-2.5 wt% HEA, Mg-5 wt.% HEA and Mg-7.5 wt% HEA composites, respectively. In comparison with traditional micron size reinforced composites, Mg-HEA composites displayed superior hardness improvement. In addition, HEA reinforcement particles also enhanced both tensile and compressive strengths of magnesium with marginally compromised tensile ductility and marginally enhanced compressive ductility. Relatively strong fiber texture of extruded pure Mg was weakened in the composites due to the presence of HEA particles effectively reducing the yield asymmetry to near unity in Mg-5 wt.% HEA and Mg-7.5 wt% HEA composites.

Prediction of the Composition and Hardness of High-Entropy Alloys by Machine Learning

Machine learning with artificial neural network (ANN)-based methods is a powerful tool for the prediction and exploitation of the subtle relationships between the composition and properties of materials. This work utilizes an ANN to predict the composition of high-entropy alloys (HEAs) based on non-equimolar AlCoCrFeMnNi in order to achieve the highest hardness in the system. A simulated annealing algorithm is integrated with the ANN to optimize the composition. A bootstrap approach is adopted to quantify the uncertainty of the prediction. Without any guidance, the design of new compositions of AlCoCrFeMnNi-based HEAs would be difficult by empirical methods. This work successfully demonstrates that, by applying the machine learning method, new compositions of AlCoCrFeMnNi-based HEAs can be obtained, exhibiting hardness values higher than the best literature value for the same alloy system. The correlations between the predicted composition, hardness, and microstructure are also discussed.

Computational design and initial corrosion assessment of a series of non-equimolar high entropy alloys

The integrated computational materials engineering approach has been employed to design a series of four single-phase non-equimolar high entropy alloys (HEAs) with systematically varied compositions (Ni38Fe20CrxMn21–0.5xCo21–0.5x with x = 6, 10, 14, and 22) and corrosion behavior. The HEAs were successfully designed, synthesized and confirmed to possess a single-phase FCC structure. Preliminary electrochemical corrosion characterization was conducted to gain fundamental understanding of the effects of HEA composition on corrosion resistance, which will be utilized to develop mechanistic corrosion models that enable the optimal design of corrosion resistant HEA. HEA containing 6 at.% Cr showed indications of passivity at a relatively low Cr content.

Effects of Mo Element on Microstructure and Mechanical Properties of CoCrFeMnNi High Entropy Alloys

In order to improve the mechanical properties of CoCrFeMnNi high entropy alloy, CoCrFeMnNiMox high entropy alloy were prepared by adding different content of Mo to re-alloying. The microstructure and phase composition were analyzed by OM, SEM, EDS and XRD, and the mechanical properties were tested by tensile test. The effects of Mo content on the phase composition and microstructure of CoCrFeMnNi high entropy alloy were investigated, and the mechanism of its influence on mechanical properties was studied. The results showed that with the increase of Mo element content, the crystalline morphology of CoCrFeMnNiMox high entropy alloy is change from dendrite crystals to cellular crystal, and the yield strength of the alloy is not change significantly. In the case of Mo content is less than 7%, Mo as substitutional atom is dissolves in the crystal structure of CoCrFeMnNi high entropy alloy substrate, which has function as solid solution strengthening, so that the strength of the alloy increases slightly, but the plasticity are not change significantly. In the case of Mo content is more than 7%, the ζ (CrMo) phase is precipitates in the alloy substrate, which has plays effect of second phase strengthening, so that strength of the alloy is increase significantly, and the plasticity is decreases obviously.

Welding of High Entropy Alloys-A Review.

High-entropy alloy (HEA) offers great flexibility in materials design with 3–5 principal elements and a range of unique advantages such as good microstructure stability, mechanical strength over a broad range of temperatures and corrosion resistance, etc. Welding of high entropy alloy, as a key joining method, is an important emerging area with significant potential impact to future application-oriented research and technological developments in HEAs. The selection of feasible welding processes with optimized parameters is essential to enhance the applications of HEAs. However, the structure of the welded joints varies with material systems, welding methods and parameters. A systemic understanding of the structures and properties of the weldment is directly relevant to the application of HEAs as well as managing the effect of welding on situations such as corrosion that are known to be a service life limiting factor of welded structures in conditions such as marine environments. In this paper, key recent work on welding of HEAs is reviewed in detail focusing on the research of main HEA systems when applying different welding techniques. The experimental details including sample preparation, sample size (thickness) and welding conditions reflecting energy input are summarized and key issues are highlighted. The microstructures and properties of different welding zones, in particular the fusion zone (FZ) and the heat affected zones (HAZ), formed with different welding methods are compared and presented in details and the structure-property relationships are discussed. The work shows that the weldability of HEAs varies with the HEA composition groups and the welding method employed. Arc and laser welding of AlCoCrFeNi HEAs results in lower hardness in the FZ and HAZ and reduced overall strength. Friction stir welding results in higher hardness in the FZ and achieves comparable/higher strength of the welded joints in tensile tests. The welded HEAs are capable of maintaining a reasonable proportion of the ductility. The key structure changes including element distribution, the volume fraction of face centered cubic (FCC) and body centered cubic (BCC) phase as well as reported changes in the lattice constants are summarized and analyzed. Detailed mechanisms governing the mechanical properties including the grain size-property/hardness relationship in the form of Hall–Petch (H–P) effect for both bulk and welded structure of HEAs are compared. Finally, future challenges and main areas to research are highlighted.