Preparation Of High-entropy Alloys Research Articles

Process optimization and microstructure of Ti3Zr1.5NbVAl0.25 high entropy alloy produced by directed energy deposition

As an emerging materials preparation method, directed energy deposition (DED) has its unique advantages in the preparation of high entropy alloys (HEAs). However, improper process parameters can lead to numerous defects, affecting material properties. In this study, a Ti3Zr1.5NbVAl0.25 HEA was produced by DED. The effects of process parameters on single track cladding layer (STCL) were studied by response surface methodology (RSM), and the parameters were optimized accordingly. It is indicated that the microstructure of the STCL is composed of columnar and equiaxed crystals and the grain growth is parallel to the heat flux direction. 〈001〉//BD (building direction), 〈111〉//SD (scanning direction) and {100}〈001〉 textures were observed in molten pool. The microhardness of the molten pool measures 299.99 HV0.2, while the estimated Young's modulus and yield stress are 87.66 GPa and 945 MPa, respectively. This work provides unique insights into optimizing the DED process parameters of HEAs, further obtaining the solidification microstructure and mechanical properties of STCL.

Classical molecular dynamics simulation of atomic structure transitions in FeSiCuMgAl high-entropy alloys under biaxial stretching

Classical molecular dynamics (MD) simulations were utilized in this study to investigate the mechanical properties of the crystal structure of high-entropy alloys under various mechanical stress conditions. The stress-strain behaviors and the merging of the crystal structure growth of FeSiCuMgAl high-entropy alloys during biaxial tensile deformation were simulated. Three stages of atomic structure transformation were observed during biaxial stretching of FeSiCuMgAl high-entropy alloys at a strain rate of 1010 s−1. In the first stage, the structure transitioned from face-centered cubic (fcc) to body-centered cubic (bcc), hexagonal close-packed (hcp), and disordered structures. The second stage involved a transition from bcc, hcp, and disordered structures to fcc. The third and final stage involved a transition from fcc, bcc, and hcp to disordered structure. To gather more information about the simulation process, this research simulated MD by varying the strain rate of tensile deformation. At a strain rate as low as 0.5×109 s−1 and below, there was minimal involvement of the bcc phase in the tensile process. During the simulation work, co-neighborhood analysis, stress-strain analysis, and dislocation analysis were utilized to accurately describe the simulated tensile behavior. The simulation results will provide more insights into the design and preparation of high-entropy alloys as foundational materials.

Corrosion resistance and tribological behavior of FeCoCrNi@GO/Ni high entropy alloy-based composite coatings prepared by electrodeposition

The preparation of high entropy alloy (HEA) coatings onto a stainless-steel substrate is crucial to pursue improved corrosion and wear resistance but are still challenging. Herein, FeCoCrNi@GO/Ni coatings are innovatively cross-integrated with stainless steel by introducing a nickel transition layer to break through the “bottleneck” of stainless steel. Meanwhile, variation of the coating corrosion resistance in 3.5 wt% NaCl solution and tribological behavior against GCr15 steel balls was also deeply explored. The addition of graphene oxide (GO) delayed the movement of corrosive medium and improved the corrosion resistance. Interestingly, the coatings with low GO content and high GO content show different wear mechanisms respectively, and the lower GO content is conducive to the formation of oxidized lubrication layer, thus protecting the substrate from severe friction and wear. However, the high content of GO coating will cause damage and spalling of oxide layer and GO, and the wear mechanism changes from oxidative wear to the co-wear of oxidative wear and abrasive wear. This work will provide a novel idea for the development of HEA coatings in the future.

The short-range ordering and atomic segregation in various phases of high-entropy alloy during the solidification process

High-entropy alloys (HEAs) are highly regarded as promising industrial materials for their unique properties, which are largely determined by their short-range ordering (SRO) and atomic segregation. However, the lack of knowledge on atomic segmentation and SRO during solidification process has hindered the design and synthesis of HEAs. Herein, we conducted molecular dynamics simulations combined with Mont Carlo calculations on FeCoNiCuAl HEA to investigate the SRO and atomic segregation during solidification process. Our findings reveal that the BCC phase exhibit stronger competitiveness during homogeneous crystallization. The SRO and atomic segregation are driven by the enthalpy of mixing for unlike atomic pairs, resulting in an energetically favorable atomic configuration and structure distribution. In the solidified FeCoNiCuAl system, Al-Fe pairs are favored in BCC phase while Co-Ni pairs dominate in the FCC phase. Cu tends to segregate at the edge of the FCC phase without forming intermetallic compounds with other elements. Furthermore, optimized atomic configuration and structure distribution at 700 K and 500 K are not energy-favorable at 300 K, indicating that atomic segregation in the HEA occur throughout the crystallization process. These findings hold significant implications for the design and preparation of HEAs with desirable structures and properties.

Design and Development of High-Entropy Alloys with a Tailored Composition and Phase Structure Based on Thermodynamic Parameters and Film Thickness Using a Novel Combinatorial Target.

This study presents a novel synthesis route for high-entropy alloys (HEAs) and high-entropy metallic glass (HEMG) using radio frequency (RF) magnetron sputtering and controlling the HEA phase selection according to atomic size difference (δ) and film thickness. The preparation of HEAs using sputtering requires either multitargets or the preparation of a target containing at least five distinct elements. In developing HEA-preparation techniques, the emergence of a novel sputtering target system is promising to prepare a wide range of HEAs. A new HEA-preparation technique is developed to avoid multitargets and configure the target elements with the required components in a single target system. Because of a customizable target facility, initially, a TiZrNbMoTaCr target emerged with an amorphous phase owing to a high δ value of 7.6, which was followed by a solid solution (SS) by lowering the δ value to 5 (≤6.6). Thus, this system was tested for the first time to prepare TiZrNbMoTa HEA and TiZrNbMoTa HEMG via RF magnetron sputtering. Both films were analyzed using X-ray diffraction (XRD), X-ray photoelectron spectroscopy, field emission scanning electron microscopy cross-sectional thickness, and atomic force microscopy (AFM). Furthermore, HEMG showed higher hardness 10.3 (±0.17) GPa, modulus 186 (±7) GPa, elastic deformation (0.055) and plastic deformation (0.032 GPa), smooth surface, lower corrosion current density (Icorr), and robust cell viability compared to CP-Ti and HEA. XRD analysis of the film showed SS with a body-centered cubic (BCC) structure with (110) as the preferred orientation. The valence electron concentration [VEC = 4.8 (<6.87)] also confirmed the BCC structure. Furthermore, the morphology of the thin film was analyzed through AFM, revealing a smooth surface for HEMG. Inclusively, the concept of configurational entropy (ΔSmix) is applied and the crystalline phase is achieved at room temperature, optimizing the processing by avoiding further furnace usage.

Preparation of high entropy alloys and application to catalytical water electrolysis

High entropy alloys are considered one of the major breakthroughs in alloying concepts. Their unique design concept and mixed entropy effects make the materials potentially valuable in many applications in various fields. This Research Update summarizes recent research progress on high entropy alloy catalysts in electrolytic water splitting. The definition and properties of high entropy alloys are described and the common preparation methods and organizational characteristics of high entropy alloys are discussed. The catalytic effects of high entropy alloys in water electrolysis are evaluated, and finally, the future development trend and application prospects of high entropy alloys are outlined.

In-situ reactive synthesis and characterization of a high entropy alloy coating by laser metal deposition

In this study, we investigate the influence of in situ reactive synthesis of Ti6Al4V– AlCoCrFeNiCu high entropy alloys by laser metal deposition on the microstructural and mechanical properties of the as-built alloy as opposed to the traditional method of mixing powders via a ball mill prone to contamination and segregation. We explore the capability of a new alloy design by combining two base alloys via in situ reactive alloying, delivering the Ti6Al4V and AlCoCrFeNiCu high entropy alloy powders from multiple powder feeders and regulating their feed rate ratios. The nano mechanical, tribological and microstructural morphologies of the alloys were characterized using a nanoindentation tester, a tribometer, XRD and SEM, respectively. The results showed that satelliting the high entropy alloys powder and the Ti6Al4V powder fraction using double powder feedstock had a homogeneous distribution with dendritic structures. Optimization was achieved at a laser power of 1600 W, a scan speed of 12 mm/s and a powder flow rate of 2 g/min. The surface roughness (Ra) for Ti–6Al–4V, AlCoCrCuFeNi and (Ti–6Al–4V)-(AlCoCrCuFeNi) alloy was 0.5 μm, 0.63 μm and 0.80 μm, respectively. The high wear resistance of the novel Ti6Al4V– AlCoCrFeNiCu alloy was influenced by the hardness of the alloy which was higher than the Ti6Al4V alloy and the AlCoCrFeNiCu alloy. This study successfully defines the capabilities of in situ fabrication of high entropy alloys and presents novel techniques for multiple powder preparation of high entropy alloys using laser additive manufacturing, to permit the next generation of compositionally graded materials for aerospace components.

Effect of raw material forms on the microstructure and mechanical properties of MoNbRe0.5TaW high-entropy alloy

MoNbRe0.5TaW high-entropy alloy with different raw material forms was synthesized using arc-melting technique. The alloy melted with bulks or powders shows different microstructure and mechanical properties. The MoNbRe0.5TaW alloy has a single body-centered-cubic (BCC) solid solution when melting metal bulks. When melting the mixture of metal powders at the same process parameters, this alloy consists of BCC solid solution and oxide particle phase. However, the particle phase disappears and the alloy turns to single BCC phase as increasing the re-melted times. The formation and element segregation of the BCC solid solution match well with the computer-aided thermodynamic calculation. The precipitation of metastable oxide particles is mainly due to the oxidation of powders originated from their high surface energy. The MoNbRe0.5TaW alloy with oxide phase shows high Vickers micro-hardness of 5674 ± 92 MPa, high yield strength of 1145 ± 10 MPa and ultimate compressive strength of 1324 ± 18 MPa. The strength mechanism is Orawan strengthening. This study provides guidance for the design and preparation of high-entropy alloys with high performance.

Laser-Ignited Self-Propagating Sintering of AlCrFeNiSi High-Entropy Alloys: An Improved Technique for Preparing High-Entropy Alloys

AlCrFeNiSi system porous high-entropy alloy material is manufactured by laser ignition and self-propagating sintering of natural chromite powder, which provides the idea of breaking the traditional synthesis procedure of high-entropy alloy compound material. The raw material powder obtained by ball milling is compacted into cylindrical compacts, and the self-propagating reaction comes from the ignition caused by the laser on the surface of compacts, the high-entropy alloy composite of chrome iron powder synthesized by laser sintering, is obtained as well. The raw material is prepared from Al, Cr, Fe, Ni and Si elements with similar effective components of natural chromite powder. The selected chromite powder is energy-saving and environment-friendly, so the preparation of high-entropy alloy by the low-cost short-process can be made for processing for pre-theoretical reserve and process design. The effect of Si content on microstructure and properties of AlCrFeNiSi high-entropy alloy is investigated.

Properties and Preparation of High Entropy Alloys

As a consequence of multi-components, the high entropy alloys embodied serious cocktail party effect and lattice distortion. So high entropy alloys have high strength and hardness possess many comprehensive properties such as thermostability and corrosion resistance. Because of excellent mechanical properties and chemical properties, high entropy alloys have immeasurable potential of development. This paper mainly introduces the properties, preparations and applications of high entropy alloys, and finally summarizes them.

Properties and Preparation of High Entropy Alloys

As a consequence of multi-components, the high entropy alloys embodied serious cocktail party effect and lattice distortion. So high entropy alloys have high strength and hardness possess many comprehensive properties such as thermostability and corrosion resistance. Because of excellent mechanical properties and chemical properties, high entropy alloys have immeasurable potential of development. This paper mainly introduces the properties, preparations and applications of high entropy alloys, and finally summarizes them.

Production of AlCrNbTaTi High Entropy Alloy via Electro-Deoxidation of Metal Oxides

High entropy alloys (HEAs) have attracted increasing attention because of their outstanding properties for various applications. During the preparation of HEAs, however, high-purity metals are usually used as raw material, which limits the large-scale application of HEAs. In this work, a simple electrochemical method is demonstrated to achieve homogeneous AlCrNbTaTi HEAs directly from metal oxide mixtures, and meanwhile the cathodic reduction mechanism is discussed. The patterns of both X-ray diffraction and selected area electron diffraction show that the as-prepared AlCrNbTaTi HEAs present body centered cubic structure. Upon electrolysis for 12 h, the oxygen content of the product is ∼0.6 wt% and the cathodic current efficiency is ∼46.7%. The results also suggest that the cathodic reduction process obeys a three-phase interline propagation mechanism, with the pellet surface initially metallized.

Microwave processing of high entropy alloys: A powder metallurgy approach

Microwaves at the ISM frequency of 2450 and 5800MHz have been exploited to prepare FeCoNiCrAl-family high entropy alloys by direct heating of pressed mixtures of metal powders. The aim of this work is to explore a new microwave assisted near-net-shape technology, using powder metallurgy approach for the preparation of high entropy alloys, able to overcome the limits of current melting technologies (defects formation) or solid state ones (time demanding). Results show that direct microwave heating of the powder precursors occurs, and further heating generation is favored by the ignition of exothermal reactions in the compound. Microwave processing, exploited both for the ignition and sustaining of such reactions, has been compared to reactive sintering in laboratory furnace and mechanical alloying in a planetary ball milling. Results demonstrate that microwave required the shortest time and lowest energy consumption, thus it is promising time- and cost-saving synthetic route.

Microwave-Assisted Preparation of High Entropy Alloys

Microwaves at the ISM (Industrial, Scientific and Medical, reserved internationally) frequency of 2450 or 5800 MHz have been used to prepare FeCoNiCuAl, FeCrNiTiAl and FeCoCrNiAl2.5 high entropy alloys by direct heating of pressed mixtures of metal powders. The aim of this work is to explore a new microwave-assisted near-net-shape technology, using a powder metallurgy approach for the preparation of high entropy alloys, able to overcome the limits of current melting technologies (defects formation) or solid state ones (time demanding). High entropy alloy compositions have been selected so as to comprise at least one ferromagnetic element and one highly reactive couple, like Ni-Al, Ti-Al, Co-Al or Fe-Al. Results show that direct microwave heating of the powder precursors occurs, and further heating generation is favored by the ignition of exothermal reactions in the load. Microwaves have been applied both for the ignition and sustaining of such reactions, showing that by the proposed technique, it is possible to control the cooling rate of the newly-synthesized high entropy alloys. Results showed also that microwave heating in predominant magnetic field regions of the microwave applicator is more effective at controlling the cooling rate. The herein proposed microwave-assisted powder metallurgy approach is suitable to retain the shape of the load imparted during forming by uniaxial pressing. The homogeneity of the prepared high entropy alloys in all cases was good, without the dendritic segregation typical of arc melting, even if some partially-unreacted powders were detected in the samples.