New Advances in High-Entropy Alloys

Synthesis of a novel AlBeSiTiV light weight HEA coating on SS316 using atmospheric plasma spray process

High Entropy Alloys (HEAs) are currently a subject of significant research interest in the fields of materials science and engineering. They are rapidly evolving due to their exceptional properties, and there is considerable focus on expanding their application potential by developing HEA coatings on various substrate materials. This area of study holds promise for advancing technology and innovation in diverse industries. In this study, a novel equiatomic AlBeSiTiV Light Weight HEA was synthesized via mechanical alloying and was sprayed on the substrate SS316 by the thermal spray process. The microstructural characterization revealed that synthesized HEA had a major FCC phase and the average coating thickness was observed to be 150 μm. The average microhardness was measured to be 975 ± 13 HV for the coating which was five times than the substrate. The coated samples' wear resistance was found out using a pin-on-disc apparatus by varying the wear process parameters and Taguchi's L27 Orthogonal Array was used to interpret the parametric influence on wear rate. ANOVA and regression analysis revealed applied load to be the most significant factor followed by distance and velocity. The major wear mechanisms observed were adhesion abrasion and oxidation, and the formation of tribolayer was observed at higher velocity and distance.

Cast Microstructure of a Complex Concentrated Noble Alloy Ag20Pd20Pt20Cu20Ni20

A complex concentrated noble alloy (CCNA) of equiatomic composition (Ag₂₀Pd₂₀Pt₂₀Cu₂₀Ni₂₀-20 at. %) was studied as a potential high-performance material. The equiatomic composition was used so that this alloy could be classified in the subgroup of high-entropy alloys (HEA). The alloy was prepared by induction melting at atmospheric pressure, using high purity elements. The degree of metastability of the cast state was estimated on the basis of changes in the microstructure during annealing at high temperatures in a protective atmosphere of argon. Characterisation of the metallographically prepared samples was performed using a scanning electron microscope (SEM) equipped with an energy dispersive spectrometer (EDS), differential scanning calorimetry (DSC), and X-ray diffraction (XRD). Observation shows that the microstructure of the CCNA is in a very metastable state and multiphase, consisting of a continuous base of dendritic solidification-a matrix with an interdendritic region without other microstructural components and complex spheres. A model of the probable flow of metastable solidification of the studied alloy was proposed, based on the separation of L-melts into L₁ (rich in Ni) and L₂ (rich in Ag). The phenomenon of liquid phase separation in the considered CCNA is based on the monotectic reaction in the Ag-Ni system.

Corrosion Behavior of CrFeCoNiVx (x = 0.5 and 1) High-Entropy Alloys in 1M Sulfuric Acid and 1M Hydrochloric Acid Solutions

CrFeCoNiV_x high-entropy alloys were prepared by arc-melting, and the microstructures and corrosion properties of these alloys were studied. The CrFeCoNiV_0.5 alloy had a granular structure; the matrix was a face-centered cubic (FCC) structure, and the second phase was a σ phase with a tetragonal structure. The CrFeCoNiV alloy had a dendritic structure; the dendrites in this alloy showed an FCC phase, and the interdendrities had a eutectic structure of FCC and σ phases. Therefore, CrFeCoNiV was much harder than the CrFeCoNiV_0.5 alloy due to the dendritic structures. The potentiodynamic polarization test and electrochemical impedance spectroscopy were used to evaluate the corrosion behavior of the CrFeCoNiV_x high-entropy alloys in deaerated 1M sulfuric acid and 1M hydrochloric acid solutions. The results indicated that the CrFeCoNiV_0.5 alloy had a better corrosion resistance because of the granular structure.

[(AlxTi1−x)-(FeCoNi)12](AlxTi1−x)0.5Cr2.5 High-Entropy Alloy Coating by Laser Cladding

To prolong the service life of the stirrer impeller made by SUS 904L austenitic super-stainless steel, a series of [(AlxTi1−x)-(FeCoNi)12](AlxTi1−x)0.5Cr2.5 high-entropy alloy (HEA) compositions were designed based on the cluster-plus-glue-atom model. The HEAs’ coatings were successfully fabricated by laser cladding technology. The microstructure, microhardness, wear resistance and corrosion resistance were measured by a scanning electron microscope, transmission electron microscope, microhardness tester, wear machine and electrochemical workstation, respectively. The experimental results indicate that the phase structures of the [(AlxTi1−x)-(FeCoNi)12](AlxTi1−x)0.5Cr2.5 (x = 0, 0.5, 1) HEA coatings mainly consist of a single face-centered-cubic solid solution and the coatings produce BCC phase with the increase of Ti content. When x = 0, the coating has the highest hardness (402.3 HV0.2) which is 1.92 times that of 904L austenitic super-stainless steel (209.0 HV0.2), the lowest wear volume (0.866 mm3) and the best corrosion resistance. The addition of Ti refined the microstructure of the coatings and promoted the formation of BCC phase, which improved the hardness and wear resistance of the coatings. Considering the wide sources of Ti, Fe and Co elements and the convenience of laser cladding, the coating can provide a cheap protective layer for 904L stainless steel.

The Effects of Niobium and Molybdenum on the Microstructures and Corrosion Properties of CrFeCoNiNbxMoy Alloys

The present work systematically investigated the effects of niobium and molybdenum on the microstructures and corrosion properties of high-entropy CrFeCoNiNb_xMo_x and CrFeCoNiNb_xMo_1−x alloys, the maximum content of (Nb + Mo) was 20 at.%. All of the alloys were prepared by arc melting under an argon atmosphere. In CrFeCoNiNb_xMo_x alloys (x = 0.15, 0.3 and 0.5), increasing Nb and Mo content would change the microstructure of the alloy from a hypoeutectic structure (x ≤ 0.3) to a hypereutectic one (x = 0.5). All of the CrFeCoNiNbxMo_1−x alloys (x = 0.25, 0.5 and 0.75) had a hypereutectic microstructure. Only two phases were analyzed in these alloys, which were face-centered cubic (FCC) and hexagonal close packing (HCP). Increasing the content of Nb and Mo increases the hardness of the alloys by the effects of the solid solution strengthening and formation of the HCP phase. The potentiodynamic polarization curves of these alloys were also measured in 1 M sulfuric acid and 1 M sodium chloride solutions to evaluate the corrosion resistance of these alloys. The CrFeCoNiNb_0.3Mo_0.3 alloy had the smallest corrosion rate (0.0732 mm/yr) in 1 M deaerated H₂SO₄ solution, and the CrFeCoNiNb_0.15Mo_0.15 alloy had the smallest corrosion rate (0.0425 mm/yr) in 1 M deaerated NaCl solution. However, the CrFeCoNiNb_0.5Mo_0.5 alloy still had the best combination of corrosion resistance and hardness in the present study.

Recent Progress with BCC-Structured High-Entropy Alloys

High-entropy alloys (HEAs) prefer to form single-phase solid solutions (body-centered cubic (BCC), face-centered cubic (FCC), or hexagonal closed-packed (HCP)) due to their high mixing entropy. In this paper, we systematically review the mechanical behaviors and properties (such as oxidation and corrosion) of BCC-structured HEAs. The mechanical properties at room temperature and high temperatures of samples prepared by different processes (including vacuum arc-melting, powder sintering and additive manufacturing) are compared, and the effect of alloying on the mechanical properties is analyzed. In addition, the effects of HEA preparation and compositional regulation on corrosion resistance, and the application of high-throughput techniques in the field of HEAs, are discussed. To conclude, alloy development for BCC-structured HEAs is summarized.

Comparative Analysis of the Phase Interaction in Plasma Surfaced NiBSi Overlays with IVB and VIB Transition Metal Carbides

Important applications of transition metal carbides (TMCs) are as wear resistant composite layers deposited by plasma transferred arc welding (PTAW) and laser methods. Growing interest in them has also been observed in additive manufacturing and in HEA technology (bulk composite materials and layers), and in the area of energy conversion and storage. This paper presents the results of comparative studies on interfacial interactions in the NiBSi−TMCs system for two border IVB and VIB TM groups of the periodic table. Model (wettability and spreadability) and application experiments (testing of the PTAW-obtained carbide particle−matrix boundaries) were performed. Fe from partially melted steel substrates is active in the liquid NiBSi−TMCs system. It was revealed that the interaction of TMCs with the liquid NiBSi matrix tends to increase with the group number, and from the top to bottom inside individual groups. Particles of IVB TMCs are decomposed by penetration of the liquid along the grain boundaries, whereas those of VIB are decomposed by solubility in the matrix and secondary crystallization. No transition zones formed at the interfacial boundaries of the matrix−IVB group TMCs, unlike in the case of the VIB group. The experimental results are discussed using the data on the TMC electronic structure and the physicochemical properties.