High entropy alloys (HEAs) for the first time were reported in 2004, however, the progress in this field was markedly realized since 2010 when a drastic upsurging demand of HEAs was observed owing to their unique properties and possibilities to form various compositions significantly grabbing the attention of academia and researchers. Initially, the main focus of researchers was on equiatomic HEAs which later paradigm shifted to non-equiatomic alloys. With progress in the research, four core effects were examined as the predominant factors enabling the distinguished properties. Amongst, mechanical alloying (MA) followed by the sintering route grabbed the attention of all the researchers dealing with the processing of HEAs. Therefore, the basic objective of this review is to critically discuss the mechanically alloyed HEAs, functional properties, and their applications. Subsequently, the existing challenges and their opportunities are also discussed which leads this review article towards uniqueness and novelty. 

Mechanically alloyed high entropy alloys: Existing challenges and opportunities

High entropy alloys are an innovative class of materials for a wide range of industrial applications due to their competitive properties such as improved mechanical properties, superior wear resistance characteristics, and excellent corrosion behavior, which are widely desired for a variety of applications considering several attributes such as economical, eco-friendly and safety. Thus, the quest for high-performance materials with exceptional properties is an unfading research topic for researchers, academia, and metallurgical scientists. HEA presents a novel alloy design idea focused on multi principal elements, a huge compositional space, and more opportunities to develop diverse alloys with exceptional properties. As universally acknowledged, the immense potential in compositions, microstructures, and properties has sparked a great interest in this field. Researchers primarily focused on equimolar HEAs, but the precedent eventually shifted to non-equimolar alloys. As the investigation over HEAs progressed, four core effects were identified as the most important aspects in enabling the distinct characteristics. Mechanical alloying (MA), followed by the sintering approach, has piqued the interest of all researchers focusing on HEA development. As a result, the main intent of this study is to examine mechanically alloyed HEAs critically for mechanical properties, tribological behavior, corrosion behavior, and functional properties. Furthermore, the predominant challenges and their conceivable prospects are also deliberated that offer novelty to this review article.

A critical review on mechanically alloyed high entropy alloys: processing challenges and properties

Multiple principal element alloys, also often referred to as compositionally complex alloys or high entropy alloys, present extreme challenges to characterize. They show a vast, multidimensional composition space that merits detailed investigation and optimization to identify compositions and to map the composition ranges where useful properties are maintained. Combinatorial thin film material libraries are a cost-effective and efficient way to create directly comparable, controlled composition variations. Characterizing them comes with its own challenges, including the need for high-speed, automated measurements of dozens to hundreds or more compositions to be screened. By selecting an appropriate thin film morphology through predictable control of critical deposition parameters, representative measured values can be obtained with less scatter, i.e., requiring fewer measurement repetitions for each particular composition. In the present study, equiatomic CoCrFeNi was grown by magnetron sputtering in different locations in the structure zone diagram applied to multinary element alloys, followed by microstructural and morphological characterizations. Increasing the energy input to the deposition process by increased temperature and adding high-power impulse magnetron sputtering (HiPIMS) plasma generators led to denser, more homogeneous morphologies with smoother surfaces until recrystallization and grain boundary grooving began. Growth at 300 °C, even without the extra particle energy input of HiPIMS generators, led to consistently repeatable nanoindentation load-displacement curves and the resulting hardness and Young's modulus values.

Structure Zone Investigation of Multiple Principle Element Alloy Thin Films as Optimization for Nanoindentation Measurements

Water electrolysis is of interest as a sustainable way to produce clean hydrogen and oxygen fuel and help mitigate the rising problems of climate change while meeting global energy demands. High-efficiency, stable, and earth-abundant bifunctional catalysts are needed to enable more effective electrochemical cells for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Here, we investigate the synthesis, composition, performance, and mechanism of multimetal catalysts serving dual functionality in both OER and HER of water electrolysis. Through a laser synthesis method, we synthesized heterogeneous catalysts of nanocrystalline multimetallic alloy pockets embedded within an amorphous oxide matrix. We evaluated the performance and composition of a range of mixed transition-metal oxide materials for both OER and HER, ultimately synthesizing a Cr0.01Fe0.27Co0.34Ni0.38Ox/Cy catalyst that has a stable, high-rate, and competitive overall water splitting performance of 1.76 V at 100 mA cm–2 in an alkaline medium. Using density functional theory to gain insight as the active site and mechanism, we propose that the inclusion of a minor amount of Cr increases the degeneracy of energetic states that lowers the cost of forming the O 2 p–d bond and H 1 s–d bond due to the hybridization of s, p, and d orbitals from Cr. Using a two-electrode water electrolysis cell with a constant potential of 1.636 V to mimic the setup for fuel production, we found the catalyst to be stable at 14–15 mA cm–2 for 40 h. This laser synthesis method allowing for facile and rapid synthesis of complex multimetal systems demonstrates how doping a Fe, Co, and Ni heterogeneous amorphous/nanocrystalline structure with small amounts of Cr is important for bifunctional catalytic behavior, particularly for increasing HER functionality in advancing our understanding for future electrocatalytic design. 

Direct Laser Writing of Multimetal Bifunctional Catalysts for Overall Water Splitting

The experimental investigation of the FINEMET samples, such as \({\text {Fe}}_{73.5}{\text {Cu}}_{1}{\text {Nb}}_{3}{\text {Si}}_{13.5}{\text {B}}_{9}\), is presented. The analysis of the chemical elements, the micro-ribbons are made of, is discussed. The experimental data for the specific susceptibility dependent on temperature, the X-ray fluorescent spectroscopy, the X-ray structural analysis, the ferromagnetic resonance spectrums in the field of super high frequency, the amplitude–frequency characteristics in the rectangular resonator in one-mode regime, the complex impedance as the function of frequency of the applied voltage, and the mechanical vibrations in the soft magnetic materials are provided. The problem of developing a comprehensive complex analytical method for the soft magnetic micro-ribbons is raised.

FINEMET Micro-ribbons: The Experimental Identification of the Object

As well known the components of CrFeCoNi alloy compose a lager number of the high entropy alloys. Application of high energetic regime of the mechanical alloying has allowed preparing this alloy by milling less than for 2 h. The resulting powder is agglomerates consisting of plate-like crystallites smaller than 50 nm. Crystallites are characterized by an increased level of packaging defects and a cell parameter that corresponds to cast alloy. Annealing at 950°C does not change the phase composition and results in a more perfect crystalline structure. Hot pressing allowed obtaining a compact, single-phase alloy, hardness of which is close to 4 GPa.

Synthesis and characterization of nanocrystalline CrFeCoNi alloy by high energetic mechanical alloying

Magnetic nanomaterials have become widely used as functional material components in power electronics. For example, the powder cores of FeCuNbSiB have better high frequency characteristic in MHz frequency band in comparison with other iron-based materials. The microwave properties of the soft magnetics have potential for microwave application. The study of the specific susceptibility dependent on temperature, the X-ray structural analysis, the ferromagnetic resonance spectrums in the field of super high frequency, and the mechanical vibrations in the soft magnetic materials can provide us with the deeper understanding of the distribution of magnetization, collective spin excitations etc. In this work the dependence of the specific susceptibility on the temperature, the Cu K a X-ray diffraction patterns, the ferromagnetic resonance spectrums in the electromagnetic field of frequency 9.4 GHz, and the mechanical vibrations of low frequency for the amorphous and nanocrystalline micro-ribbons of Fe73.5Cu1Nb3Si13.5B9 are discussed.

About Some Resonance, Structural, and Magnetic Properties of Amorphous and Nanocrystalline FeCuNbSiB Ribbons

Main factors that determine the steam-turbine rotors work lifetime which work in a long-term load circumstances under an influence of high temperatures are the accumulation of creep deformations and the concomitant accumulation of the damage of the material. The heterogeneity of the material leads to the nascence of the defect that is the concentrator of stresses. To describe the dispersed damages impact on the construction material strength the function of the damage is applied the value of what changes during the operation process. To perform the research discrete models with N = 235, N = 731 and N = 2047 are considered. Using thicker discrete models allows clarifying the rotor basic lifetime value on 2% and 1% respectively. For the finite-element model with N = 731 the rotor basic lifetime value in the presence of the defect is 104000 hours that is less by 15% than in the absence of the defect. The rotor additional lifetime value is 6000 hours. The maximum value of the damage parameter can be observed on isolines nearby the defect at the moment of time 2200 hours and the difference between maximum values of the damage parameter for the rotor with the defect and hereunto is 30% that increases over time to 70%. After 104000 hours the accumulation region of the maximum values of the damage parameter increases in both directions of the rotor cross-section over time in such way that the augment of the maximum values of the damage parameter occurs more intensively in the rotation axis direction. At the moment of time 107000 hours the correlation between dimensions of the continuous fracture region in axes directions is 2/3 and to the moment of time 110000 hours is about 1/2. The value of the damage parameter in finite elements those border with the fracture region doesn’t exceed 0,3 scilicet the accumulation of the damage is local.

The Analysis of the Continuous Fracture Process of the Steam-Turbine Rotor with the Local Defect

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Papers

Synthesis and characterization of nanocrystalline CrFeCoNi alloy by high energetic mechanical alloying

FINEMET Micro-ribbons: The Experimental Identification of the Object

About Some Resonance, Structural, and Magnetic Properties of Amorphous and Nanocrystalline FeCuNbSiB Ribbons

The Analysis of the Continuous Fracture Process of the Steam-Turbine Rotor with the Local Defect