Refractory High-Entropy Alloys: Design, Fabrication, Characterization, and Nanoparticle Synthesis

High-Entropy Alloys have been a highly researched area of metals ever since their introduction in 2004 by Brian Cantor and Jien-Weh Yeh. In the continued research of High-Entropy Alloys (HEAs), a specific area concerning Refractory High-Entropy Alloys (RHEAs) has emerged for their high-temperature applications. Although RHEAs have maintained high strength and toughness at high temperatures, their low ductility still needs to be addressed. A dataset was created to find correlations between various characteristics of RHEAs and their composition. A set of seven compositions were selected and fabricated. Mechanical tests were run on the seven compositions, and a proposal was written for neutron diffraction tests at Oak Ridge National Lab. In-situ neutron diffraction was performed during mechanical testing on the seven compositions. X-ray diffraction was performed on as-cast samples and post-mortem samples of each composition. The microstructural analysis led to one out of the seven RHEAs being confirmed to contain Transformation Induced Plasticity (TRIP). The six remaining RHEAs were confirmed as a single-phase BCC. Nanoparticles were fabricated from three of the confirmed single-phase compositions and characterized using X-ray diffraction and Scanning Electron Microscope (SEM) Imaging. All three were found to be a single-phase FCC nanoparticle

Microstructural Characterization and Mechanical Behaviors of High Entropy Alloys at Room and Elevated-Temperatures

High entropy alloys (HEAs) are proposed as solid-solution alloys containing five or more principal elements in equimolar or near-equimolar ratios, possessing a single crystal structure rather than several ordered phases. Several studies of HEAs have been performed, with focus on the mechanical behavior and characterization of microstructures. The mechanical behavior and properties of HEAs under various conditions, i.e., strain rates, grain sizes, and temperatures, exhibit great differences, such as strong work hardening, homogeneous macroscopic flow, and excellent compression or tension ductility with obvious serrations at room temperature, and partial or complete dynamic recrystallization at high temperatures. The strong and ductile single-phase body-centered-cubic (BCC) HfNbTaTiZr refractory high-entropy alloy (RHEA) is a potential structural material for high-temperature applications. The present work will focus the mechanical properties and serration behavior in HfNbTaTiZr HEAs, by applying transmission electron microscopy (TEM), atom probe tomography (APT), synchrotron diffraction, and scanning electron microscopy (SEM) to the study of plastic deformation and fatigue behaviors in HEAs under different conditions (covering a wide range of strain rates, temperatures, and tension behaviors), in order to reveal the underlying mechanisms of the plastic deformation for HEAs and to predict the fracture stress. Specifically, an anomaly in strain hardening was observed at elevated temperatures–the strain-hardening exponent decreases expectedly from 77 K to 298 K but reverts to an anomalous ascending trend afterwards. Flow serrations at 673 and 773 K implied the dynamic strain aging (DSA) as an extra strengthening mechanism contributing to the intensified strain hardening at elevated temperatures. The superior fatigue properties during cyclic loading were investigated at room temperature, which present a series of substructures, including dislocation loops, jogs, and dislocation network. The resulting dislocation network was formed by the interaction between dislocations with different Burgers vectors, which can act as the obstacle to dislocation motion to strengthen the fatigue behavior and release the strain energy and stress concentration to improve the resistance to cyclic loading. Moreover, the recrystallization, grain growth and phase transformation of HfNbTaTiZr HEAs were investigated as well in the certain range of temperatures to better understand their grain growth kinetics and phase stability in body centered-cubic (bcc) HEAs, which will be helpful for the materials design and optimization

Characterizing High Entropy Alloys for Hypersonic Applications

In this paper, the properties of a new and broad class of materials, high entropy alloys (HEAs), were investigated and evaluated for hypersonic applications. The plan was to identify candidate hypersonic HEAs and model the high-temperature strength using new advanced material models that account for asymmetry and anisotropy characterized with available test data. After accessing a local database of HEAs and their material properties in collaboration with Dr. Gorsse et al., it was realized the knowledge of HEAs is currently very broad but lacks depth. While hundreds of HEAs have been created and tested, none so far have both sufficient data and the desired properties to more accurately model high-temperature strength. Further research is required to begin bringing HEAs into the world of constitutive modeling and simulation, which is the next step to real world applications. Several HEAs have been identified as having high potential for further research

Influence of Temperature and Plastic Strain on Deformation Mechanisms and Kink Band Formation in Homogenized HfNbTaTiZr

Due to its outstanding ductility over a large temperature range, equiatomic HfNbTaTiZr is well-suited for investigating the influence of temperature and plastic strain on deformation mechanisms in concentrated, body centered cubic solid solutions. For this purpose, compression tests in a temperature range from 77 up to 1073 K were performed and terminated at varying plastic strains for comparison of plastic deformation behavior. The microstructure and chemical homogeneity of a homogenized HfNbTaTiZr ingot were evaluated on different length scales. The compression tests reveal that test temperature significantly influences yield strength as well as work hardening behavior. Electron backscatter diffraction aids in shedding light on the acting deformation mechanisms at various temperatures and strains. It is revealed that kink band formation contributes to plastic deformation only in a certain temperature range. Additionally, the kink band misorientation angle distribution significantly differs at varying plastic strains

Grain Boundary Wetting by a Second Solid Phase in the High Entropy Alloys: A Review

In this review, the phenomenon of grain boundary (GB) wetting by the second solid phase is analyzed for the high entropy alloys (HEAs). Similar to the GB wetting by the liquid phase, the GB wetting by the second solid phase can be incomplete (partial) or complete. In the former case, the second solid phase forms in the GB of a matrix, the chain of (usually lenticular) precipitates with a certain non-zero contact angle. In the latter case, it forms in the GB continuous layers between matrix grains which completely separate the matrix crystallites. The GB wetting by the second solid phase can be observed in HEAs produced by all solidification-based technologies. The particle chains or continuous layers of a second solid phase form in GBs also without the mediation of a liquid phase, for example by solid-phase sintering or coatings deposition. To describe the GB wetting by the second solid phase, the new GB tie-lines should be considered in the two- or multiphase areas in the multicomponent phase diagrams for HEAs. The GB wetting by the second solid phase can be used to improve the properties of HEAs by applying the so-called grain boundary engineering methods.This research was funded by the Russian Ministry Of Science And Higher Education (contract no. 075-15-2021-945 grant no. 13.2251.21.0013). Support from the University of the Basque Country under the GIU19/019 project is also acknowledged

Superfunctional high-entropy alloys and ceramics by severe plastic deformation

High-entropy alloys and ceramics containing at least five principal elements have received high attention in recent years for various mechanical and functional applications. The application of severe plastic deformation (SPD), particularly the high-pressure torsion (HPT) method combined with the CALPHAD and first-principles calculations, resulted in the development of numerous superfunctional high-entropy materials with superior properties compared to the normal functions of engineering materials. This article reviews the recent advances in the application of SPD to achieving superfunctional high-entropy materials. These superfunctional properties include (i) ultrahigh hardness levels in high-entropy alloys which are comparable to ceramics, (ii) high yield strength and good hydrogen embrittlement resistance in high-entropy alloys; (iii) high strength, low elastic modulus, and high biocompatibility in high-entropy alloys, (iv) fast and reversible hydrogen storage in high-entropy alloys and corresponding hydrides, (v) photovoltaic performance and photocurrent generation on high-entropy semiconductors, (vi) photocatalytic oxygen and hydrogen production on high-entropy oxides and oxynitrides from water splitting, and (vii) CO2 photoreduction on high-entropy ceramics. These findings introduce SPD as not only a processing tool to improve the properties of existing high-entropy materials but also as a synthesis tool to synthesize novel high-entropy materials with superior properties compared with conventional engineering materials

Effect of Ti addition on the structural, thermodynamic, and elastic properties of Tix(HfNbTaZr)(1−x)/4Ti_{x}(HfNbTaZr)_{(1-x)/4} alloys

The structure and thermodynamic properties of $Ti_{x}(HfNbTaZr)_{(1-x)/4}$ refractory highly entropy multicomponent alloys have been studied using a comprehensive Monte-Carlo Special Quasi-random Structure (MCSQS) realization of the disordered atomic structure and DFT calculations. We have shown that to model the random structure in a small supercell, it is necessary to study a large space of random configurations with respect to the nearest shells. Mimicking the randomness with the many-body terms does not lead to significant improvements in the formation energy but modeling the random structure with the few nearest neighbor pairs leads to improvements in the formation energy. We have also demonstrated the existence of weak to intermediate SRO for equimolar compositions. Chemical ordering is studied by linking a large number of MCSQS realizations to DFT calculations, and the SRO results are rationalized in terms of the crystallographic structure of the element pairs and binary phase diagrams. The formation energy of $Ti_{x}(HfNbTaZr)_{(1-x)/4}$ alloys remains slightly positive for all $x$ when Ti is added. For $x$ > 0.5, a phase transition in favor of an hcp structure is observed in agreement with the Bo-Md diagram. A dual phase is predicted at $x$ = 0.5. The Ti content in this class of alloys appears to be a practical way to select the phase structure and tailor the structure and elastic properties to specific applications

Severe Plastic Deformation and Phase Transformations in High Entropy Alloys: A Review

This review discusses an area of expertise that is at the intersection of three large parts of materials science. These are phase transformations, severe plastic deformation (SPD), and high-entropy alloys (HEA). First, SPD makes it possible to determine the borders of single-phase regions of existence of a multicomponent solid solution in HEAs. An important feature of SPD is that using these technologies, it is possible to obtain second-phase nanoparticles included in a matrix with a grain size of several tens of nanometers. Such materials have a very high specific density of internal boundaries. These boundaries serve as pathways for accelerated diffusion. As a result of the annealing of HEAs subjected to SPD, it is possible to accurately determine the border temperature of a single-phase solid solution area on the multicomponent phase diagram of the HEA. Secondly, SPD itself induces phase transformations in HEAs. Among these transformations is the decomposition of a single-phase solid solution with the formation of nanoparticles of the second phase, the formation of high-pressure phases, amorphization, as well as spinodal decomposition. Thirdly, during SPD, a large number of new grain boundaries (GBs) are formed due to the crystallites refinement. Segregation layers exist at these new GBs. The concentration of the components in GBs differs from that in the bulk solid solution. As a result of the formation of a large number of new GBs, atoms leave the bulk solution and form segregation layers. Thus, the composition of the solid solution in the volume also changes. All these processes make it possible to purposefully influence the composition, structure and useful properties of HEAs, especially for medical applications