An Overview of High-Entropy Alloys as Biomaterials

Exploring the potential of intermetallic alloys as implantable biomaterials: A comprehensive review

This review delves into the utilization of intermetallic alloys (IMAs) as advanced biomaterials for medical implants, scrutinizing their conceptual framework, fabrication challenges, and diverse manufacturing techniques such as casting, powder metallurgy, and additive manufacturing. Manufacturing techniques such as casting, powder metallurgy, additive manufacturing, and injection molding are discussed, with specific emphasis on achieving optimal grain sizes, surface roughness, and mechanical properties. Post-treatment methods aimed at refining surface quality, dimensional precision, and mechanical properties of IMAs are explored, including the use of heat treatments to enhance biocompatibility and corrosion resistance. The review presents an in-depth examination of IMAs-based implantable biomaterials, covering lab-scale developments and commercial-scale implants. Specific IMAs such as Nickel Titanium, Titanium Aluminides, Iron Aluminides, Magnesium-based IMAs, Zirconium-based IMAs, and High-entropy alloys (HEAs) are highlighted, with detailed discussions on their mechanical properties, including strength, elastic modulus, and corrosion resistance. Future directions are outlined, with an emphasis on the anticipated growth in the orthopedic devices market and the role of IMAs in meeting this demand. The potential of porous IMAs in orthopedics is explored, with emphasis on achieving optimal pore sizes and distributions for enhanced osseointegration. The review concludes by highlighting the ongoing need for research and development efforts in IMAs technologies, including advancements in design and fabrication techniques.

Design and Development of Stable Nanocrystalline High-Entropy Alloy: Coupling Self-Stabilization and Solute Grain Boundary Segregation Effects

Grain growth is prevalent in nanocrystalline (NC) materials at low homologous temperatures. Solute element addition is used to offset excess energy that drives coarsening at grain boundaries (GBs), albeit mostly for simple binary alloys. This thermodynamic approach is considered complicated in multi-component alloy systems due to complex pairwise interactions among alloying elements. Guided by empirical and GB-segregation enthalpy considerations for binary-alloy systems, a novel alloy design strategy, the "pseudo-binary thermodynamic" approach, for stabilizing NC-high entropy alloys (HEAs) and other multi-component-alloy variants is proposed. Using Al25Co25Cr25Fe25 as a model-HEA to validate this approach, Zr, Sc, and Hf, are identified as the preferred solutes that would segregate to HEA-GBs to stabilize it against growth. Using Zr, NC-Al25Co25Cr25Fe25 HEAs with minor additions of Zr are synthesized, followed by annealing up to 1123 K. Using advanced characterization techniques- in situ X-ray diffraction (XRD), scanning/transmission electron microscopy (S/TEM), and atom probe tomography, nanograin stability due to coupling self-stabilization and solute-GB segregation effects is reported in HEAs up to substantially high temperatures. The self-stabilization effect originates from the preferential GB-segregation of constituent HEA-elements that stabilizes NC-Al25Co25Cr25Fe25 up to 0.5Tm (Tm-melting temperature). Meanwhile, solute-GB segregation originates from Zr segregation to NC-Al25Co25Cr25Fe25 GBs; this results in further stabilization of the phase and grain-size (≈14 nm) up to ≈0.58 and ≈0.64Tm, respectively.

Solid-State Processing of CoCrMoNbTi High-Entropy Alloy for Biomedical Applications

High-entropy alloys (HEAs) gained interest in the field of biomedical applications due to their unique effects and to the combination of the properties of the constituent elements. In addition to the required property of biocompatibility, other requirements include properties such as mechanical resistance, bioactivity, sterility, stability, cost effectiveness, etc. For this paper, a biocompatible high-entropy alloy, defined as bio-HEA by the literature, can be considered as an alternative to the market-available materials due to their superior properties. According to the calculation of the valence electron concentration, a majority of body-centered cubic (BCC) phases were expected, resulting in properties such as high strength and plasticity for the studied alloy, confirmed by the XRD analysis. The tetragonal (TVC) phase was also identified, indicating that the presence of face-centered cubic (FCC) phases in the alloyed materials resulted in high ductility. Microstructural and compositional analyses revealed refined and uniform metallic powder particles, with a homogeneous distribution of the elemental particles observed from the mapping analyses, indicating that alloying had occurred. The technological characterization of the high-entropy alloy-elaborated powder revealed the particle dimension reduction due to the welding and fracturing process that occurs during mechanical alloying, with a calculated average particle size of 45.12 µm.

Machine Learning Paves the Way for High Entropy Compounds Exploration: Challenges, Progress, and Outlook

Machine learning (ML) has emerged as a powerful tool in the research field of high entropy compounds (HECs), which have gained worldwide attention due to their vast compositional space and abundant regulatability. However, the complex structure space of HEC poses challenges to traditional experimental and computational approaches, necessitating the adoption of machine learning. Microscopically, machine learning can model the Hamiltonian of the HEC system, enabling atomic-level property investigations, while macroscopically, it can analyze macroscopic material characteristics such as hardness, melting point, and ductility. Various machine learning algorithms, both traditional methods and deep neural networks, can be employed in HEC research. Comprehensive and accurate data collection, feature engineering, and model training and selection through cross-validation are crucial for establishing excellent ML models. ML also holds promise in analyzing phase structures and stability, constructing potentials in simulations, and facilitating the design of functional materials. Although some domains, such as magnetic and device materials, still require further exploration, machine learning's potential in HEC research is substantial. Consequently, machine learning has become an indispensable tool in understanding and exploiting the capabilities of HEC, serving as the foundation for the new paradigm of Artificial-intelligence-assisted material exploration.

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.

Effect of oxidation at an elevated temperature on the evolution of phases, microstructure, and properties of the oxide films formed on the surface of TiZr

This study examined the evolution of the microstructure, microhardness, corrosion resistance, and selective leaching properties of oxide films formed on the surface of a Ti-50Zr (%) alloy during heat treatment at 600 °C for various time intervals. According to our experimental results, the growth and evolution of oxide films can be divided into three stages. In stage I (heat treatment for less than 2 min), ZrO₂ was first formed on the surface of the TiZr alloy, which slightly improved its corrosion resistance. In stage II (heat treatment for 2-10 min), the initially generated ZrO₂ is gradually transformed into ZrTiO₄ from the top to the bottom of the surface layer. The formation of ZrTiO₄ significantly improves the microhardness and corrosion resistance of the alloy. In stage III (heat treatment for more than 10 min), microcracks appeared and propagated on the surface of the ZrTiO₄ film, deteriorating the surface properties of the alloy. The ZrTiO₄ began to peel off after heat treatment for more than 60 min. The untreated and heat-treated TiZr alloys exhibited excellent selective leaching properties in Ringer's solution, whereas a trace amount of suspended ZrTiO₄ oxide particles formed in the solution after soaking the 60 min heat-treated TiZr alloy for 120 days. Surface modification of the TiZr alloy by generating an intact ZrTiO₄ oxide film effectively improved its microhardness and corrosion resistance; however, oxidation should be performed appropriately to obtain materials with optimal properties for biomedical applications.

Biomaterial types, properties, medical applications, and other factors: a recent review

Biomaterial research has been going on for several years, and many companies are heavily investing in new product development. However, it is a contentious field of science. Biomaterial science is a field that combines materials science and medicine. The replacement or restoration of damaged tissues or organs enhances the patient’s quality of life. The deciding aspect is whether or not the body will accept a biomaterial. A biomaterial used for an implant must possess certain qualities to survive a long time. When a biomaterial is used for an implant, it must have specific properties to be long-lasting. A variety of materials are used in biomedical applications. They are widely used today and can be used individually or in combination. This review will aid researchers in the selection and assessment of biomaterials. Before using a biomaterial, its mechanical and physical properties should be considered. Recent biomaterials have a structure that closely resembles that of tissue. Anti-infective biomaterials and surfaces are being developed using advanced antifouling, bactericidal, and antibiofilm technologies. This review tries to cover critical features of biomaterials needed for tissue engineering, such as bioactivity, self-assembly, structural hierarchy, applications, heart valves, skin repair, bio-design, essential ideas in biomaterials, bioactive biomaterials, bioresorbable biomaterials, biomaterials in medical practice, biomedical function for design, biomaterial properties such as biocompatibility, heat response, non-toxicity, mechanical properties, physical properties, wear, and corrosion, as well as biomaterial properties such surfaces that are antibacterial, nanostructured materials, and biofilm disrupting compounds, are all being investigated. It is technically possible to stop the spread of implant infection.

Aspects of Applied Chemistry Related to Future Goals of Safety and Efficiency in Materials Development for Nuclear Energy

The present paper is a narrative review focused on a few important aspects and moments of trends surrounding materials and methods in sustainable nuclear energy, as an expression of applied chemistry support for more efficiency and safety. In such context, the paper is focused firstly on increasing alloy performance by modifying compositions, and elaborating and testing novel coatings on Zr alloys and stainless steel. For future generation reactor systems, the paper proposes high entropy alloys presenting their composition selection and irradiation damage. Nowadays, when great uncertainties and complex social, environmental, and political factors influence energy type selection, any challenge in this field is based on the concept of increased security and materials performance leading to more investigations into applied science.

Recent Developments in Additive-Manufactured Intermetallic Compounds for Bio-Implant Applications

Purpose

This paper reviews the recent developments of two newly developed intermetallic compounds (IMCs) of metallic glasses (MGs) and high-entropy alloys (HEAs) as potential implantable biomaterials.

Methods

The paper commences by summarizing the fundamental properties of recently developed MGs and high-entropy alloys (HEAs). A systematic review is presented of the recent literature about the use of AM technology in fabricating MG and HEA components for biological implant applications.

Results

The high strength, low Young’s modulus, and excellent corrosion resistance make these IMCs good candidates as bio-implantable materials. Recent studies have shown that additive manufacturing (AM) techniques provide an advantageous route for the preparation of glassy metallic components due to their intrinsically rapid cooling rates and ability to fabricate parts with virtually no size or complexity constraints. A practical example is conducted by AM producing a porous gradient Ti-based MG spinal cage. The produced MG powders and the in vivo test results on an 18 M-old Lanyu pig confirm the feasibility of the AM technique for producing implantable IMC-based prosthesis.

Conclusion

The non-crystalline structure of MGs alloy and the random crystalline composition of HEAs provide unique material properties that will substantially impact the development of future implantable prostheses.

Promoted mechanical properties and functionalities via Ta-tailored Ti-Au-Cr shape memory alloys towards biomedical applications

In view of the urgent demands of shape memory alloys (SMAs) for biomedical applications due to the world population aging issue, the mechanical properties and functionalities of the biocompatible Ti-Au-Cr-based SMAs, which are tailored by Ta additions, have been developed in this study. The quaternary SMAs were successfully manufactured by physical metallurgy techniques and their mechanical properties and functionalities were examined. In the continuous tensile tests, it was found that the correlation between the yielding strength and phase stability followed a typical trend of mechanical behavior of SMAs, showing the lowest yielding strength at the metastable β-parent phase. Functional mappings between the alloy strength and elongation revealed that compared to the Ta-free specimen, the ductility was promoted 50% while the strength remained intact through the 4 at.% introduction of Ta. Slight shape recovery was observed in the cyclic loading-unloading tensile tests during the unloading process and the highest shape recovery was found in the Ti-4 at.% Au-5 at.% Cr-4 at.% Ta specimen. This indicates that the 4 at.% Ta tailored Ti-Au-Cr SMAs could be a promising material for biomedical applications.

High Entropy Materials: Challenges and Prospects

Entropy is an important concept in thermodynamics, measuring the disorder in a system or, more precisely, the number of possible microscopic configurations of individual atoms or molecules of a system, i.e., microstates [...]

The Effects of Annealing at Different Temperatures on Microstructure and Mechanical Properties of Cold-Rolled Al0.3CoCrFeNi High-Entropy Alloy

In this work, cold-rolling was utilized to induce a high density of crystal defects in Al0.3CoCrFeNi high-entropy alloys. The effects of annealing temperature on static recrystallization, precipitation behavior and mechanical properties were investigated. With increasing annealing temperature from 590 °C to 800 °C, the area fraction of recrystallized region increases from 26.9% to 93.9%. Cold-rolling deformation largely promotes the precipitation of B2 phases during annealing, and the characteristics of the precipitates are linked to recrystallization level. The coarse and equiaxed B2 phases exist in the recrystallized region and the fine and elongated B2 phases occupy the non-recrystallized region. Combined use of cold-rolling and annealing can remarkably enhance the strength and toughness. A partially recrystallized microstructure in a cold-rolled sample annealed at 700 °C exhibits a better combination of strength and toughness than a fully recrystallized microstructure in a cold-rolled sample annealed at 800 °C. Finally, related mechanisms are discussed.