Design and fabrication of Ti–Zr-Hf-Cr-Mo and Ti–Zr-Hf-Co-Cr-Mo high-entropy alloys as metallic biomaterials

Novel TiZrHfCr0.2Mo and TiZrHfCo0.07Cr0.07Mo high-entropy alloys for metallic biomaterials (bio-HEAs) were developed based on the combination of Ti–Nb–Ta–Zr–Mo alloy system and Co–Cr–Mo alloy system as commercially-used metallic biomaterials. Ti–Zr-Hf-Cr-Mo and Ti–Zr-Hf-Co-Cr-Mo bio-HEAs were designed using (a) a tree-like diagram for alloy development, (b) empirical alloy parameters for solid-solution-phase formation, and (c) thermodynamic calculations focused on solidification. The newly-developed bio-HEAs overcomes the limitation of classical metallic biomaterials by the improvement of (i) mechanical hardness and (ii) biocompatibility all together. The TiZrHfCr0.2Mo and TiZrHfCo0.07Cr0.07Mo bio-HEAs showed superior biocompatibility comparable to that of commercial-purity Ti. The superior biocompatibility, high mechanical hardness and low liquidus temperature for the material processing in TiZrHfCr0.2Mo and TiZrHfCo0.07Cr0.07Mo bio-HEAs compared with the Ti–Nb–Ta–Zr–Mo bio-HEAs gave the authenticity of the application of bio-HEAs for orthopedic implants with multiple functions.Nagase T., Iijima Y., Matsugaki A., et al. Design and fabrication of Ti–Zr-Hf-Cr-Mo and Ti–Zr-Hf-Co-Cr-Mo high-entropy alloys as metallic biomaterials. Materials Science and Engineering C, 107, 110322. https://doi.org/10.1016/j.msec.2019.110322

High temperature properties of equiatomic FeAl with ternary additions

The aluminide intermetallic compounds are considered potential structural materials for aerospace applications. The B2 binary aluminide FeAl has a melting point in excess of 1500 K, is of simple cubic structure, exits over a wide range of composition with solubility for third elements and is potentially self-protecting in extreme environments. The B2 FeAl compound has been alloyed with 1 to 5 at % ternary additions of Si, Ti, Zr, Hf, Cr, Ni, Co, Nb, Ta, Mo, W, and Re. The alloys were prepared by blending a third elemental powder with prealloyed binary FeAl powder. Consolidation was by hot extrusion at 1250 K. Annealing studies on the extruded rods showed that the third element addition can be classified into three categories based upon the amount of homogenization and the extent of solid solutioning. Constant strain rate compression tests were performed to determine the flow stress as a function of temperature and composition. The mechanical strength behavior was dependent upon the third element homogenization classification

High Entropy Alloys for Medical Applications

A wide variety of metallic biomaterials have been developed so far, including various types of alloys. However, there is a strong need in the medical field for new solutions in what concerns metallic biomaterials with superior biocompatibility and mechanical properties in order to meet future requirements, including the recently developed high entropy alloys (HEAs). This chapter presents some characteristics of high entropy biocompatible metallic alloys produced in an electric-arc remelting furnace in argon inert atmosphere. The effects of the chemical elements used, the microstructural features, and some mechanical characteristics, both in the cast state or after some heat treatments, are highlighted

High Entropy Materials: Challenges and Prospects

This book is a reprint of a special issue of Metals (ISSN 2075-4701), titled High Entropy Materials: Challenges and Prospects. It is a compilation of nine articles from different aspects of high-entropy materials. The book primarily focuses on high-entropy alloys, the first emergent high-entropy materials, but also covers high-entropy ceramics and high-entropy composites, which are the extensions of high-entropy alloys. The articles on high-entropy alloys cover some important facets in the field such as phase structures, mechanical properties, laser beam welding, design of soft magnetic alloys, and potential as biomaterials. In addition, there are one article introducing the potential of using high-entropy carbides as hard metals for machining, and one another on high-entropy composite studying the microstructures and tribological properties of the FeCoNiCuAl-TiC composite. The goal of this reprinted book is essentially two-fold. In the first place, it offers a platform for researchers in the broad field of high-entropy materials to communicate their views and recent research on the subject. Next, it reports challenges in the sub-fields of high-entropy materials and inspires researchers to continue to practice diligence to resolve these challenges and advance high-entropy materials solidly. We hope that readers in the field feel encouraged, inspired, and challenged by the book, and readers outside the field can grasp some basic ideals of high-entropy materials and their potential to the society as a family of novel materials

Microstructure, lattice strain and mechanical properties of single phase multi-component alloys

The high entropy alloys (HEAs) have been developed based on the concept of entropic stabilization associated with a large number of constituent elements. The high configurational entropy in HEAs is expected to cause promising characteristic properties, i.e. high microstructural stability and high mechanical properties. In this study, the equiatomic fcc-structured FeNiCoCrMn and the bcc-structured TiNbHfTaZr single phase high entropy alloys (HEAs) were investigated regarding the effect of multiple atom species on microstructure, intrinsic lattice strain and mechanical properties. In a comparison with the HEAs, the sub-alloys having less chemical complexity were studied. The selected sub-alloys of the FeNiCoCrMn HEA were FeNiCoCr, FeNiCo, FeNi alloys and pure Ni, while equiatomic TiNbHfTa, TiNbHf, TiNb alloys and pure Nb were studied to compare with the TiNbHfTaZr HEA. The samples in this study were prepared by arc-melting, cold-crucible casting and thermomechanical treatment. The thermal phase stability of the FeNiCoCrMn HEA, TiNbHfTaZr HEA and their sub-alloys were observed and no second phase was formed between 300 - 1500 K. In high entropy alloys, the random arrangement of multiple atom species is assumed to cause large atomic displacements at lattice sites, which give rise to a severe lattice distortion. The evidences of lattice distortion in HEAs have been limitedly reported due to a difficulty of experimental investigation. In this work, the pair distribution function (PDF) method was used to assess local strain with analysis of diffuse intensities on total synchrotron X-ray scattering data. The current study found that the level of local lattice strain associated with atomic displacement was a function of atomic size misfit. The local lattice strain of the FeNiCoCrMn HEA was small and comparable to that of the sub-alloys which obtain similar values of the atomic size misfit. In contrast to the FeNiCoCrMn system, the magnitude of the local lattice strain increased with the value of atomic size misfit from the unary Nb sample to the quinary TiNbHfTaZr HEA. The lattice distortion was evident in the TiNbHfTaZr HEA due to large local lattice strain, but the local lattice strain of the FeNiCoCrMn HEA was not anomalously large. The level of lattice strain determines the solid solution hardening as a consequence of the elastic interaction between dislocations and atoms. The comparable level of the lattice strain in the FeNiCoCrMn HEA, its sub-alloys and Ni sample led to narrow range of hardness (64 – 132 HV) and tensile yield strength (60 – 192 MPa). For the bcc-structured samples, the hardness and the yield strength significantly varied depending on the level of local lattice strain, between 80 – 327 HV of hardness and 207 – 985 MPa of tensile yield strength. It is clear from the result that the atomic size misfit influences the level of the local lattice strain and the solid solution hardening. Cold rotary swaging was used to study the work hardening in the HEAs because it can delay fracture by large hydrostatic stresses. The large plastic deformability was observed in the FeNiCoCrMn and TiNbHfTaZr HEAs. The TiNbHfTaZr HEA was cold-swaged by 90% reduction of the cross-sectional area without intermediate annealing. The FeNiCoCrMn HEA was swaged until 85% reduction of the cross-sectional area; however, it was observed that it could be further deformed. The dislocation densities of the HEAs and its sub-alloys after the cold deformation were calculated as in the range between 1014 - 1015 m-2, in a good agreement with reported values of conventional metals after severe plastic deformation. This finding suggested that the level of dislocation density storage was correlated with the number of the constituent elements, the level of lattice distortion associated with atomic size misfit and the intrinsic properties (i.e. the stacking fault energy and the melting point). Whereas the intrinsic lattice strains of the FeNiCoCrMn HEA and its sub-alloys were comparable, the levels of dislocation storage were different possibly due to a difference of stacking fault energy. For the bcc-structured samples, the dislocation densities of the TiNbHfTaZr HEA, TiNbHfTa and TiNbHf alloys were large due to the large atomic size misfits. The high dislocation density leads to strong interactions between dislocations, which results in high resistance to dislocation motions. The high mechanical properties (hardness and yield strength) in the as-deformed FeNiCoCrMn and TiNbHfTaZr HEA were presented with the evidence of high dislocation densities. Moreover, the hardness and yield strength of the FeNiCoCrMn HEA significantly increased by the deformation, while those of the TiNbHfTaZr HEA after the deformation were slightly changed from the undeformed HEA. The large work hardenability of the FeNiCoCrMn HEA is possibly caused by small solid solution hardening and ease of twin formation. The research results suggest a further step towards designing an alloy composition for a development of microstructure and mechanical properties of high entropy alloys. It is evidently clear from the findings that the large number of constituent elements (in a term of high configurational entropy) is not only a factor in the determination of lattice distortion, microstructure and mechanical properties, but the type and the combination of constituent elements including the atomic interactions (i.e. atomic size misfit) have also an effect.:Abstract v Zusammenfassung ix Contents xiii 1. Motivation and objectives 1 2. Fundamentals 5 2.1 Concept of high entropy alloys 5 2.1.1 Phase formation and thermodynamic 5 2.1.2 Four core effects 10 2.2 Alloy classification of high entropy alloys 13 2.3 Mechanical properties of high entropy alloys 14 3. Experiments 19 3.1 Alloy preparation 19 3.1.1 Alloy selection 19 3.1.2 Melting and casting 21 3.1.3 Thermomechanical treatment 23 3.2 Sample characterization 27 3.2.1 Chemical analyses 27 3.2.2 Differential scanning calorimetry (DSC) 27 3.2.3 Scanning electron microscopy and microstructural analyses 28 3.2.4 X-ray diffraction (XRD) 29 3.2.5 High energy synchrotron X-ray diffraction 29 3.2.6 Mechanical Properties 33 4. Thermal phase stability of single phase high entropy alloys 35 5. An assessment of lattice strain in single phase high entropy alloys 49 5.1 Analysis of micro lattice strain on fcc- and bcc-structured high entropy alloys 50 5.2 Analysis of local lattice strain on fcc- and bcc-structured high entropy alloys 56 6. Solid solution hardening in single phase high entropy alloys 65 6.1 Hardness of fcc- and bcc-structured high entropy alloys 65 6.2 Tensile strength of fcc- and bcc-structured high entropy alloys 70 6.3 Correlation between atomic size misfit and solid solution hardening in Ti-Nb-Hf-Ta-Zr system 82 7. Work hardening in single phase high entropy alloys 91 7.1 Work hardenability of fcc- and bcc-structured high entropy alloys 91 7.2 Dislocation density of fcc- and bcc-structured high entropy alloys after cold swaging 93 8. Summary and outlook 109 8.1 Summary 109 8.2 Outlook 112 References 113 Acknowledgements 131 Erklärung 133Die Hochentropielegierungen (HELen) wurden auf der Grundlage des Konzepts der entropischen Stabilisierung entwickelt, was eine große Anzahl von Legierungselementen beinhaltet. Es wird erwartet, dass die hohe Konfigurationsentropie in HELen vielversprechende charakteristische Eigenschaften hervorruft, d.h. hohe mikrostrukturelle Stabilität und hohe mechanische Eigenschaften. In dieser Studie wurden die äquiatomare kfz-strukturierte FeNiCoCrMn und die krz-strukturierte TiNbHfTaZr Einphasen-Hochentropielegierung hinsichtlich der Wirkung mehrerer Atomarten auf das Gefüge, die intrinsische Gitterdehnung und die mechanischen Eigenschaften untersucht. Im Vergleich zu den HELen wurden die Sublegierungen mit geringerer chemischer Komplexität untersucht. Die ausgewählten Sublegierungen der FeNiCoCrMn HEL waren FeNiCoCr, FeNiCo, FeNi-Legierungen und reines Ni, während äquiatomare TiNbHfTa, TiNbHf, TiNbHf, TiNb-Legierungen und reines Nb im Vergleich zur TiNbHfTaZr HEL untersucht wurden. Die Proben in dieser Studie wurden durch Lichtbogenschmelzen, Kalttiegelguss und thermomechanische Behandlung hergestellt. Die thermische Phasenstabilität der FeNiCoCrMn HEL, der TiNbHfTaZr HEL und ihrer Sublegierungen wurde untersucht und es wurde keine zweite Phase zwischen 300 - 1500 K gebildet. Bei Hochentropielegierungen wird angenommen, dass die zufällige Anordnung mehrerer Atomarten zu großen Atomverschiebungen an den Gitterplätzen führt, die eine starke Gitterverzerrung hervorrufen. Aufgrund der Schwierigkeit der experimentellen Untersuchung wurden Beweise für Gitterverzerrungen bei HELen nur begrenzt berichtet. In dieser Studie wurde die Methode der Paarverteilungsfunktion (PDF) verwendet, um die lokale Dehnung mit Analyse der diffusen Intensitäten der gesamten Synchrotron-Röntgenstreuungsdaten zu beurteilen. Die aktuelle Studie ergab, dass die Höhe der lokalen Gitterdehnung, die mit der atomaren Verschiebung einhergeht, eine Funktion der Differenz der Atomgröße ist. Die lokale Gitterdehnung der FeNiCoCrMn HEL war klein und vergleichbar mit der der Sublegierungen, für die ähnliche Werte für die Atomgrößen-Unterschiede ermittelt wurden. Im Gegensatz zum FeNiCoCrMn-System stieg die Größe der lokalen Gitterdehnung mit dem Wert der Atomgrößendifferenz von der unären Nb-Probe bis zur quinären TiNbHfTaZr HEL. Die Gitterverzerrung war in der TiNbHfTaZr HEL aufgrund der großen lokalen Gitterdehnung offensichtlich, wohingegen die lokale Gitterdehnung der FeNiCoCrMn HEL nicht ungewöhnlich groß war. Die Höhe der Gitterdehnung bestimmt die Mischkristallverfestigung als Folge der elastischen Wechselwirkung zwischen Versetzungen und Atomen. Der vergleichbare Wert der Gitterdehnung in der FeNiCoCrMn HEL, seinen Sublegierungen und den Ni-Proben führte zu einem engen Härte- (64 - 132 HV) und Streckgrenzenbereich (60 - 192 MPa). Für die krz-strukturierten Proben variierten die Härte und die Streckgrenze dagegen je nach Höhe der lokalen Gitterdehnung signifikant, d.h zwischen 80 - 327 HV hinsichtlich der Härte und zwischen 207 - 985 MPa hinsichtlich der Streckgrenze. Aus dem Ergebnis ist ersichtlich, dass die Differenz der Atomgröße einen Einfluss auf die Höhe der lokalen Gitterdehnung und die Mischkristallverfestigung hat. Das Kalthämmen wurde für die Untersuchung der Kaltverfestigung in den HELen genutzt, da es den Bruch durch die großen hydrostatischen Spannungen verzögern kann. Die große plastische Verformbarkeit wurde bei den FeNiCoCrMn und TiNbHfTaZr HELen beobachtet. Die TiNbHfTaZr HEL wurde ohne Zwischenglühen um 90% der Querschnittsfläche kaltgehämmert. Die FeNiCoCrMn HEL wurde bis zu einer Verkleinerung der Querschnittsfläche von 85% gehämmert, wobei jedoch eine weitere Verformung möglich gewesen wäre. Die Versetzungsdichten der HELen und ihrer Sublegierungen wurden nach dem Verformung in einem Bereich zwischen 1014 - 1015 m-2 berechnet, was in guter Übereinstimmung mit den berichteten Werten konventioneller Metalle nach starker plastischer Verformung ist. Dieses Ergebnis deutete darauf hin, dass die Höhe der gespeicherten Versetzungsdichte mit der Anzahl der beinhaltenden Elemente, dem Grad der Gitterverzerrung im Zusammenhang mit der Differenz der Atomgröße und den intrinsischen Eigenschaften (d.h. der Stapelfehlerenergie und dem Schmelzpunkt) korreliert. Obwohl die intrinsischen Gitterdehnungen der FeNiCoCrMn HEL und seiner Sublegierungen vergleichbar waren, waren die Werte der gespeicherten Versetzungen unterschiedlich, was möglicherweise an einer Differenz der Stapelfehlerenergie lag. Für die krz-strukturierten Proben waren die Versetzungsdichten der TiNbHfTaZr HEL, der TiNbHfTa- und der TiNbHf-Legierungen aufgrund der großen Atomgrößenunterschiede hoch. Die hohe Versetzungsdichte bewirkt starke Wechselwirkungen zwischen den Versetzungen, was zu einem hohen Widerstand gegen Versetzungsbewegungen führt. Die hohen mechanischen Eigenschaften (Härte und Streckgrenze) in den verformten FeNiCoCrMn und TiNbHfTaZr HELen wurden mit dem Nachweis hoher Versetzungsdichten belegt. Darüber hinaus wurden die Härte und die Streckgrenze des FeNiCoCrMn HEL durch das Kalthämmern deutlich erhöht, während die der TiNbHfTaZr HEL nach dem Hämmerprozess nur leicht gegenüber der unverformten HEL verändert wurden. Die große Kaltverfestigung der FeNiCoCrMn HEL ist möglicherweise auf eine geringe Mischkristallhärtung und eine geringfügige Zwillingsbildung zurückzuführen. Die Forschungsergebnisse empfehlen für die Entwicklung des Gefüges und der mechanischen Eigenschaften von Hochentropielegierungen weitere Schritte hinsichtlich eines zielführenden Legierungsdesigns durchzuführenhin. Aus den Ergebnissen geht eindeutig hervor, dass die große Anzahl an Legierungselementen ( hinsichtlich einer hochkonfigurativen Entropie) nicht die einzige Einflussgrößebei der Bestimmung von Gitterverzerrungen, dem Gefüge und der mechanischen Eigenschaften darstellt, sondern auch die Art und die Kombination der Legierungselementen einschließlich der atomaren Wechselwirkungen (d.h. Atomgrößenunterschiede) einen Effekt haben.:Abstract v Zusammenfassung ix Contents xiii 1. Motivation and objectives 1 2. Fundamentals 5 2.1 Concept of high entropy alloys 5 2.1.1 Phase formation and thermodynamic 5 2.1.2 Four core effects 10 2.2 Alloy classification of high entropy alloys 13 2.3 Mechanical properties of high entropy alloys 14 3. Experiments 19 3.1 Alloy preparation 19 3.1.1 Alloy selection 19 3.1.2 Melting and casting 21 3.1.3 Thermomechanical treatment 23 3.2 Sample characterization 27 3.2.1 Chemical analyses 27 3.2.2 Differential scanning calorimetry (DSC) 27 3.2.3 Scanning electron microscopy and microstructural analyses 28 3.2.4 X-ray diffraction (XRD) 29 3.2.5 High energy synchrotron X-ray diffraction 29 3.2.6 Mechanical Properties 33 4. Thermal phase stability of single phase high entropy alloys 35 5. An assessment of lattice strain in single phase high entropy alloys 49 5.1 Analysis of micro lattice strain on fcc- and bcc-structured high entropy alloys 50 5.2 Analysis of local lattice strain on fcc- and bcc-structured high entropy alloys 56 6. Solid solution hardening in single phase high entropy alloys 65 6.1 Hardness of fcc- and bcc-structured high entropy alloys 65 6.2 Tensile strength of fcc- and bcc-structured high entropy alloys 70 6.3 Correlation between atomic size misfit and solid solution hardening in Ti-Nb-Hf-Ta-Zr system 82 7. Work hardening in single phase high entropy alloys 91 7.1 Work hardenability of fcc- and bcc-structured high entropy alloys 91 7.2 Dislocation density of fcc- and bcc-structured high entropy alloys after cold swaging 93 8. Summary and outlook 109 8.1 Summary 109 8.2 Outlook 112 References 113 Acknowledgements 131 Erklärung 13

An overview of high‐entropy alloys as biomaterials

This work was partially funded by national funds through the FCT-Fundacao para a Ciencia e a Tecnologia, I.P., under the scope of the project UIDB/50025/2020-2023.High‐entropy alloys (HEAs) have been around since 2004. The breakthroughs in this field led to several potential applications of these alloys as refractory, structural, functional, and biomedical materials. In this work, a short overview on the concept of high‐entropy alloys is provided, as well as the theoretical design approach. The special focus of this review concerns one novel class of these alloys: biomedical high‐entropy alloys. Here, a literature review on the potential high‐entropy alloys for biomedical applications is presented. The characteristics that are required for these alloys to be used in biomedical‐oriented applications, namely their mechanical and biocompatibility properties, are discussed and compared to commercially available Ti6Al4V. Different processing routes are also discussed.publishersversionpublishe