Atomic Scale Microscopy of Zr-based Bulk Metallic Glasses Processed by Various Routes

Bulk metallic glasses (BMGs) exhibit a rare combination of strength and toughness that is difficult to achieve by other materials. These properties make them favourable for a diverse range of engineering applications. However, their disordered amorphous structure invokes catastrophic failure with shear bands localisation, limiting their industrial development as structural materials. Moreover, it is not yet clear how to quantitatively link their microstructural features to processing and mechanical properties. The aim of this thesis was to quantitatively analyse the structural features contributing to local hardness variations in thermomechanically processed zirconium (Zr)-based BMGs. Advanced atom probe tomography (APT) techniques were used to observe structural and chemical changes in these BMGs. APT operational parameters were optimised and tested for robust data outcomes. APT cluster analysis was effectively utilised in the characterisation of nanoscale heterogeneities in the BMG microstructure. The chemical composition of the nanoscale heterogeneities was roughly Zr27Cu29Al21Ni19Nb4 (at. %) in Zr63.96Cu13.36Ni10.29Al11.04Nb1.25 (at. %), and Zr22Cu29Al17Ni23Ti9 (at. %) in Zr52.5Cu17.9Ni14.6Al10Ti5 (at. %). Their chemistry was experimentally reported herein for the first time. Additionally, an ab-initio molecular dynamic (AIMD) simulation was used to simulate the atomistic distribution in a Zr-based BMG. Clusters observed in APT assigned as MRO regions were found synonymous to the shear band nucleation zones. Beyond the novel methodological rigor introduced here, the findings provide a new, independent validation of the inverse correlation between local hardness and size of the MRO regions, with their chemical compositions, providing a novel handle on the quest for understanding microstructure- property-processing relationship in BMGs

Improving Mechanical Properties of Bulk Metallic Glasses by Approaches of In-situ Composites and Thin Films

Bulk-metallic glasses (BMGs) exhibits lots of unique properties, such as, high strengths, high hardness, high specific strengths, superior elastic limits, high corrosion resistance, etc. However, the applications of BMGs are still quite limited due to their intrinsic brittleness and low ductility at room temperature. Many efforts have been conducted to improve the plasticity of BMGs, in which metallic-glass-matrix composites (MGMCs) and thin-film metallic glasses (TFMGs) are two popular and effective approaches. Nevertheless, the deformation mechanisms for the improved plasticity of MGMCs and TFMGs are still far from satisfactory understanding, which will be investigated using both experimental and simulation methods in the present work. For the MGMCs, in situ high-energy synchrotron X-ray diffraction experiments and micromechanics-based finite element simulations have been conducted to examine their lattice strain evolution. The entire lattice-strain evolution curves can be divided into elastic-elastic (denoting deformation behavior of matrix and inclusion, respectively), elastic-plastic, and plastic-plastic stages. Characteristics of these three stages are governed by the constitutive laws of the two phases (modeled by free-volume theory and crystal plasticity) and geometric information (crystalline phase morphology and distribution). The deformation behavior, especially the fatigue behavior, of TFMG materials has been investigated on the some substrates, including 316L stainless steel, BMG, etc. The results show that the four-point-bending fatigue life of the substrates is greatly improved by Zr- and Cu-based TFMGs, while Fe-based TFMG, TiN, and pure-Cu films are not so beneficial in extending the fatigue life of 316L stainless steel. However, quite limited work is reported on the fatigue behavior of TFMG coated on the BMG substrate, which can be a very interesting topic. Moreover, a synergistic experimental/theoretical study are conducted to investigate the micro-mechanisms of the fatigue behavior of TFMGs adhered to BMG substrates. Furthermore, shear-band initiation and propagation under deformation are investigated using the Rudnicki-Rice instability theory and the free-volume models employing finite-element simulations

Laser-based Additive Manufacturing of Bulk Metallic Glasses: Recent Advances and Future Perspectives for Biomedical Applications

Bulk metallic glasses (BMGs) are non-crystalline class of advanced materials and have found potential applications in the biomedical field. Although there are numerous conventional manufacturing approaches for processing BMGs, the most commonly used like copper-mould casting have some limitations. It is not easy to manage and control the critical cooling rate, especially when the fabrication of complex BMG geometries is involved. Other limitations of these techniques include the size constraints, non-flexibility, and the tooling and accessories are costly. The emergence of additive manufacturing (AM) has opened another promising manufacturing route for processing BMGs. AM processes, particularly laser powder-bed fusion (PBF-LB/M) builds parts layer-by-layer and successively fused the powder-melted feedstocks using prescribed computer-controlled laser scanner system, thereby forming a BMGs part upon sufficiently rapid cooling to ensure the glass forming-ability. PBF-LB/M overcomes the limitations of the pre-existing BMGs processing techniques by not only improving the part size, but also produces exceptionally complex structures and patient-specific implants. This review article aims to summarise and discuss the mechanism of BMGs formation through PBF-LB/M for biomedical applications and to highlight the current scientific and technological challenges as well as the future research perspectives towards overcoming the pore-mediated microcracks, partial crystallisation, brittleness and BMG size constraint

Electrochemical and Corrosion Behavior of Metallic Glasses

Metallic glasses are multi-component metallic alloys with disordered atomic distribution unlike their crystalline counterparts with long range periodicity in arrangement of atoms. Metallic glasses of different compositions are being commercially used in bulk form and as coatings because of their excellent corrosion resistance. This book was written with the objective of providing a comprehensive understanding of the electrochemical and corrosion behavior of metallic glasses for a wide range of compositions. Corrosion in structural materials leads to rapid deterioration in the performance of critical components and serious economic implications including property damage and loss in human life. Discovery and development of metallic alloys with enhanced corrosion resistance will have a sizable impact in a number of areas including manufacturing, aerospace, oil and gas, nuclear industry, and load-bearing bioimplants. The corrosion resistance of many metallic glass systems is superior compared to conventionally used alloys in different environments. In this book, we discuss in detail the role of chemistry, processing conditions, environment, and surface state on the corrosion behavior of metallic glasses and compare their performance with conventional alloys. Several of these alloy systems consist of all biocompatible and non-allergenic elements making them attractive for bioimplants, stents, and surgical tools. To that end, critical insights are provided on the bio-corrosion response of some metallic glasses in simulated physiological environment

Effects of Irradiation and Thermal Annealing on the Mechanical and Microstructural Properties of Bulk Metallic Glasses

A series of ion irradiation and annealing experiments have been performed on Zr52.5Cu17.9Ni14.6Al10Ti5 “BAM-11” and Cu60Zr20Hf10Ti10 bulk metallic glass (BMG) specimens to evaluate their irradiation- and temperature-induced microstructural and mechanical property evolution. These experiments covered four main themes, namely, ion irradiation, neutron irradiation, thermal annealing, and helium implantation. For the ion irradiations, samples were exposed to 9 MeV Ni and 5.5 MeV C ions at temperatures ranging from room temperature to 360 oC. For the Ni ion irradiations the samples were exposed to midrange (~1.5 m depth) doses of 0.5 and 10 displacements per atom (dpa), while the C ion irradiations samples were irradiated to a midrange dose of 0.5 dpa. For the neutron irradiations, samples were irradiated by neutrons (E \u3e 0.1 MeV) at ~70 oC to fluences of 1.4 × 1020 n/cm2 and 1.4 × 1021 n/cm2 (doses of 0.1 and 1 dpa). Thermal annealing experiments involved heating the samples to various temperatures ranging from 25 - 770 oC. For the helium implantation experiments, amorphous and partially crystallized BMGs were exposed to helium fluences of 2 × 1015 and 5 × 1015 cm-2 . The mechanical property and microstructural characterization included nanoindentation, compression testing, bend testing, Xray diffraction (XRD), neutron diffraction, thermal desorption analysis (TDS), and nuclear reaction analysis. From the experiments, several important conclusions were obtained. The results of the XRD and nanoindentation characterizations of the ion irradiated and thermal annealed specimens indicate good stability during irradiation at 25 to 290 oC up to at least 10 dpa but suggest that the BAM-11 BMG is not suitable for irradiation environments where temperatures exceed 300 oC for prolonged periods of time. As for the neutron irradiation and thermal annealing experiments, significant softening was observed in the sample irradiated by neutrons, while postirradiation annealing led to a marked increase in hardening. Neutron diffraction results indicated vi that primary knock-on events caused rejuvenation (disordering) while annealing resulted in structural relaxation. The results of the TDS experiments found that for the lower He implantation fluence, He desorbed more quickly in the partially crystallized alloy, indicating a structural effect on the mobility of He

Unraveling the microstructural heterogeneity and plasticity of Zr50Cu40Al10bulk metallic glass by nanoindentation

Unraveling the microstructural heterogeneity is an important issue to understand the physical and mechanical properties of metallic glasses. Structural relaxation below the glass transition tem- perature Tg and cold rolling at ambient temperature are effective ways to tune the state within the potential energy landscape of metallic glasses, modifying the microstructural heterogeneity. With the help of nanoindentation technique, by scrutinizing the local reduced modulus and hardness of a typical Zr50Cu40Al10 metallic glasses in different energy states (from structural relaxed state to rejuvenated state), we demonstrate that the enhancement of microstructural heterogeneity plays an important role in tailoring the mechanical behavior. The internal friction coefficient is ob- tained by fitting the experimental load-displacement curves with finite element simulation. It is found that internal friction coefficient decreases with the increase of concentration of flow de- fects. The stress exponent obtained from the creep stage shows a strong dependence on the structural state of metallic glasses. Effects of structural relaxation and rejuvenation on the structural state are rationalized in terms of the relative flow defects concentration. Finally, the statistics of discrete displacement bursts during the deformation process were probed. We find that the displacement bursts behavior is sensitive to the structural energy state (microstructural heterogeneity). The displacement bursts phenomenon changes from obvious (relaxed state) to unapparent behaviors (rejuvenated state). By given a physical schematic, the correlation between the displacement bursts size and the shear band size in the local plastic zones is well established, which will lead to a deeper understanding of the rejuvenation-induced plasticity of metallic glasses.This work is supported by the National Natural Science Foundation of China (NSFC) (Grant Nos. 51971178 and 51871132), Natural Science Basic Research Plan for Distinguished Young Scholars in Shaanxi Province (Grant No. 2021JC-12) and the Natural Science Foundation of Chongqing (Grant No. cstc2020jcyj-jqX0001). E. Pineda acknowledges financial support from MICINN (grant PID2020- 112975GB-I00) and Generalitat de Catalunya (grant 2017SGR0042).Peer ReviewedPostprint (author's final draft

Investigation of new Ti-based metallic glasses with improved mechanical properties and corrosion resistance for implant applications

The glass-forming Ti75Zr10Si15 alloy is regarded as a potential new material for implant applications due to its composition of non-toxic, biocompatible elements and many interesting mechanical properties. The effects of partial substitution of 15 at.-% Ti by Nb on the microstructure and mechanical behavior of the alloy have been investigated. The limited glass-forming ability (GFA) of the Ti75Zr10Si15 alloy results for melt-spun ribbons mainly in nanocomposite structures with β-type nanocrystals being embedded in a glassy matrix. Addition of Nb increases the glass-forming ability. Raising the overheating temperature of the melt prior to melt-spinning from 1923 K to 2053 K yields a higher amorphous phase fraction for both alloys. A decrease of hardness (H), ultimate stress and reduced Young’s modulus (Er) is observed for Ti60Zr10Nb15Si15 rods as compared to Ti75Zr10Si15 ones. This is attributed to an increase of the fraction of the β-type phase. The melt-spun ribbons show an interesting combination of very high hardness values (H) and moderate reduced elastic modulus values (Er). This results in comparatively very high H/Er ratios of >0.075 which suggests these new materials for applications demanding high wear resistance. The corrosion and passivation behavior of these alloys in their homogenized melt-spun states have been investigated in Ringer solution at 37°C in comparison to their cast multiphase crystalline counterparts and to cp-Ti and β-type Ti-40Nb. All tested materials showed very low corrosion rates. Electrochemical and surface analytical studies revealed a high stability of their passive states in a wide potential range. The addition of Nb does not only improve the glass-forming ability and the mechanical properties but also supports a high pitting resistance even at extreme anodic polarization. With regard to the corrosion properties, the Nb-containing nearly single-phase glassy alloy can compete with the β-type Ti-40Nb alloy. In addition, it has been demonstrated that thermal oxidation could be well applied to Ti75Zr10Si15 and Ti60Zr10Nb15Si15 melt-spun ribbons. Thermal oxidation treatment is one of the simple and cost-effective surface modification methods to improve the surface characteristics of these alloys. In the first tests, ribbon samples of the ternary and the quaternary alloy which were oxidized at 550°C in synthetic air showed suitable fundamental properties for implant applications, i.e. high hardness, good wettability and hydroxyapatite-forming ability after 10 days. All these properties recommend the new glass-forming alloys for application as wear- and corrosion-resistant coating materials for implants.Die glasbildende Legierung Ti75Zr10Si15 wird wegen ihrer biokompatiblen Zusammensetzung ohne toxische Elemente und auf Grund interessanter mechanischer Eigenschaften als potentielles neues Implantatmaterial betrachtet. Es wurden 15 at.-% Ti durch Nb partiell substituiert und die Effekte auf die Mikrostruktur und die mechanischen Eigenschaften der Legierung untersucht. Auf Grund der eingeschränkten Glasbildungsfähigkeit von Ti75Zr10Si15 bestehen die schmelzgeschleuderten Bänder dieser Legierung hauptsächlich aus Nanokomposit-Strukturen mit β-phasigen Nanokristallen in einer glasartigen Matrix. Die Zugabe von Nb steigert die Glasbildungsfähigkeit. Das Anheben der Überhitzungstemperatur der Schmelze vor dem Schmelzschleudern von 1923 auf 2053 K führt für beide Legierungen zu einem höheren Anteil amorpher Phase. Es wird bei der Legierung Ti60Zr10Nb15Si15 im Vergleich zur Ti75Zr10Si15-Legierung eine Abnahme der Härte (H), Bruchfestigkeit und ein reduzierter E-Modul (Er) beobachtet. Dies wird mit dem Anstieg des beta-Phasenanteils erklärt. Die schmelzgeschleuderten Bänder zeigen eine interessante Kombination aus sehr hoher Härte und moderaten E-Modul Werten (Er). Dies führt zu vergleichsweise sehr hohen H/Er-Verhältnissen von >0,075, wodurch diese Materialien für Anwendungen mit hohen Verschleißanforderungen geeignet sind. Das Korrosions- und Passivierungsverhalten dieser Legierungen in ihrem homogenisierten schmelzgeschleuderten Zustand wurde in Ringer-Lösung bei 37°C untersucht und mit dem gegossenen vielphasigen kristallinen Zustand dieser Legierungen sowie mit cpTi und beta-Typ Ti-40Nb verglichen. Alle untersuchten Materialien zeigten sehr niedrige Korrosionsraten. Elektrochemische Studien und Oberflächenanalysen belegen eine hohe Stabilität der Passivfilme in einem weiten Potentialbereich. Die Zugabe von Niob verbessert nicht nur die Glasbildungsfähigkeit und die mechanischen Eigenschaften, sondern erhöht weiterhin die Lochfraßbeständigkeit, selbst bei stark anodischer Polarisation. Bezüglich der Korrosionseigenschaften konkurriert die Nb-haltige fast einphasige glasartige Legierung mit β-phasigem Ti-40Nb. Weiterhin wurde gezeigt, dass an schmelzgeschleuderten Bändern der Legierung Ti75Zr10Si15 und Ti60Zr10Nb15Si15 eine thermische Oxidation erfolgreich durchgeführt werden konnte. Die thermische Oxidation ist eine der einfachsten und kosteneffektivsten Möglichkeiten der Oberflächenmodifikation um die Eigenschaften der Oberflächen dieser Legierungen zu verbessern. In den ersten Tests zeigten die Bänder-Proben der ternären und der quaternären Legierung, die bei 550°C in synthetischer Luft oxidiert wurden, entsprechende Eigenschaften für Implantat-Anwendungen, d.h. hohe Härte, gute Benetzbarkeit und die Fähigkeit nach 10 Tagen Hydroxylapatit auf der Oberfläche zu bilden. Alle zuvor genannten Eigenschaften machen diese neuen glasbildenden Legierungen zu geeigneten Materialien für die Anwendung als verschleiß- und korrosionsbeständige Beschichtung für Implantate

Electrochemical and Corrosion Behavior of Metallic Glasses

Metallic glasses are multi-component metallic alloys with disordered atomic distribution unlike their crystalline counterparts with long range periodicity in arrangement of atoms. Metallic glasses of different compositions are being commercially used in bulk form and as coatings because of their excellent corrosion resistance. This book was written with the objective of providing a comprehensive understanding of the electrochemical and corrosion behavior of metallic glasses for a wide range of compositions. Corrosion in structural materials leads to rapid deterioration in the performance of critical components and serious economic implications including property damage and loss in human life. Discovery and development of metallic alloys with enhanced corrosion resistance will have a sizable impact in a number of areas including manufacturing, aerospace, oil and gas, nuclear industry, and load-bearing bioimplants. The corrosion resistance of many metallic glass systems is superior compared to conventionally used alloys in different environments. In this book, we discuss in detail the role of chemistry, processing conditions, environment, and surface state on the corrosion behavior of metallic glasses and compare their performance with conventional alloys. Several of these alloy systems consist of all biocompatible and non-allergenic elements making them attractive for bioimplants, stents, and surgical tools. To that end, critical insights are provided on the bio-corrosion response of some metallic glasses in simulated physiological environment

Microyielding of Core-Shell Crystal Dendrites in a Bulk-metallic-glass Matrix Composite

In-situ synchrotron x-ray experiments have been used to follow the evolution of the diffraction peaks for crystalline dendrites embedded in a bulk metallic glass matrix subjected to a compressive loading-unloading cycle. We observe irreversible diffraction-peak splitting even though the load does not go beyond half of the bulk yield strength. The chemical analysis coupled with the transmission electron microscopy mapping suggests that the observed peak splitting originates from the chemical heterogeneity between the core (major peak) and the stiffer shell (minor peak) of the dendrites. A molecular dynamics model has been developed to compare the hkl-dependent microyielding of the bulk metallic-glass matrix composite. The complementary diffraction measurements and the simulation results suggest that the interface, as Maxwell damper, between the amorphous matrix and the (211) crystalline planes relax under prolonged load that causes a delay in the reload curve which ultimately catches up with the original path