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The irradiation of metallic glasses to induce nanocrystallization was studied in two metallic glass compositions, Cu50Zr45Ti5 and Zr55Cu30Al10Ni5. Atomic mobility was described using a model based on localized excess free volume due to displace- ment cascades created by energetic particle irradiation. Due to the di erence in cascade size among di erent masses of projectiles, a mass-dependent study was per- formed. Metallic glass ribbon samples produced by melt-spinning were bombarded with electron, He, Ar, and Cu particles. Electron irradiation and characterization was performed "in-situ" by means of transmission electron microscopy. The di erent metallic glasses showed dissimilar levels of radiation stability under electron irradi- ation by Cu50Zr45Ti5 forming crystals 1-10 nm in diameter embedded in the amor- phous matrix after about 30 minutes of irradiation, while Zr55Cu30Al10Ni5 showed no such crystallization. Increasing projectile mass caused an increase in the maximum nanocrystal diameter up to approximately 100 nm in Cu irradiated Zr55Cu30Al10Ni5. Studies of di raction patterns of irradiated specimens showed nucleation of Cu10Zr7 phases in both specimens, as well as evidence of CuZr2 in Cu50Zr45Ti5 and both CuZr2 and NiZr2 in Zr55Cu30Al10Ni5. Crystal sizes in irradiated Zr55Cu30Al10Ni5 specimens showed bimodal distribution with many large (50-100 nm) crystals and many small (1-5 nm) crystals. The small crystals in irradiated Zr55Cu30Al10Ni5 were determined to be NiZr2 phase because of the low abundance of Ni. After exposure to 2 keV Ar ions, areas of composition roughly Cu10Zr7 were found by energy-dispersive X-ray spectroscopy but no crystallization was found. Further crystallization was achieved in decomposed specimens after electron irradiation. This shows that atomic segregation is a necessary step before nucleation in metallic glasses.

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Aluminum based amorphous metallic glass powders were produced and tested as the anode materials for the lithium ion rechargeable batteries. Ground Al₈₀Ni&#x2081₀La&#x2081₀ was found to have a low first cycle capacity of about 100 Ah/Kg. The considerable amount of intermetallic formed in the amorphous glass makes the aluminum inactive towards the lithium. The ball milled Al₈₈Ni₉Y₃ powders contain pure aluminum crystalline particles in the amorphous matrix and have first cycle capacity of about 500 Ah/Kg. Nevertheless, polarization was caused by oxidation introduced by the ball-milling process. The electrochemical performances of these amorphous metallic glasses need to be further investigated. Their full lithium insertion capacities cannot be confirmed until the compositions and particle size inside the metallic glass anodes, the conformation of the electrodes and the mechanical milling processes are optimized. / Singapore-MIT Alliance (SMA)

Des de principis dels anys seixanta, els materials coneguts com a vidres metàl·lics han estat objecte d’un gran número d’investigacions centrades principalment en la comprensió de la seva estructura i de les propietats tèrmiques, magnètiques i mecàniques. Tal com el seu nom indica, sent vidres no presenten ordenament atòmic a llarg abast donant lloc a la seva estructura amorfa. Aquesta falta d’ordre els confereix propietats i comportaments considerablement diferents respecte als aliatges cristal·lins. En el camp de les propietats mecàniques, els vidres metàl·lics es caracteritzen per posseir un alt límit elàstic que, juntament amb la possibilitat d’obtenir-los en forma màssica, ha obert un nou camp d’interès en la utilització d’aquests com a materials estructurals. No obstant, la deformació plàstica a temperatura ambient succeeix de forma molt localitzada mitjançant la formació de bandes de cisalla. Com a conseqüència, els vidres metàl·lics, enlloc d’experimentar enduriment per deformació típica dels materials cristal·lins, s’ablaneixen impedint una deformació plàstica estable. Com a resultat, malgrat el seu alt límit de fluència, els vidres metàl·lics tendeixen a fracturar-se després d’una deformació macroscòpica limitada, restringint d’aquesta manera la seva utilització en molts camps d’aplicació. Així doncs, en els darrers temps, la millora de la plasticitat d’aquest tipus de materials s’ha convertit en l’objecte de molts treballs d’investigació. Apart d’alguns casos puntuals on s’han trobat vidres metàl·lics amb plasticitat intrínseca, el concepte més explorat per tal d’evitar tal ruptura catastròfica ha estat la precipitació d’una segona fase. Vàries rutes han estat provades per obtenir aliatges amb matriu amorfa amb més ductilitat: afegir directament una fase reforçant al material fos, dissenyar una composició adequada que resulti en un material compost al solidificar-lo o precipitar la segona fase durant un tractament tèrmic o de deformació. Seguint una d’aquestes rutes, un aliatge heterogeni format, ja sigui per una fase cristal·lina micro- o nano-mètrica, una fase quasicristal·lina o fins i tot una segona fase amorfa integrada dins una matriu amorfa ha estat fabricat. L’estudi desenvolupat al llarg d’aquesta Tesi es centra bàsicament en la millora de les propietats mecàniques dels vidres metàl·lics, donant especial èmfasi a la millora de la plasticitat, mitjançant algunes de les tècniques esmentades anteriorment. Principalment, s’han estudiat els canvis estructurals induïts per deformació, tractament tèrmic i afegint un nou element en diferents famílies de vidres metàl·lics i l’efecte de tals en les propietats mecàniques resultants. Els canvis estructurals han estat caracteritzats per tècniques de calorimetria, difracció de raig-X i per microscòpia electrònica. Tot seguit, la influència d’aquests en el comportament mecànic dels vidres s’ha portat a terme mitjançant principalment tests de compressió i nanoindentació. Els canvis mecànics i estructurals ocasionats per tractaments tèrmics en dues famílies de vidres metàl·lics (basats en Ti i en Fe) han estat estudiats. D’aquesta secció podríem concloure que bé si s’ha observat una bona millora en les propietats mecàniques (mòdul de Young, duresa, resistència al desgast,etc) quan el tractament tèrmic s’ha dut a terme al voltant de la temperatura de transició vítria (microstructura formada per nanocristalls dispersos en una matriu amorfa) la plasticitat continua sent limitada. Tot seguit, hem estudiat els canvis ocasionats per l’addició d’un element, Nb, en una composició inicialment amorfa. Hem pogut observar, com a mesura que introduïm Nb a l’aliatge format per Ti, Zr, Cu i Pd la capacitat de formació vítria va disminuint fins que per un percentatge de Nb del 4 % atòmic ens trobem amb un aliatge pràcticament cristal·lí. No obstant, en les composicions amb el 2% i el 3% de Nb, on s’observa una microstructura formada per petits cristalls integrats en una matriu amorfa, s’ha observat un increment notable (al voltant del 10%) en la plasticitat de l’aliatge. Aquesta Tesi també engloba un estudi detallat de la resposta de materials inicialment amorfs al ser sotmesos a diferents nivells de deformació plàstica. Tot i ser sotmesos a les mateixes condicions de deformació, en l’aliatge basat en titani es detecta ablaniment, mentre que en l’aliatge format principalment per zirconi s’observa enduriment. Els possibles efectes d’aquest comportament dissimilar són explicats en termes de volum lliure, cristal·lització i canvis en l’ordre a curt abast. / Metallic glasses have been the subject of widespread research since the 1960's with significant progress in the understanding of their behaviour. They are amorphous metallic alloys; as the name suggests, being glasses, they do not possess long range order because of their amorphous nature; formed by metals, they are not transparent to light. As a result, they exhibit unique chemical, physical, mechanical and magnetic properties compared to conventional crystalline metallic alloys. In particular, their high yield strength together with the possibility of casting them into bulk form has triggered the interest in using them as structural materials. However, plastic deformation at room temperature occurs in a highly localized manner by the formation of a few shear bands. Instead of work hardening, metallic glasses soften upon deformation, which prevents stable plastic elongation. Once yielding has set in, most metallic glasses tend to fracture in a brittle manner, with almost total lack of plasticity, restricting their use in many applications. As expected, in the last decade, many works were focused on the circumvention of this limited plasticity. Apart from some specific cases where intrinsic plasticity was observed, precipitation of a second phase was found to be the most effective way to promote multiple shear band formation and obtain both, high strength and enhanced plasticity. Many methods were developed to achieve such heterogeneous microstructure. For instance, the second phase can be physically added to the alloy prior to casting, precipitated directly from the melt or by thermal or mechanical treatments of the as-cast alloy. Following one of those procedures a microstructure composed of either a micro- or nano- sized crystalline, quasi-crystalline or non-crystalline phases embedded in an amorphous matrix can be synthesized. The structural and mechanical changes induced in metallic glasses by annealing, element addition and deformation form the main topic of the work presented in this Thesis. The microstructural changes have been characterized by calorimetry, X-ray diffraction and electron microscopy. The influence of these changes on the mechanical behaviour of these glasses has been mainly investigated through compression tests and nanoindentation tests. Structural and mechanical changes upon annealing have been studied in two families of metallic glasses: a Ti40Zr10Cu38Pd12 alloy and a Fe36Co36B19.2Si4.8Nb4 alloy. Annealing treatments of these alloys around Tg, when disperse nanacrystallites are embedded in an amorphous matrix, results in enhanced mechanical properties. However, the former is believed to deteriorate plasticity due to structural relaxation of the amorphous structure. On the other hand, the appearance of tiny nanocrystals embedded in an amorphous matrix is found to be the responsible of the increase above 10% of plastic strain when 3 % of Nb is added to the amorphous Ti40Zr10Cu38Pd12 alloy. However, a brittle behaviour is observed if the Nb content is exceedingly high (4%). The structural and mechanical changes experienced by metallic glasses upon deformation treatments have also been subject of study in this thesis. While strain softening in the Ti40Zr25Ni8Cu9Be18 metallic glass alloy takes place as deformation proceeds (by means of compression test as well as during nanoindentation), in the Zr62Cu18Ni10Al10 BMG the opposite effect, strain hardening, is observed. The possible reasons of the observed dissimilar behaviour have been explained in terms of free volume, crystallization and changes in the short range order.

The tensile behavior of the monolithic amorphous ZrCu and crystalline Cu thin films and the ZrCu/Cu multilayered thin films, coated on polyimide (PI) substrates in different layer thicknesses has been investigated. The scanning electron microscope (SEM) morphology of the as-deposited thin film is composed of sphere domains. Between the domains, stress concentration is induced. The cracks perpendicular to the loading direction would propagate along the domains. The constituent component examined by energy dispersive X-ray spectrometer (EDS) shows that the average composition (in atomic percent) amorphous thin film is Zr46.78Cu53.22, closed to the designed Zr50Cu50 goal. The X-ray diffraction (XRD) results show that the multilayered specimens are composed of both amorphous ZrCu and nanocrystalline Cu crystal structure. As the monolayer thickness become lower, the normalized peak height and grain sizes of Cu become lower. To obtain the mechanical properties of the coated films, deducting the contribution of substrates is used in this study. The tensile Young¡¦s moduli of monolithic amorphous ZrCu and nanocrystalline Cu thin films are close to the results extracted from micro-compression. Based on the current tensile results for the moduli of multilayered thin films, the obtained mechanical data are demonstrated to be reliable and are consistent with the theoretical values predicted by Rule of Mixture. As the thickness decreases from 100 nm down to 10 nm, the tensile Young¡¦s moduli do not vary much. On the other hand, the maximum tensile stress shows strong variation, being highest for the layer thickness of 25 nm. The deformed surface morphologies characterized by scanning electron microscopy also exhibit a similar trend. The optimum tensile properties of the monolithic and multilayered thin film combinations are examined and discussed in this thesis.

The processing and characterization of thin film metallic glasses (TFMGs) and bulk metallic glass foams (BMGFs), prepared by the dealloying corrosion process, have been studied. The TFMGs were fabricated by the co-sputtering with the Zr65Cu25Ti10 (wt%) alloy target and the pure Ta target. For BMG, there are two kinds of amorphous metallic powders adopted, namely, Zr53Cu30Ni9Al8, and Ti40Cu36Pd14Zr10. The Zr- and Ti-based powders were sintered at the temperature that Zr- and Ti-based powders overlap their supercooled regions. To fabricate the porous structure, these materials were selectively dissolved using electrochemical treatments in 0.1 M HNO3 solution. The glassy nature and the pore morphology of the corroded materials were confirmed by X-ray diffraction and scanning electron microscopy. In the current study, the pore size is about 10-30 £gm and the porosity volume faction is about 43%. By using different combinations of the powders size and volume fraction, the resulting pore size and porosity fraction can be upgraded to 200-500 £gm and 60-80%, respectively.

Over the past decades, bulk metallic glasses (BMGs) have attracted extensive interests because of their unique properties such as good corrosion resistance, large elastic limit, as well as high strength and hardness. However, with the advent of micro-electro-mechanical systems (MEMS) and other microscaled devices, the fundamental properties of micrometer-sized BMGs have become increasingly more important. Thus, in this study, a methodology for performing uniaxial compression tests on BMGs having micron-sized dimensions is presented. Micropillar with diameters of 3.8, 1 and 0.7 £gm are fabricated successfully from the Mg65Cu25Gd10 and Zr63.8Ni16.2Cu15Al5 BMGs using focus ion beam, and then tested in microcompression at room temperature and strain rates from 1 x 10-4 to 1 x 10-2 s-1. Microcompression tests on the Mg- and Zr-based BMG pillar samples have shown an obvious sample size effect, with the yield strength increasing with decreasing sample diameter. The strength increase can be rationalized by the Weibull statistics for brittle materials, and the Weibull moduli of the Mg- and Zr-based BMGs are estimated to be about 35 and 60, respectively. The higher Weibull modulus of the Zr-based BMG is consistent with the more ductile nature of this system. In additions, high temperature microcompression tests are performed to investigate the deformation behavior of micron-sized Au49Ag5.5Pd2.3Cu26.9Si16.3 BMG pillar samples from room to their glass transition temperature (~400 K). For the 1 £gm Au-based BMG pillars, a transition from inhomogeneous flow to homogeneous flow is clearly observed at or near the glass transition temperature. Specifically, the flow transition temperature is about 393 K atthe strain rate of 1 x 10-2 s-1. For the 3.8 £gm Au-based BMG pillars, in order to investigate the homogeneous deformation behavior, microcompression tests are performed at 395.9-401.2 K. The strength is observed to decrease with increasing temperature and decreasing strain rate. Plastic flow behavior can be described by a shear transition zone model. The activation energy and the size of the basic flow unit are deduced and compared favorably with the theory.

Macroscopic ductility is promoted in bulk metallic glasses by both composite reinforcements (at low temperatures) and by the activation of viscous flow mechanisms (at high temperatures). It is of fundamental interest to understand deformation physics when both of these strategies are employed at the same time. Despite the quickly growing literature around the room-temperature mechanical properties of metallic glass matrix composites (MGMCs), the deformation behavior of MGMCs over a wide range of temperatures and strain rates has yet to be systematically investigated, especially at high temperatures close to Tg. Here the high temperature compressive behavior of Zr-based MGMCs with in-situ reinforcements is explored systematically over a series of strain rates. Additionally, the volume fraction of second-phase reinforcements was tailored to explore its effect on both inhomogeneous and homogeneous deformation modes. / Singapore-MIT Alliance (SMA)

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