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Metallic glass is a class of materials which have a disordered structure of atoms, due to this, glasses lack grains and grain boundaries, which are present in their crystalline counterparts. Metallic glasses have many interesting properties worth investigating, such as high corrosion resistance or high mechanical strength. However, metallic glasses are metastable and will therefore crystallise if heated above the crystallisation temperature. MoSiZr alloys have been studied and to gain knowledge of how the composition affects the crystallisation temperature, which enables further improvement of thermal stability. Crystallisation temperatures of the MoSiZr alloys were investigated by heat treatments in vacuum and ex-situ X-ray diffraction and X-ray reflectivity analysis. The highest thermal stability of the alloys was exhibited by M48Si48Zr4, Mo43Si50Zr7, Mo50Si40Zr10 and Mo45Si43Zr12, they remained amorphous after heat treatment at 1073 K. The resulting crystalline phases are Mo3Si, Mo5Si3 and ZrO2. Oxidation of Zr in the alloys is present only when the Zr content is at least 10 at%, crystallisation is otherwise mainly driven by formation of Mo3Si. Further improvement of the thermal stability is possible by introducing new alloying elements at the cost of those that promote crystallisation. Keeping the content of Zr below 10 at% is of great importance to prevent oxidation.

Bulk metallic glasses (BMGs) have received significant research interest due to their completely amorphous structure which results in unique structural and functional properties. Absence of grain boundaries and secondary phases in BMGs results in high corrosion resistance in many different environments. Understanding and tailoring the corrosion behavior can be significant for various structural applications in bulk form as well as coatings. In this study, the corrosion behavior of several Zr-based and Fe-Co based BMGs was evaluated to understand the effect of chemistry as well as quenched in free volume on corrosion behavior and mechanisms. Presence of Nb in Zr-based alloys was found to significantly improve corrosion resistance due to the formation of a stable passive oxide. Relaxed glasses showed lower rates compared to the as-cast alloys. This was attributed to lowering of chemical potential from the reduced fraction of free volume. Potentiodynamic polarization and Electrochemical Impedance Spectroscopy (EIS) techniques helped in quantifying the corrosion rate and polarization resistance. The effect of alloy composition was quantified by extensive surface analysis using Raman spectroscopy, energy dispersive x-ray spectroscopy and auger spectroscopy. Pitting intensity was higher in the as-cast glasses than the relaxed glasses. The electrochemical behavior of a Zr-Ti-Cu-Ni-Be bulk metallic glass subjected to high strain processing was studied. High strain processing caused shear band formation and an increase in the free volume. Potentiodynamic polarization and EIS showed a strong correlation between the enthalpy of structural relaxation and corrosion rate and polarization resistance. Pitting was observed to preferentially occur on shear bands in the processed samples, while it was stochastic in unprocessed glass. The corrosion analysis of Co-Fe glasses showed an increase in corrosion current density when Fe content was increased from 0 to 7 at%. The corrosion resistance improved when Fe content was further increased to 15 at%. Similar trend was seen in EIS studies. The improved corrosion resistance at 15 at% Fe can be attributed to the large supercooled region that facilitates the formation of completely amorphous alloy, in contrast to lower Fe containing alloys, where short range ordering may deteriorate the corrosion resistance. Porous metallic glass structure was developed by electrochemical dealloying via cyclic voltammetry. Mechanical properties and changes in electrical conductivity were measured as a function of depth from surface by nano-indentation and nano electrical contact resistance technique. The nanoporous layer was found have hardness of 0.41 GPa and elastic modulus of nearly 22 GPa. The resistivity of the nanoporous layer continuously decreased when moving towards the substrate as the indentation depth increased which is attributed to the gradient in pore size.

Amorphous ribbons of alloy composition Fe75B10Si15 are cast by melt spinning and annealed to partially nanocrystalline states. The magnetic properties are investigated by VSM and MTGA. Structure is examined using XRD and SEM. Results obtained show nanostructured material with excellent soft magnetism in samples annealed at temperatures below the crystallization temperature as well as enhancement of magnetic hardness for annealing at high temperatures. This validates Herzer’s Random Anisotropy model of magnetism in nanostructured materials and provides basis for further inquiry into tweaking alloy compositions and/or manipulating annealing parameters. Also, increase of Curie temperature is noted with respect to increasing annealing temperatures arising from stress relaxation, validating a study on the relationship between the two.

Equiatomic composition of Ni and Ti was cryomilled with varying milling times to create Ni-Ti lamella structures with average spacings of 50 nm, 470 nm, and 583 nm in powder particles to vary the interfacial surface area per volume. These surfaces form interfaces for diffusion that are essential for solid state amorphization during low temperature annealing. To compare solid state amorphization in a relatively defect free multilayer system, elemental Ni and Ti were deposited by electron beam physical vapor deposition on titanium plates with comparable spacing as above. Both milled and deposited multilayers were annealed between 225 and 350°C for up to 50 hours. X-ray diffraction characterization and in situ annealing was conducted on cryomilled and deposited multilayers of Ni-Ti. Based on this characterization, an amorphization model based on the Johnson-Mehl-Avrami nucleation and growth equation has been established to predict the amorphization of both cryomilled and deposited multilayers. Cryomilled powders experienced much larger amorphization rates during annealing than that of deposited multilayer structures, for all layer spacings. This superior amorphization is seen despite the formation of amorphous phase during the milling process; the amount of which increases with increasing milling time. The difference in amorphization rates between cryomilled and deposited multilayers is attributed to excess driving force due to the extensive preexisting defects in the powders caused by cryomilling. Serial 3D reconstruction of cryomilled Ni-Ti powders was done by scanning electron microscopy and focused ion beam. Through 3D reconstruction it was observed that a random and non-linear lamella structure has been formed in cryomilled powders. Furthermore, lamellar spacing was seen to become smaller with increased milling time while at the same time becoming more homogeneous through the material's volume. 3D reconstruction of cryomilled Ni-Ti offers a unique insight into the microstructures and surface areas of cryomilled powder particles that has never been accomplished. / Ph. D.

En la presente memoria se describe el método para la medida de la variación térmica de la resistividad entre 10 y 300 K que hemos puesto a punto en nuestros laboratorios de la Universidad de Cantabria. Dicho método es el de cuatro puntas con corriente alterna y detección síncrona. También se analizan, mediante el método de los elementos finitos, los efectos de la geometría de la muestra y posición de los contactos en la relación entre la resistividad observada experimentalmente y la resistividad real, haciendo hincapié en el caso de muestras anisótropas. Posteriormente aplicamos esta técnica al estudio de diversas series de materiales: 1) Los compuestos RPt que cumplen la Ley de De Gennes y en los que hemos podido determinar los valores de la masa efectiva m* y de la constante de canje. 2) Los compuestos RNi2Si2 en las que se puede observar una relación entre la resistividad magnética y las estructuras magnéticas. 3) Los compuestos CeyLa1-yNixPt1-x, en las que se estudia el efecto Kondo 4) Los amorfos metálicos (Co1-x(Fe5Ni5)x)75Si15B10 en los que se observa un mínimo en la resistividad a bajas temperaturas así como variaciones en lnT, T2 y T a temperaturas bajas, intermedias y altas respectivamente / The present report describes the method for measuring the temperature dependence of the resistivity between 10 and 300 K that we have developed in our laboratories at the University of Cantabria. This is a four-probe method using ac current and synchronous detection. Also it is analyzed, using the finite element method, the effects of the sample geometry and contacts position in the relationship between the real resistivity and resistivity observed experimentally, even in the case of anisotropic samples. Subsequently we apply this technique to the study of different family of materials: 1) RPt compounds which follow the De Gennes Law, allowing us to determine the values of the effective mass m * and exchange constant. 2) The compounds RNi2Si2 in which it is possible to establish a relationship between the magnetic resistivity and the magnetic structures. 3) The compounds CeyLa1-yNixPt1-x, in which we study the Kondo effects. 4) The amorphous Co1-x(Fe5Ni5)x)75Si15B10 metals, in which there is a minimum in the resistivity at low temperatures as well as variations in LnT, T2 and T at low, intermediate and high temperatures respectively.

Structural characterization is essential for the understanding of the magnetic properties of thin films and multilayers. In this thesis, both crystalline and amorphous thin films and multilayers were analyzed utilizing transmission electron microscopy (TEM). High resolution TEM and electron diffraction studies emphasize on the growth of amorphous Fe91Zr9 and Co68Fe24Zr8 on both Al2O3 and Al70Zr30 in multilayer structures by magnetron sputtering. The properties of the growth surfaces were found to strongly influence the formation of nano-crystallites of the magnetic material at interfaces. Field induced uniaxial magnetic anisotropy was found to be possible to imprint into both fully amorphous and partially crystallized Co68Fe24Zr8 layers, yielding similar magnetic characteristics regardless of the structure. These findings are important for the understanding of both growth and magnetic properties of these amorphous thin films. As magnetic systems become smaller, new analysis techniques need to be developed. One such important step was the realization of electron energy-loss magnetic circular dichroism (EMCD) in the TEM, where information about the ratio of the orbital to spin magnetic moment (mL/mS) of a sample can be obtained. EMCD makes use of angular dependent inelastic scattering, which is characterized using electron energy-loss spectroscopy. The work of this thesis contributes to the development of EMCD by performing quantitative measurements of the mL/mS ratio. Especially, methods for obtaining energy filtered diffraction patterns in the TEM together with analysis tools of the data were developed. It was found that plural inelastic scattering events modify the determination of the mL/mS ratio, wherefore a procedure to compensate for it was derived. Additionally, utilizing special settings of the electron gun it was shown that EMCD measurements becomes feasible on the nanometer level through real space maps of the EMCD signal.

Metallische Gläser erreichen aufgrund ihrer ungeordneten, amorphen Struktur eine Streckgrenze, welche andere metallische Materialien in den Schatten stellt (bis zu 5 GPa). Dabei wird aufgrund ihres geringen E-Moduls eine elastische Verformung von circa zwei Prozent erreicht und damit eine sonst unerreichte Menge elastischer Energie aufgenommen. Leider besitzen die metallischen Gläser auch einen enormen Nachteil: Aufgrund der Erweichung der Gläser bei plastischer Verformung kommt es zur Lokalisierung dieser in sogenannten Scherbändern (SBs). Dies führt nahezu ohne plastische Verformung zum Versagen des Materials. Es gibt bereits viele Methoden diesen Nachteil zu überwinden, indem man die Verformung durch das Einbringen von strukturellen Heterogenitäten auf eine Vielzahl von Scherbändern verteilt: Beispielsweise durch elastische Belastung, plastische Verformung mit eingeschränkten Freiheitsgraden, oder das Einbringen einer Fremdphase. Allerdings sind die dabei wirkenden Mechanismen und Wechselwirkungen zwischen der Struktur und der Verformung noch nicht vollständig verstanden. In dieser Arbeit wurde daher zum einen die Struktur des Glases beeinflusst und diese Änderungen charakterisiert und zum anderen die Wirkung dieser Veränderung auf das Verformungsverhalten untersucht. Als Ausgangszustand wurden dabei gegossene Stäbe und Platten vier verschiedener Legierungen (mit ansteigender Glasbildungsfähigkeit: Cu47,5Zr47,5Al5, Cu46Zr46Al8, Cu45Zr45Al5Ag5, Cu36Zr48Al8Ag8; von 1,5 mm bis 25 mm kritischem Gießdurchmesser) verwendet, welche mechanischen Verformungen unterzogen wurden, um strukturelle Heterogenitäten einzubringen. Zudem wurden B2-Glasmatrixkomposite mit der Formgedächtnisphase B2-CuZr beim Abschrecken der Schmelze (Cu47,5Zr46,5Al5-Sc1) bzw. beim Rascherhitzen über die Kristallisationstemperatur (Cu44Zr44Al8Hf2-Co2) hergestellt. Die angewandten Methoden decken ein breites Spektrum der in der Literatur diskutierten Vorbehandlungen mit einem Einfluss auf die Struktur und die mechanischen Eigenschaften ab. Die systematischen und umfangreichen Untersuchungen in dieser Arbeit beleuchten detailliert den Zusammenhang zwischen der Struktur und der Verformung metallischer Gläser. Die Struktur wird gezielt manipuliert um eine Veränderung ihrer Heterogenität zu induzieren. Dabei wird auf Veränderungen in atomaren Längenskalen ebenso eingegangen, wie auf die makroskopischen Unterschiede. Außerdem wird die Abhängigkeit der induzierten strukturellen Änderungen von der Stabilität des Glases betrachtet. Dies alles stellt einen wichtigen Beitrag zum Verständnis des Verformungsverhaltens metallischer Gläser dar. In letzter Konsequenz ermöglichen die Erkenntnisse, durch eine gezieltere Manipulation der Struktur, eine erhöhte plastische Verformung im metallischen Glas zu erreichen.

Fe-based amorphous powders are fabricated through gas and water atomization using industrial grade raw materials. The atomic structure of the powder is examined by X-Ray Diffraction (XRD). Eight of totally thirteen different compositions are proved completely amorphous or amorphous with traces of crystalline phase in the desired powder particle size (d > 75 μm) and five are crystalline. It reveals that the Glass Forming Ability (GFA) of atomized powders is well correlated to the GFA of as-casted rods or melt-span ribbons. In the present study at least 1.5-2 mm critical size of GFA for a target composition is necessary for the formation of amorphous powders in the desired particle size. The thermal stability of the amorphous powder is examined by Differential Scanning Calorimetry (DSC). Applying the conventional powder metallurgy process the amorphous powders are mixed with the crystalline Somaloy® 110i, a commercial Soft Magnetic Composite (SMC) material from Höganäs AB in Sweden, and made into toroid-shaped components. The components are annealed aiming for improved soft-magnetic properties. The magnetic measurements are taken on copper-wire double coiled toroids. As a result, the total magnetic flux (B), coercivity (HC) and permeability (μmax) is reduced due to the addition of amorphous powders to Somaloy® 110i powder but the core losses (P) is at the same level despite reduced density. An improved soft magnetic property and core loss is revealed by the comparison to recent literature reports on SMC mixing of crystalline and amorphous powders.

Metallic glasses possess attractive properties, such as high strength, good corrosion resistance, and superior soft magnetic performance. They also serve as precursors for synthesizing nanocrystalline materials. In addition, a new class of composites having crystalline phases embedded in amorphous matrix is evolving based on selective crystallization of metallic glasses. Therefore, crystallization of metallic glasses and its effects on properties has been a subject of interest. Previous investigations from our research group related to laser assisted crystallization of Fe-Si-B metallic glass (an excellent soft magnetic material by itself) showed a further improvement in soft magnetic performance. However, a fundamental understanding of crystallization and mechanical performance of laser treated metallic glass was essential from application point of view. In light of this, the current work employed an integrated experimental and computational approach to understand crystallization and its effects on tensile behavior of laser treated Fe-Si-B metallic glass. The time temperature cycles during laser treatments were predicted using a finite element thermal model. Structural changes in laser treated Fe-Si-B metallic glass including crystallization and phase evolution were investigated with the aid of X-ray diffraction, differential scanning calorimetry, resistivity measurements, and transmission electron microscopy. The mechanical behavior was evaluated by uniaxial tensile tests with an InstronTM universal testing machine. Fracture surfaces of the metallic glass were observed using scanning electron microscopy and site specific transmission electron microscopy. Fe-Si-B metallic glass samples treated with lower laser fluence (<0.49 J/mm2) underwent structural relaxation while higher laser flounces led to partial crystallization. The crystallization temperature experienced an upward shift due to rapid heating rates of the order of 104 K/s during laser treatments. The heating cycle was followed by termination of laser upon treatment attainment of peak temperature and rapid cooling of the similar order. Such dynamic effects resulted in premature arrest of the crystallite growth leading to formation of fine crystallites/grain (~32 nm) of α-(Fe,Si) as the major component and Fe2B as the minor component. The structural relaxation, crystallization fractions of 5.6–8.6 Vol% with α-(Fe,Si) as the main component, and crystallite/grain size of the order of 12 nm obtained in laser fluence range of 0.39-0.49 J/mm2 had minimal/no influence on tensile behavior of the laser treated Fe-Si-B metallic glass foils. An increase in laser fluence led to progressive increase in crystallization fractions with considerable amounts of Fe2B (2-6 Vol%) and increase in grain size to ~30 nm. Such a microstructural evolution severely reduced the strength of Fe-Si-B metallic glass. Moreover, there was a transition in fracture surface morphology of laser treated Fe-Si-B metallic glass from vein pattern to chevron pattern. Tensile loading lacked any marked influence on the crystallization behavior of as-cast and structurally relaxed laser-treated metallic glass foils. However, a significant crystallite/grain growth/coarsening of the order of two and half times was observed in the fractured region compared to the region around it for the laser-treated partially crystallized metallic glass foils. The simultaneous effects of stress generation and temperature rise during tensile loading were considered to play a key role in crystallite/grain growth/coarsening.