It is of great importance for a materials scientist both from fundamental and applicability aspects to have better understanding of solid-state phase transformations and its kinetics responsible for micro-/nano-structure development in alloys and corresponding physical and mechanical properties. Transformation kinetics can be analyzed by various experimental techniques such as thermal analysis, laborious electron microscopy combined with extensive image analysis or by measuring changes in electrical resistivity, specimen volume and relative intensities of diffraction lines caused by the phase transformation. Beyond these conventional techniques, this dissertation provides a novel magnetic monitoring approach to study the isothermal kinetics of phase transformations in multicomponent alloy systems involving measurable changes in overall magnetic moment as the transformation proceeds. This dissertation focuses on understanding the microstructural evolution, macro- and micro-alloying behavior, magnetic properties, thermal characteristics, mechanical properties and kinetics of solid-state transformations, i.e. nanoscale precipitation and nanocrystallization, in nickel aluminides and Fe-based bulk amorphous alloys. Microstructural characterization of alloys was done by X-ray diffraction, scanning electron microscopy and transmission electron microcopy techniques. Magnetic properties were analyzed by vibrating sample magnetometry whereas thermal characteristics were evaluated by differential scanning calorimetry. Mechanical properties of alloys were determined by microhardness measurements and compression tests. The influence of Fe macroalloying and 3d transition metal microalloying on the microstructure and properties of Ni-Al-Fe alloys were studied for as-cast and annealed states and it is shown that desired microstructure and related properties can be obtained by proper selection of the type and concentration of macro- or micro-alloying elements together with an appropriate annealing procedure. Thermomagnetic characterization reveals the nanoscale precipitation of a ferromagnetic second phase with annealing. In conjunction with saturation magnetization dependence on annealing, an optimum temperature is identified where nanoscale precipitates impart the highest extent of precipitation strengthening. The isothermal kinetics of ferromagnetic second phase precipitation reveals invariant Avrami exponents close to unity, indicating that nanoscale precipitation is governed by a diffusion-controlled growth process with decreasing growth rate, which closely resembles continuous precipitation kinetics. Appropriate annealing of the Fe-based bulk amorphous alloy precursor produced by suction casting demonstrated extremely fine microstructures containing uniformly distributed and densely dispersed nanocrystals inside a residual amorphous matrix. In order to have better understanding of nanocrystallization mechanisms, kinetic parameters were determined via isothermal magnetic monitoring and non-isothermal differential scanning calorimetry where excellent agreement was obtained in Avrami exponent and activation energy. Analyzing the local kinetics, the nanocrystalline phase was found to evolve through distinct transformation regimes during annealing which were discussed on the basis of transformation kinetics theory and microscopical investigations on each characteristic transformation regime.

Fe-based bulk metallic glasses (BMGs) have gained considerable interest due to their excellent soft magnetic properties with high saturation magnetization, high electrical resistivity, very good corrosion resistance, low materials cost, extremely high mechanical strength and hardness. In spite of having excellent strength, Fe-based BMGs are not used as structural materials in service, so far. The major obstacle is their inherent brittleness under mechanical loading, once a crack is developed the material fails catastrophically. Owing to the ever growing industrial demand for the materials with outstanding properties, aside from exploring new alloy compositions, it is pertinent to understand why or why-not the existing system work and how to improve their properties. Recent reports suggested that the plastic deformability can be enhanced by introducing different microstructural heterogeneities such as free volume enhanced regions, separated phases, nano-crystals, atomic clusters caused by for instance additions of small amount of soft elements. Understanding the effect of addition of soft elements to Fe-based BMGs on thermal stability, structural evolution, magnetic and mechanical properties are the main point which this work addresses. In this work, a study on two different soft ferromagnetic Fe-based glass forming alloys are presented, both of them known to have very high mechanical strength and excellent soft magnetic properties but so far have not been used in any industrial applications. The important issue is with the brittle behavior of this BMGs, particularly under mechanical loading. In each glass forming alloys, the aim was to find out the optimum quantity of the soft elements (Cu and Ga), which can be added to improve their room temperature plastic deformability without affecting the glass forming ability (GFA) and soft magnetic properties. The first glass forming alloy that is studied is Fe36Co36B19.2Si4.8Nb4. This glass forming alloy is highly sensitive to the impurities, only pure elements were used to form this alloy. The addition of only 0.5 at.% Cu completely changes the thermal stability and structural evolution but it also improves the mechanical properties. In case of Ga addition up to 1.5 at.% the crystallization behavior remains unaltered and the thermal stability improves marginally. The addition of Ga improves the plastic deformability of the glass by forming soft zones, whose melting point is much lower compared to rest of the alloy. These soft zones are responsible for the plastic deformation of this glass. Thus addition of Ga is very beneficial in improving the mechanical properties of this Fe-based BMG. In the second part, Fe74Mo4P10C7.5B2.5Si2 glass forming alloy is studied. Unlike the aforementioned alloy, this glass forming alloy is not very sensitive to the impurities, industrial grade alloy elements can also be used to form this alloy. In this alloy addition of Cu is beneficial only up to 0.5 at.%, beyond that Cu addition deteriorates GFA and magnetic properties. In case of Ga addition up to 2 at.% the crystallization behavior remains unaltered and the thermal stability improves marginally. Similar to the FeCoBSiNb glass, the addition of Ga in FeMoPCBSi glass also improves the plastic deformability of the glass by formation of soft zones. Addition of small at.% Ga proved be an viable solution to improve the plastic deformability in the ferromagnetic Fe-based metallic glasses without compromising on thermal and magnetic properties of the glass. In the final part we tried to cast the Fe74Mo4P10C7.5B2.5Si2 glass in a complex shape using an industrial high pressure die casting (HPDC) set up. The important issues were with the casting alloy temperature, casting speed and die material. The aim of our work was to optimize the die material suitable for casting the BMGs and then address the issues with casting temperature and casting speed. We have thus attempted to gain a basic knowledge in casting the Fe-based BMG in industrial scale. Our effort was tremendously successful, we were able to produce fully amorphous complex shaped samples with excellent surface finish. We have thus made a considerable advancement towards understanding the basics behind improving the room temperature plastic deformability in Fe36Co36B19.2Si4.8Nb4 and Fe74Mo4P10C7.5B2.5Si2 ferromagnetic BMGs. We have also made a considerable progress in industrialization of bulk ferromagnetic BMGs.

Many modern technologies are enabled by the use of thin films and/or nanostructured composite materials. For example, many thermoelectric devices, solar cells, power electronics, thermal barrier coatings, and hard disk drives contain nanostructured materials where the thermal conductivity of the material is a critical parameter for the device performance. At the nanoscale, the mean free path and wavelength of heat carriers may become comparable to or smaller than the size of a nanostructured material and/or device. For nanostructured materials made from semiconductors and insulators, the additional phonon scattering mechanisms associated with the high density of interfaces and boundaries introduces additional resistances that can significantly change the thermal conductivity of the material as compared to a macroscale counterpart. Thus, better understanding and control of nanoscale heat conduction in solids is important scientifically and for the engineering applications mentioned above. In this dissertation, I discuss my work in two areas dealing with nanoscale thermal transport: (1) I describe my development and advancement of important thermal characterization tools for measurements of thermal and thermoelectric properties of a variety of materials from thin films to nanostructured bulk systems, and (2) I discuss my measurements on several materials systems done with these characterization tools. First, I describe the development, assembly, and modification of a time-domain thermoreflectance (TDTR) system that we use to measure the thermal conductivity and the interface thermal conductance of a variety of samples including nanocrystalline alloys of Ni-Fe and Co-P, bulk metallic glasses, and other thin films. Next, a unique thermoelectric measurement system was designed and assembled for measurements of electrical resistivity and thermopower of thermoelectric materials in the temperature range of 20 to 350 °C. Finally, a commercial Anter Flashline 3000 thermal diffusivity measurement system is used to measure the thermal diffusivitiy and heat capacity of bulk materials at high temperatures. With regards to the specific experiments, I examine the thermal conductivity and interface thermal conductance of two different types of nanocrystalline metallic alloys of nickel-iron and cobalt-phosphorus. I find that the thermal conductivity of the nanocrystalline alloys is reduced by a factor of approximately two from the thermal conductivity measured on metallic alloys with larger grain sizes. With subsequent molecular dynamics simulations performed by a collaborator, and my own electrical conductivity measurements, we determine that this strong reduction in thermal conductivity is the result of increased electron scattering at the grain boundaries, and that the phonon component of the thermal conductivity is largely unchanged by the grain boundaries. We also examine four complex bulk metallic glass (BMG) materials with compositions of Zr₅₀Cu₄₀Al₁₀, Cu_46.25Zr_44.25Al_7.5Er₂, Fe₄₈Cr₁₅Mo₁₄C₁₅B₆Er₂, and Ti_41.5Zr_2.5Hf₅Cu_42.5Ni_7.5Si₁. From these measurements, I find that the addition of even a small percentage of heavy atoms (i.e. Hf and Er) into complex disordered BMG structures can create a significant reduction in the phonon thermal conductivity of these materials. This work also indicates that the addition of these heavy atoms does not disrupt electron transport to the degree with which thermal transport is reduced. / Ph. D.

No description available.

A systematic study of iron-based binary and multi-component alloys was undertaken to study the structural evolution in these powders as a function of milling time during mechanical alloying. Blended elemental powders of Fe100-XBX (where x = 5, 10, 17, 20, 22, 25, 37.5 and 50 at. %) and a bulk metallic glass (BMG) composition (Fe60Co8Zr10Mo5W2B15) were subjected to mechanical alloying in a SPEX 8000 mixer mill. X-ray diffraction technique was employed to study the phase evolution, crystallite size, lattice strain and also to determine the crystal structure(s) of the phases. Depending on the milling time, formation of supersaturated solid solutions, intermetallics, and amorphous phases was noted in the binary Fe-B powder mixtures. A maximum of about 22 at. % B was found to dissolve in Fe in the solid state, and formation of FeB and Fe2B intermetallics was noted in some of the powder blends. However, an interesting observation that was made, for the first time, related to the formation of a crystalline phase on continued milling of the amorphous powder in the BMG composition. This phenomenon, termed mechanical crystallization, has been explored. Reasons for the mechanical crystallization of the amorphous powder using the X-ray diffraction and electron microscopy methods have been discussed. External heat treatments of the milled powder were also conducted to study the complete crystallization behavior of the amorphous phase. Preliminary attempts were made to consolidate the milled BMG powder to bulk shape by hot isostatic pressing (HIP) and magnetic compaction techniques. Full densification was not achieved. Nanoindentation and microhardness tests were performed to characterize the mechanical properties of the glassy alloy. Nanoindentation results gave an elastic modulus of 59 GPa, lower than the expected value of 184 GPa; due to the presence of porosity in the consolidated sample. Optimization of the consolidation parameters is required to achieve a fully dense material.

Os vidros metálicos baseados em Cu-Zr representam uma classe bastante promissora para a categoria de materiais estruturais, tendo em vista suas interessantes propriedades resultantes da natureza amorfa. Sabe-se que o oxigênio tem grande influência na formação da estrutura amorfa e consequentemente nas propriedades dessa classe de materiais. No presente trabalho, ligas amorfas baseadas em Cu-Zr foram analisadas termicamente a fim de se observar o comportamento das mesmas frente à contaminação com oxigênio. As análises térmicas foram realizadas em um equipamento de calorimetria exploratória diferencial (DSC), e as temperaturas características como de transição vítrea, cristalização, fusão e líquidus foram determinadas. Concluiu-se que conforme a literatura apresenta, o processo de cristalização é favorecido pela presença de oxigênio, causando uma queda na energia de ativação dos processos de cristalização das ligas trabalhadas. Através de ensaios que simularam tratamentos térmicos, cristalizou-se controladamente as amostras amorfas baseadas em Cu-Zr a fim de formar compósitos entre cristais e vidros metálicos buscando diminuir a fragilidade das ligas. / Cu-Zr-based bulk metallic glasses represent a very promising class of structural materials with interesting properties resulting from the amorphous nature. It is known that oxygen has a great influence on the formation of the amorphous structure and consequently on the properties of these materials. In the present work, Cu-Zr-based amorphous alloys were thermally analyzed in order to observe their behavior against oxygen contamination. Thermal analyzis were performed on a differential scanning calorimetry (DSC) equipment, and characteristic temperatures as glass transition, crystallization, melting and liquidus were determined. It was concluded that, according to the literature, the crystallization process is favored by the presence of oxygen, causing a decrease in the activation energy of the crystallization processes of the worked alloys. Through tests that simulated heat treatments, the amorphous samples were crystallized in order to form composites between crystals and metallic glasses in order to reduce the brittleness of the alloys.

The aim of this study is to investigate the bulk glass forming ability (BGFA) of Ti-based alloy systems. These investigations were carried out in two main parts that are complementary to each other: theoretical and experimental. For theoretical studies, which are based on electronic theory of alloys in pseudopotential approximation, Ti-Zr, Ti-Co and Ti-Cu alloys were chosen as the binary systems. Alloying element additions were performed to each binary for the investigation of the BGFA of multicomponent Ti-based alloys. Among the three studied binary systems, Ti-Cu was found to exhibit better BGFA, and Mn, Al and Ni elements were found to be suitable for improving the BGFA of Ti-Cu binary alloy system. BGFA of Ti-Cu binary and Ti-Cu-(Mn, Al, Ni) multicomponent alloys were investigated with the experimental studies that were carried out with performing arc melting and centrifugal casting operations. The characterizations of these alloys were done with scanning electron microscopy, X-ray diffraction analysis and differential scanning calorimetry. Ti60Cu35Mn5, Ti60Cu35Al5 and Ti60Cu35Ni5 alloys were produced and characterized as examples for ternary systems. Among them, Ti60Cu35Mn5 system was found to have better indications regarding to BGFA. Therefore, it was chosen as the main composition and multicomponent alloys of Ti59Cu35Mn5Al1, Ti59Cu35Mn5Ni1 and Ti58Cu35Mn5Al1Ni1 were synthesized and characterized.

Το θέμα αυτής της διπλωματικής είναι παρασκευή άμορφων μεταλλικών συμπαγών κραμάτων με τήξη σε βολταϊκό τόξο και ο χαρακτηρισμός της δομής τους και των μηχανικών ιδιοτήτων τους. Στο πρώτο κεφάλαιο γίνεται εισαγωγή στη δομή των υλικών και επεξηγούνται βασικές έννοιες των άμορφων υλικών και επίσης παρουσιάζονται οι κυριότερες εφαρμογές των άμορφων μεταλλικών συμπαγών κραμάτων. Στο δεύτερο κεφάλαιο περιγράφονται οι τεχνικές που χρησιμοποιούνται για την παρασκευή άμορφων συμπαγών μεταλλικών κραμάτων και στο τρίτο κεφάλαιο αναφέρονται οι κυριότερες φυσικές ιδιότητες των άμορφων μεταλλικών συμπαγών κραμάτων. Στο τέταρτο κεφάλαιο παρουσιάζεται η βαθμονόμηση της συσκευής περίθλασης ακτίνων Χ και της συσκευής παραγωγής και μέτρησης υπερήχων που χρησιμοποιήθηκαν για την μέτρηση των δειγμάτων που παρασκευάστηκαν στο εργαστήριο. Στο πέμπτο κεφάλαιο περιγράφεται ο τρόπος παρασκευής των άμορφων κραμάτων ζιρκονίου (Zr) και σιδήρου (Fe) και επίσης παρουσιάζονται και αναλύονται τα XRD διαγράμματα τους και τα μέτρα ελαστικότητας Young (E), διάτμησης (G) και όγκου (B). Τέλος στο έκτο κεφάλαιο αναφέρονται άλλες μετρήσεις που θα μπορούσαν να γίνουν για να χαρακτηριστούν τα παραπάνω δείγματα και περιγράφονται οι προοπτικές των άμορφων κραμάτων σιδήρου ως αντικείμενο έρευνας για το μέλλον. / The subject of this diploma thesis is the preparation of bulk amorphous metallic alloys with arc-melting technique and the characterization of their structure and their mechanical properties. The first chapter is an introduction to the structure of materials and it is explaine the basic concepts of amorphous materials. It is also present the main applications of bulk amorphous metallic alloys. The second chapter describes the techniques that used in the preparation of bulk amorphous metallic alloys and the third chapter refers the main physical properties of bulk amorphous metallic alloys. The fourth chapter describes the calibration of X-ray diffractοmeter and ultrasound measurement system that used for measuring the samples that have been prepared in the laboratory. In the fifth chapter is described the preparation of amorphous zirconium based (Zr) and iron based (Fe) alloys and also is presented and isanalyzed their XRD patterns and also their elastic moduli, like Young modulus(E), shear modulus (G) and bulk modulue (B). Finally the sixth chapter refers to other measurements that could be used to charactirize the samples that prepared and describes the prospects of amorphous iron based alloys as a research subject for the future.

Os vidros metálicos baseados em Cu-Zr representam uma classe bastante promissora para a categoria de materiais estruturais, tendo em vista suas interessantes propriedades resultantes da natureza amorfa. Sabe-se que o oxigênio tem grande influência na formação da estrutura amorfa e consequentemente nas propriedades dessa classe de materiais. No presente trabalho, ligas amorfas baseadas em Cu-Zr foram analisadas termicamente a fim de se observar o comportamento das mesmas frente à contaminação com oxigênio. As análises térmicas foram realizadas em um equipamento de calorimetria exploratória diferencial (DSC), e as temperaturas características como de transição vítrea, cristalização, fusão e líquidus foram determinadas. Concluiu-se que conforme a literatura apresenta, o processo de cristalização é favorecido pela presença de oxigênio, causando uma queda na energia de ativação dos processos de cristalização das ligas trabalhadas. Através de ensaios que simularam tratamentos térmicos, cristalizou-se controladamente as amostras amorfas baseadas em Cu-Zr a fim de formar compósitos entre cristais e vidros metálicos buscando diminuir a fragilidade das ligas. / Cu-Zr-based bulk metallic glasses represent a very promising class of structural materials with interesting properties resulting from the amorphous nature. It is known that oxygen has a great influence on the formation of the amorphous structure and consequently on the properties of these materials. In the present work, Cu-Zr-based amorphous alloys were thermally analyzed in order to observe their behavior against oxygen contamination. Thermal analyzis were performed on a differential scanning calorimetry (DSC) equipment, and characteristic temperatures as glass transition, crystallization, melting and liquidus were determined. It was concluded that, according to the literature, the crystallization process is favored by the presence of oxygen, causing a decrease in the activation energy of the crystallization processes of the worked alloys. Through tests that simulated heat treatments, the amorphous samples were crystallized in order to form composites between crystals and metallic glasses in order to reduce the brittleness of the alloys.

Fe-based bulk metallic glasses (BMGs) have gained considerable interest due to their excellent soft magnetic properties with high saturation magnetization, high electrical resistivity, very good corrosion resistance, low materials cost, extremely high mechanical strength and hardness. In spite of having excellent strength, Fe-based BMGs are not used as structural materials in service, so far. The major obstacle is their inherent brittleness under mechanical loading, once a crack is developed the material fails catastrophically. Owing to the ever growing industrial demand for the materials with outstanding properties, aside from exploring new alloy compositions, it is pertinent to understand why or why-not the existing system work and how to improve their properties. Recent reports suggested that the plastic deformability can be enhanced by introducing different microstructural heterogeneities such as free volume enhanced regions, separated phases, nano-crystals, atomic clusters caused by for instance additions of small amount of soft elements. Understanding the effect of addition of soft elements to Fe-based BMGs on thermal stability, structural evolution, magnetic and mechanical properties are the main point which this work addresses. In this work, a study on two different soft ferromagnetic Fe-based glass forming alloys are presented, both of them known to have very high mechanical strength and excellent soft magnetic properties but so far have not been used in any industrial applications. The important issue is with the brittle behavior of this BMGs, particularly under mechanical loading. In each glass forming alloys, the aim was to find out the optimum quantity of the soft elements (Cu and Ga), which can be added to improve their room temperature plastic deformability without affecting the glass forming ability (GFA) and soft magnetic properties. The first glass forming alloy that is studied is Fe36Co36B19.2Si4.8Nb4. This glass forming alloy is highly sensitive to the impurities, only pure elements were used to form this alloy. The addition of only 0.5 at.% Cu completely changes the thermal stability and structural evolution but it also improves the mechanical properties. In case of Ga addition up to 1.5 at.% the crystallization behavior remains unaltered and the thermal stability improves marginally. The addition of Ga improves the plastic deformability of the glass by forming soft zones, whose melting point is much lower compared to rest of the alloy. These soft zones are responsible for the plastic deformation of this glass. Thus addition of Ga is very beneficial in improving the mechanical properties of this Fe-based BMG. In the second part, Fe74Mo4P10C7.5B2.5Si2 glass forming alloy is studied. Unlike the aforementioned alloy, this glass forming alloy is not very sensitive to the impurities, industrial grade alloy elements can also be used to form this alloy. In this alloy addition of Cu is beneficial only up to 0.5 at.%, beyond that Cu addition deteriorates GFA and magnetic properties. In case of Ga addition up to 2 at.% the crystallization behavior remains unaltered and the thermal stability improves marginally. Similar to the FeCoBSiNb glass, the addition of Ga in FeMoPCBSi glass also improves the plastic deformability of the glass by formation of soft zones. Addition of small at.% Ga proved be an viable solution to improve the plastic deformability in the ferromagnetic Fe-based metallic glasses without compromising on thermal and magnetic properties of the glass. In the final part we tried to cast the Fe74Mo4P10C7.5B2.5Si2 glass in a complex shape using an industrial high pressure die casting (HPDC) set up. The important issues were with the casting alloy temperature, casting speed and die material. The aim of our work was to optimize the die material suitable for casting the BMGs and then address the issues with casting temperature and casting speed. We have thus attempted to gain a basic knowledge in casting the Fe-based BMG in industrial scale. Our effort was tremendously successful, we were able to produce fully amorphous complex shaped samples with excellent surface finish. We have thus made a considerable advancement towards understanding the basics behind improving the room temperature plastic deformability in Fe36Co36B19.2Si4.8Nb4 and Fe74Mo4P10C7.5B2.5Si2 ferromagnetic BMGs. We have also made a considerable progress in industrialization of bulk ferromagnetic BMGs.