abstract: Broadband dielectric spectroscopy is a powerful technique for understanding the dynamics in supercooled liquids. It generates information about the timescale of the orientational motions of molecular dipoles within the liquid. However, dynamics of liquids measured in the non-linear response regime has recently become an area of significant interest, because additional information can be obtained compared with linear response measurements. The first part of this thesis describes nonlinear dielectric relaxation experiments performed on various molecular glass forming-liquids, with an emphasis on the response at high frequencies (excess wing). A significant nonlinear dielectric effect (NDE) was found to persist in these modes, and the magnitude of this NDE traces the temperature dependence of the activation energy. A time resolved measurement technique monitoring the dielectric loss revealed that for the steady state NDE to develop it would take a very large number of high amplitude alternating current (ac) field cycles. High frequency modes were found to be ‘slaved’ to the average structural relaxation time, contrary to the standard picture of heterogeneity. Nonlinear measurements were also performed on the Johari-Goldstein β-relaxation process. High ac fields were found to modify the amplitudes of these secondary modes. The nonlinear features of this secondary process are reminiscent of those found for the excess wing regime, suggesting that these two contributions to dynamics have common origins. The second part of this thesis describes the nonlinear effects observed from the application of high direct current (dc) bias fields superposed with a small amplitude sinusoidal ac field. For several molecular glass formers, the application of a dc field was found to slow down the system via reduction in configurational entropy (Adam-Gibbs relation). Time resolved measurements indicated that the rise of the non-linear effect is slower than its decay, as observed in the electro-optical Kerr effect. A model was discussed which quantitatively captures the observed magnitudes and time dependencies of the NDE. Asymmetry in these rise and decay times was demonstrated as a consequence of the quadratic field dependence of the entropy change. It was demonstrated that the high bias field modifies the polarization response to the field, even including the zero field limit. / Dissertation/Thesis / Doctoral Dissertation Chemistry 2016

No description available.

Bulk metallic glasses have interesting combination of physical, chemical, mechanical, and magnetic properties which make them attractive for a variety of applications. Consequently there has been a lot of interest in understanding the structure and properties of these materials. More varied applications can be sought if one understands the reasons for glass formation and the methods to control them. The glass-forming ability (GFA) of alloys can be substantially increased by a proper selection of alloying elements and the chemical composition of the alloy. High GFA will enable in obtaining large section thickness of amorphous alloys. Ability to produce glassy alloys in larger section thicknesses enables exploitation of these advanced materials for a variety of different applications. The technique of mechanical alloying (MA) is a powerful non-equilibrium processing technique and is known to produce glassy (or amorphous) alloys in several alloy systems. Metallic amorphous alloys have been produced by MA starting from either blended elemental metal powders or pre-alloyed powders. Subsequently, these amorphous alloy powders could be consolidated to full density in the temperature range between the glass transition and crystallization temperatures, where the amorphous phase has a very low viscosity. This Dissertation focuses on identifying the various Fe-based multicomponent alloy systems that can be amorphized using the MA technique, studying the GFA of alloys with emphasis on improving it, and also on analyzing the effect of extended milling time on the constitution of the amorphous alloy powder produced at earlier times. The Dissertation contains seven chapters, where the lead chapter deals with the background, history and introduction to bulk metallic glasses. The following four chapters are the published/to be published work, where the criterion for predicting glass formation, effect of Niobium addition on glass-forming ability (GFA), lattice contraction on amorphization, effect of Carbon addition on GFA, and observation of mechanical crystallization in Fe-based systems have been discussed. The subsequent chapter briefly mentions about the consolidation of amorphous powders and presents results of hot pressing and spark plasma sintering on one of the alloy systems. The final chapter summarizes the Dissertation and suggests some prospective research work that can be taken up in future. The Dissertation emphasizes the glass-forming ability, i.e., the ease with which amorphization can occur. In this work the milling time required for amorphization was the indicator/measure of GFA. Although the ultimate aim of this work was to consolidate the Fe-based amorphous alloy powders into bulk so as to undertake mechanical characterization, however, it was first necessary to study the glass forming aspect in the different alloy systems. By doing this a stage has been reached, where different options are available with respect to amorphous phase-forming compositions and the knowledge to improve glass-forming ability via the mechanical alloying technique. This will be ultimately useful in the powder compaction process into various shapes and sizes at optimum pressure and temperature. The study on mechanical crystallization indicates, or in a way defines, a limit to the process of amorphization, and it was also demonstrated that this phenomenon is more common in occurrence than and not as restricted as it was earlier reported to be.

Bulk metallic glasses (BMGs) are known to have various advantageous chemical and physical properties. However, the condition of producing BMGs is critical. From a melt to congealing into a glass, the nucleation and growth of crystals has to be suppressed, which requires a fast removal of the heat. Such high cooling rates inevitably confine the casting dimensions (so-called critical casting thickness). To overcome this shortcoming, additive manufacturing proves to be an interesting method for fabricating metastable alloys, such as bulk metallic glasses. Selective laser melting (SLM), one widely used additive manufacturing technique, is based on locally melting powder deposited on the powder bed layer by layer. During the SLM process, the interaction between laser beam and alloys is completed with a high energy density (105 - 107 W/cm2) in very short duration (10-3 - 10-2 s), which results in a high cooling rate (103 - 108 K/s). Such high cooling rates favour vitrification and to date, various glass-forming alloys have been prepared. The approach to prepare bulk metallic glasses (BMGs) by SLM bears the indisputable advantage that the size of the additively manufactured glassy components can exceed the typical dimensions of cast bulk metallic glasses. Simultaneously, also delicate and complex geometries can be obtained, which are otherwise inaccessible to conventional melt quenching techniques. By using such advantages of SLM, Ti47Cu38Zr7.5Fe2.5Sn2Si1Ag2 (at.%) and Zr52.5Cu17.9Ni14.6Al10Ti5 (at.%) BMGs have been successfully fabricated via SLM in the current work. The SLM process yields material with very few and small defects (pores or cracks) while the conditions still have to render possible vitrification of the molten pool. This confines the processing window of the fully amorphous SLM samples. By additively manufacturing different BMG systems, it is revealed that the non-linear interrelation is differently pronounced for varied compositions. The only way to obtain glassy and dense products is optimizing all the process parameters. However, it is difficult to obtain fully dense sample (100%). The relative density of the additively manufactured BMGs can reach 98.5% (Archimedean method) in current work. The residual porosity acts as structural heterogeneities in the additively manufactured BMGs. The structures of BMGs are sensitive to the thermal history, i.e. to the cooling rate and to the thermal treatment. During SLM process, the laser beam not only melts the topmost powder, but also the adjacent already solidified parts. Such complicated thermal history may lead to locally more/less relaxed structure of the additively manufactured BMGs. Thus, systematic and extensive calorimetric measurements and nanoindentation tests were carried out to detect these structural heterogeneities. The relaxation enthalpies, which can reveal the free volume content and average atomic packing density in the additively manufactured BMGs are much higher than that in the as-cast samples, indicating an insufficient duration for structural relaxation. The nanoindentation tests indicate that the structure of additively manufactured BMG is more heterogeneous than that of as-cast sample. Nevertheless, no obvious heat-affected zone which corresponds to the more/less relaxed structure is visible in the hardness map. In order to reveal the origin of such heterogeneity, the thermal field of the additively manufactured BMGs was simulated via finite volume method (FVM). Owing to the different process parameters and varied thermophysical properties of Ti47Cu38Zr7.5Fe2.5Sn2Si1Ag2 and Zr52.5Cu17.9Ni14.6Al10Ti5 BMGs, the heat-affected zone (HAZ) is differently pronounced, resulting in the varied heterogeneities of both additively manufactured BMGs. Afterwards, the physical and chemical properties of the additively manufactured BMGs were systematically studied. The additively manufactured BMGs tend to fail in a premature manner. The heterogeneities (defects, crystalline phases and relaxed/rejuvenated regions) can determine the mechanical and chemical properties of the BMGs. In the current work, the additively manufactured BMGs are fully amorphous. Thus, the effects of crystalline phases can be ruled out. The effect of residual porosity and more/less relaxed state on the deformation of additively manufactured and as-cast BMGs has been studied. The analysis of the observed serrations during compressive loading implies that the shear-band dynamics in the additively manufactured samples distinctly differ from those of the as-cast glass. This phenomenon appears to originate from the presence of uniformly dispersed spherical pores as well as from the more pronounced heterogeneity of the glass itself as revealed by instrumented indentation. Despite these heterogeneities, the shear bands are straight and form in the plane of maximum shear stress. Additive manufacturing, hence, might not only allow for producing large BMG samples with complex geometries but also to manipulate their deformation behaviour through tailoring porosity and microstructural heterogeneity. Different from the compressive tests, the heterogeneities of additively manufactured BMGs have no significant effect on the tribological and corrosion properties. The similar specific wear rate and the worn surfaces demonstrate that similar wear mechanisms are active in the additively manufactured and the as-cast samples. The same holds for the corrosion tests. The anodic polarization curves of SLM samples and as-cast samples illustrate a similar corrosion behaviour. However, the SLM samples have a slightly reduced susceptibility to pitting corrosion and reveal an improved surface healing ability, which might be attributed to an improved chemical homogeneity of the additively manufactured BMGs. In order to improve plasticity, bulk metallic glasses composites (BMGCs) have been developed, in which crystals precipitate in a glassy matrix. The crystalline phases can alter the local stress state under loading, thereby, impacting the initiation and propagation of the shear bands. However, it is difficult to control the crystalline volume fraction as well as the size and spacing between the crystals by using the traditional melt-quenching method. One approach is to mix glass-forming powder with conventional alloy powder. In this way, a large degree of freedom for designing the microstructure can be gained. Thus, SLM was chosen to prepare such “ideal” BMGCs in the present work. The β-phase stabilizer Nb powder was mixed with Zr52.5Cu17.9Ni14.6Al10Ti5 powder. After SLM processing, the irregular-shaped Nb particles are distributed uniformly within the glassy matrix and bond well to it. At the higher Nb content, diffusion of Nb during processing locally deteriorates the glass-forming ability of the matrix and results in the formation of several brittle intermetallic phases around the Nb particles. The size of these precipitates covers a wide range from nanometres to micrometres. Despite the fact that the soft Nb particles increase the heterogeneity of the glassy matrix, none of the samples deforms plastically. This is attributed to the network-like distribution of the intermetallic phases, which strongly affects the fracture process. Besides the ex-situ method of mixing powders, designing in-situ ductile phases and controlling the fraction of the crystalline phases by altering process parameters can also prepare optimized BMGCs. Cu46Zr46Al8 (at.%) was processed via SLM to produce in-situ BMGCs. It is revealed that the microstructure of the nearly fully dense additively manufactured BMGs is strongly affected by the energy input. By increasing the energy input, the amount of the crystalline phases was raised. By optimizing the energy input, the B2 CuZr phase was particularly deliberately introduced. Due to the residual porosity and brittle phases, no plasticity is visible in the additively manufactured samples. Generally, selective laser melting opens a gateway to design the microstructure of the BMG matrix composites.:Abstract I Kurzfassung IV Symbols and abbreviations VIII Aims and objectives VIII CHAPTER 1 Metallic glasses and selective laser melting 1 1.1 Formation of metallic glasses from the melt 1 1.2 Mechanical properties of BMGs and their composites 4 1.2.1 Shear banding in metallic glasses 4 1.2.2 Effect of structural heterogeneities on plastic deformation 7 1.2.2.1 Nanoscale heterogeneities 8 1.2.2.2 Microscale heterogeneities 11 1.2.3 Shear band dynamics 13 1.2.4 Tribological properties of BMGs 15 1.3 Corrosion behaviour of bulk metallic glasses 16 1.4 Selective laser melting (SLM) 20 1.4.1 The SLM process 20 1.4.1.1 Powder properties 21 1.4.1.2 Process parameters 22 1.4.2 Solidification and thermal history 25 1.5 Selectively laser-melted glass formers 28 1.5.1 Selective laser melting of a single alloy powder 28 1.5.2 Heterogeneities and mechanical properties of additively manufactured BMGs 32 CHAPTER 2 Experimental 36 2.1 Sample preparation 36 2.1.1 Arc melting 36 2.1.2 Suction casting 36 2.1.3 Gas atomization 37 2.1.4 Powder mixtures 37 2.1.5 Selective laser melting (SLM) 38 2.1.5 Heat treatment 39 2.2 Sample characterization methods 39 2.2.1 Composition analysis 40 2.2.2 X-ray diffraction 40 2.2.3 Calorimetry 40 2.2.4 Density measurements (Archimedean method) 41 2.2.5 µ-CT 41 2.2.6 Scanning electron microscopy (SEM) 41 2.2.7 Transmission electron microscopy (TEM) 42 2.2.8 Hardness measurements 42 2.2.9 Compression tests 43 2.2.10 Sliding wear tests 43 2.2.11 Corrosion tests 44 2.2.12 Finite volume method modelling 45 CHAPTER 3 Selective laser melting of glass-forming alloys 46 3.1 Selective laser melting of a Ti47Cu38Zr7.5Fe2.5Sn2Si1Ag2 BMG 46 3.1.1 Powder analysis 47 3.1.2 Parameter optimization and microstructural characterization 48 3.1.3 Mechanical properties 55 3.1.3.1 Compression tests 55 3.1.3.2 Microhardness and structural relaxation 57 3.1.3.3 Nanoindentation 59 3.1.4 Corrosion properties 61 3.2 Selective laser melting of a Zr52.5Cu17.9Ni14.6Al10Ti5 BMG 62 3.2.1 Powder analysis 62 3.2.2 Microstructural characterization 63 3.2.3 Mechanical properties 66 3.2.3.1 Compression tests 66 3.2.3.2 Microhardness and structural relaxation 68 3.2.3.3 Nanoindentation 71 3.2.4 Shear band dynamics and shear band propagation 74 3.2.5 Tribological and corrosion properties 80 3.3 Structural heterogeneities of BMGs produced by SLM 87 CHAPTER 4 Selective laser melting of ex-situ Zr-based BMG matrix composites 97 4.1 Phase formation 97 4.2 Microstructures 101 4.3 Mechanical properties 110 CHAPTER 5 Selective laser melting of in-situ CuZr-based BMG matrix composites 115 5.1 Powder analysis 115 5.2 Parameter optimization 116 5.3 Microstructure 120 5.4 Mechanical properties 124 5.4.1 Compression tests 124 5.4.2 Microhardness and structural relaxation 127 5.4.3 Nanoindentation 129 CHAPTER 6 Summary 132 CHAPTER 7 Outlook 132 Acknowledgements 137 Bibliography 139 Publications 163 Eidesstattliche Erklärung 164

Desde a descoberta das ligas amorfas em 1960, os motivos pelos quais algumas ligas podem ser facilmente amorfizadas enquanto outras não podem, não é claramente conhecido, assim não há teoria universal para predizer a habilidade de formação vítrea em sistemas metálicos. No presente trabalho, um critério de seleção foi aplicado ao sistema Ni-Nb-Zr com o objetivo de predizer as melhores estequiometrias com as mais altas tendências de formação vítrea. As habilidades de formação vítrea das ligas foram avaliadas pelo parâmetro térmico γm e os resultados mostraram uma pobre correlação com as predições. Este critério preditivo correlaciona as taxas de resfriamento para a formação vítrea com a instabilidade topológica de estruturas cristalinas, as diferenças médias de função trabalho e densidade eletrônica entre os elementos constituintes da liga. O parâmetro térmico depende de temperaturas características de transformações de fases que podem ser facilmente determinadas a partir de curvas de calorimetria exploratória diferencial dos vidros metálicos. As hipóteses iniciais para explicar a pobre correlação entre os resultados e as predições foram atribuídas às influências de fatores não considerados nos cálculos como os compostos intermetálicos desconhecidos e contaminação por oxigênio. Assim, algumas ligas solidificadas rapidamente foram investigadas com mais rigor com o objetivo de entender a formação das fases cristalinas que competem contra a formação vítrea. As fases cristalinas foram caracterizadas e comparadas com estruturas cristalinas encontradas na literatura como também alguns diagramas de fases. Os diagramas de fases foram utilizados como guias para o melhor entendimento do comportamento de cristalização. Em adição, o critério de seleção também foi utilizado para predizer o melhoramento da tendência de formação vítrea de uma liga do sistema Ni-Nb-Zr com pequenas adições dos elementos terras-raras Y ou Er. É bem conhecido que uma pequena adição de um elemento terra-rara apropriado pode aumentar significativamente a habilidade de formação vítrea de algumas ligas. As tendências de formação vítrea da liga base e das ligas dopadas com terras-raras também foram avaliadas pelo parâmetro térmico &#947m e os resultados concordaram muito bem com a tendência predita pelo cálculo. As amostras amorfas volumosas foram produzidas por injeção em molde de cobre. A natureza amorfa foi analisada por difração de raios-X e calorimetria exploratória diferencial. As fases cristalinas foram analisadas por microscopia eletrônica de transmissão, microscopia eletrônica de varredura, espectroscopia de raios-X por dispersão em energia e difração de raios-X. A contaminação por oxigênio foi quantificada pelo método de fusão em gás inerte. / Since the discovering of amorphous alloys in 1960, the actual causes of why some alloys can be easily formed into glasses while others cannot, are not clearly known, thus there is no universal theory to predict the glass forming ability in metallic systems. In the present work, a selection criterion was applied in the Ni-Nb-Zr system in order to predict the best stoichiometries with high glass forming ability. The actual glass forming ability of alloys were evaluated by the thermal parameter γm and the results have shown a poor correlation with the predictions. This criterion correlates critical cooling rate for glass formation with topological instability of stable crystalline structures; average work function difference and average electron density difference among the constituent elements of the alloy. The thermal parameter depends on the characteristic temperatures of phase transformations which can be easily measured from differential scanning calorimetry curves of metallic glasses. The initial hypotheses to explain the poor correlation between the experimental results and the predictions concerned with the influence of factors not considered in the calculation, such as unknown intermetallic compounds and oxygen contamination. Thus some rapidly quenched alloys were investigated with more accuracy in order to understand the formation of crystalline phases which compete against the glass formation. We characterized the crystalline phases and compared them to crystalline structures found in literature as well as some phase diagrams. The phase diagrams were used as guides in order to understand the crystallization behavior. In addition, the selection criterion also was used to predict the glass forming ability improvement of a Ni-Nb-Zr alloy with minor additions of rare-earth elements Y or Er. It is well known that the minor amount addition of proper rare-earth elements can greatly enhance the glass forming ability of some glass-forming alloys. The actual glass forming ability of the base alloy and rare-earth doped alloys also were evaluated by the thermal parameter &#947m and the results agree very well with the tendency predicted by the calculation. Bulk amorphous specimens were produced by injection casting. The amorphous nature was analyzed by X-ray diffraction and differential scanning calorimetry. The crystalline phases were analyzed by transmission electron microscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy and x-ray diffraction. Oxygen contamination was quantified by the inert gas fusion method.

Desde a descoberta das ligas amorfas em 1960, os motivos pelos quais algumas ligas podem ser facilmente amorfizadas enquanto outras não podem, não é claramente conhecido, assim não há teoria universal para predizer a habilidade de formação vítrea em sistemas metálicos. No presente trabalho, um critério de seleção foi aplicado ao sistema Ni-Nb-Zr com o objetivo de predizer as melhores estequiometrias com as mais altas tendências de formação vítrea. As habilidades de formação vítrea das ligas foram avaliadas pelo parâmetro térmico γm e os resultados mostraram uma pobre correlação com as predições. Este critério preditivo correlaciona as taxas de resfriamento para a formação vítrea com a instabilidade topológica de estruturas cristalinas, as diferenças médias de função trabalho e densidade eletrônica entre os elementos constituintes da liga. O parâmetro térmico depende de temperaturas características de transformações de fases que podem ser facilmente determinadas a partir de curvas de calorimetria exploratória diferencial dos vidros metálicos. As hipóteses iniciais para explicar a pobre correlação entre os resultados e as predições foram atribuídas às influências de fatores não considerados nos cálculos como os compostos intermetálicos desconhecidos e contaminação por oxigênio. Assim, algumas ligas solidificadas rapidamente foram investigadas com mais rigor com o objetivo de entender a formação das fases cristalinas que competem contra a formação vítrea. As fases cristalinas foram caracterizadas e comparadas com estruturas cristalinas encontradas na literatura como também alguns diagramas de fases. Os diagramas de fases foram utilizados como guias para o melhor entendimento do comportamento de cristalização. Em adição, o critério de seleção também foi utilizado para predizer o melhoramento da tendência de formação vítrea de uma liga do sistema Ni-Nb-Zr com pequenas adições dos elementos terras-raras Y ou Er. É bem conhecido que uma pequena adição de um elemento terra-rara apropriado pode aumentar significativamente a habilidade de formação vítrea de algumas ligas. As tendências de formação vítrea da liga base e das ligas dopadas com terras-raras também foram avaliadas pelo parâmetro térmico &#947m e os resultados concordaram muito bem com a tendência predita pelo cálculo. As amostras amorfas volumosas foram produzidas por injeção em molde de cobre. A natureza amorfa foi analisada por difração de raios-X e calorimetria exploratória diferencial. As fases cristalinas foram analisadas por microscopia eletrônica de transmissão, microscopia eletrônica de varredura, espectroscopia de raios-X por dispersão em energia e difração de raios-X. A contaminação por oxigênio foi quantificada pelo método de fusão em gás inerte. / Since the discovering of amorphous alloys in 1960, the actual causes of why some alloys can be easily formed into glasses while others cannot, are not clearly known, thus there is no universal theory to predict the glass forming ability in metallic systems. In the present work, a selection criterion was applied in the Ni-Nb-Zr system in order to predict the best stoichiometries with high glass forming ability. The actual glass forming ability of alloys were evaluated by the thermal parameter γm and the results have shown a poor correlation with the predictions. This criterion correlates critical cooling rate for glass formation with topological instability of stable crystalline structures; average work function difference and average electron density difference among the constituent elements of the alloy. The thermal parameter depends on the characteristic temperatures of phase transformations which can be easily measured from differential scanning calorimetry curves of metallic glasses. The initial hypotheses to explain the poor correlation between the experimental results and the predictions concerned with the influence of factors not considered in the calculation, such as unknown intermetallic compounds and oxygen contamination. Thus some rapidly quenched alloys were investigated with more accuracy in order to understand the formation of crystalline phases which compete against the glass formation. We characterized the crystalline phases and compared them to crystalline structures found in literature as well as some phase diagrams. The phase diagrams were used as guides in order to understand the crystallization behavior. In addition, the selection criterion also was used to predict the glass forming ability improvement of a Ni-Nb-Zr alloy with minor additions of rare-earth elements Y or Er. It is well known that the minor amount addition of proper rare-earth elements can greatly enhance the glass forming ability of some glass-forming alloys. The actual glass forming ability of the base alloy and rare-earth doped alloys also were evaluated by the thermal parameter &#947m and the results agree very well with the tendency predicted by the calculation. Bulk amorphous specimens were produced by injection casting. The amorphous nature was analyzed by X-ray diffraction and differential scanning calorimetry. The crystalline phases were analyzed by transmission electron microscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy and x-ray diffraction. Oxygen contamination was quantified by the inert gas fusion method.

Amorphous and bulk amorphous metallic alloys are an intriguing class of structural materials and possess a range of interesting properties, including near theoretical strength, high hardness, extremely low damping characteristics, excellent wear properties, high corrosion resistance, low shrinkage during cooling and almost perfect as-cast surfaces with good potential for forming and shaping. In this study, new Ti-based bulk amorphous alloys are tried to be modeled and synthesized. For that purpose, electronic theory of alloys in the pseudo potential approximation was used as a tool for understanding the theory lying beneath the bulk glass forming ability (BGFA). The results from this approach were evaluated both separately and together with the other theories supposed by our colleagues. Glass forming parameters of ordering energy, &amp / #916 / HM, viscosity, mismatch entropy, Rc was calculated for various Ti-based binary and ternary and the change in these parameters in both cases was evaluated. The results of the theoretical calculations of glass forming parameters has shown good relation with the literature data that the predicted alloying elements, i.e. Mo, Hf, Zr, B, Fe, Sn, and Be, to increase GFA for Ti2Ni binary system were generally used in the production of Ti-based bulk amorphous alloys. In the second part of this thesis, new Ti-based compositions with high GFA were tried to be synthesized with light of these results and encouraging conclusions were drawn. The production of these alloys were made with centrifugal casting method which is relatively a new technique for producing such alloys and the characterization of these alloys were made with metallographic, X-ray and thermal means.

The aim of this study is to form a theoretical model for simulation of glass forming ability of Fe &amp / #65533 / Based bulk amorphous alloys, to synthesize Fe &amp / #65533 / based multicomponent glassy alloys by using the predictions of the theoretical study, and to analyze the influence of crystallization and solidification kinetics on the microstructural features of this amorphous alloys. For this purpose, first, glass forming ability of Fe &amp / #65533 / (Mo, B, Cr, Nb, C) &amp / #65533 / X ( X = various alloying elements, selected from the periodic table) ternary alloy systems were simulated for twenty different alloy compositions by using the electronic theory of alloys in pseudopotential approximation and regular solution theory. Then, by using the results of the theoretical study, systematic casting experiments were performed by using centrifugal casting method. The alloying elements were melted with induction under argon atmosphere in alumina crucibles and casted into copper molds of different shapes. Characterization of the cast specimens were performed by using DSC, XRD, SEM, and optical microscopy. Comparison of equilibrium and nonequilibrium solidification structures of cast specimens were also performed so as to verify the existence of the amorphous phase. Good agreement of the results of experimental work, with the predictions of the theoretical study, and the related literature was obtained.

In recent years, bulk amorphous alloys and nanocrystalline materials have been synthesized in a number of ferrous and non-ferrous based alloys systems, which have gained some applications due to their unique physico-chemical and mechanical properties. In the last decade, Zr-based alloys with a wide supercooled liquid region and excellent glass forming ability have been discovered. These systems have promising application fields due to their mechanical properties / high tensile strength, high fracture toughness, high corrosion resistance and good machinability. In this study, the aim is to model, synthesize and characterize the Zr-based bulk amorphous alloys. Initially, theoretical study on the basis of the semi-empirical rules well known in literature and the electronic theory of alloys in pseudopotential approximation has been provided in order to predict the potential impurity elements that would lead to an increase in the GFA of the selected Zr-Ni, Zr-Fe, Zr-Co and Zr-Al based binary systems. Furthermore, thermodynamic and structural parameters were calculated for mentioned binary and their ternary systems. According to the theoretical study, Zr67Ni33 binary system was selected and its multicomponent alloys were formed by adding its potential impurity elements / Mo, W and Al. Centrifugal casting method was used to produce alloy systems. Structural characterizations were performed by DSC, XRD, SEM and EDS methods. In the near-surface regions of Zr60Ni25Mo10W5 and Zr50Ni20Al15Mo10W5 alloys, amorphous structure has been observed. Experimental studies have shown that Zr-Ni based systems with impurity elements Mo, W and Al, not widely used in literature, might be good candidates for obtaining high GFA.

The aim of this study is to synthesize and characterize new bulk amorphous alloys in the Ni- based systems. Theoretical studies on the basis of semi-empirical rules and the electronic theory of alloys in pseudopotential approximation has been provided in order to predict the impurity elements that will lead to an increase in the glass forming ability of Ni-based alloy systems. Glass forming ability of ten different compositions of alloys of Ni-Nb, Ni-Fe, Ni-B, Ni-Hf and Ni-Cr was simulated by using FORTRAN programs based on pseudopotential theory. In addition to the binary alloys, ternary alloys, which were formed by addition of 1 at% of third element to these systems, were also simulated. Since ordering energy is an indicator of glass forming ability, theoretical studies allowed to predict the effect of various third elements on the formation of amorphous phase. Furthermore, ordering energies were also used to calculate other parameters important for glass forming ability. In the second part of the study, on the basis of theoretical results, a series of casting experiments were done. Different compositions of Ni-Nb, Ni-Nb-Sn and Ni-Nb-Al alloys were cast in the centrifugal casting machine. Alloys were melted in alumina crucibles and cast into the copper moulds. Characterizations of cast alloys were done by the use of Metallography, SEM, XRD and DSC. Fully amorphous Ni52Nb41Al7 alloy was synthesized in bulk form with 0.8 mm thickness.