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    THẠC SĨ A thesis submitted to the graduate school of natural and applied sciences of middle east technical u

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  6. A thesis submitted to the graduate school of natural and applied sciences of middle east technical u

    TABLE OF CONTENTS
    ABSTRACT iv
    ÖZ . vi
    DEDICATION . vii
    ACKNOWLEDGEMENTS ix
    TABLE OF CONTENTS xi
    LIST OF TABLES xiv
    LIST OF FIGURES . xv
    CHAPTERS
    1.INTRODUCTION 1
    2.THEORY . 3
    2.1HISTORY OF METALLIC GLASSES . 3
    2.2 BASIC CONCEPTS OF METALLIC GLASSES 8
    2.2.1 Conventional Glasses and Glass Transition . 8
    2.2.2 Glass Formation . 12
    2.2.2.1 Thermodynamics of glass formation . 13
    2.2.2.2 Kinetics of glass formation . 15
    2.3 GLASS-FORMING ABILITY CRITERIA FOR BULK METALLIC
    GLASSES . 16
    2.3.1 Topological Criterion . 18
    2.3.2 Parameters Involving Characteristic Temperatures . 19
    2.3.2.1 φcriterion 22
    2.3.2.2 γcriterion 22
    2.3.2.3 δcriterion 24
    2.3.2.4 αand βcriteria 25
    2.3.3 The Use of Phase Diagrams in Evaluating the GFA 26
    2.3.4 Bulk Glass Forming Ability . 27
    2.3.5 Theoretical Studies Concerning GFA 28
    xii
    2.4 PRODUCTION METHODS OF BULK METALLIC GLASSES . 28
    2.5 CRYSTALLIZATION OF BULK METALLIC GLASSES 30
    2.5.1 Phase Separation 32
    2.5.2 Structural Relaxation 32
    2.5.3 Crystallization Kinetics 33
    2.5.3.1 Isothermal crystallization kinetics-JMAK analysis 34
    2.5.3.2 Non-isothermal crystallization kinetics: Kissenger
    and Ozawa Methods 36
    2.5.4 Methods Used in Critical Cooling Rate Calculations 39
    2.5.4.1 Quantitative evaluation ofcritical cooling rate . 39
    2.5.4.2 Measuring the critical cooling rate by analyzing
    crystallization peaks from continuously cooled
    melts 40
    2.5.5 Nanocrystallization of Bulk Metallic Glasses 43
    2.6 PROPERTIES AND APPLICATIONS OF BULK METALLIC
    GLASSES . 45
    2.6.1 Mechanical Properties 46
    2.6.2 Magnetic Properties . 47
    2.6.3 Chemical Properties . 48
    2.6.4 Applications . 48
    3.EXPERIMENTAL PROCEDURE . 50
    3.1 ALLOY PREPARATION . 50
    3.1.1 Raw Materials 50
    3.1.2 Alloy Preparation Methods 50
    3.2 BULK METALLIC GLASS FORMATION 54
    3.2.1 Quenching from the Liquid State . 54
    3.2.2 Quenching from the Semi-Solid State . 57
    3.3 EQUILIBRIUM SOLIDIFICATION OF THE MASTER ALLOY . 58
    3.4 SAMPLE CHARACTERIZATION . 58
    3.4 CRYSTALLIZATION EXPERIMENTS . 61
    4. RESULTS AND DISCUSSIONS 63
    xiii
    4.1 THE SOLIDIFICATION BEHAVIOR OF Fe60Co8Mo5Zr10W2B15
    ALLOY . 63
    4.2 BULK METALLIC GLASS FORMATION 72
    4.2.1 Quenching from the Liquid State . 73
    4.2.2 Quenching from the Semi-solid State 84
    4.3 EXPERIMENTAL ESTIMATION OF CRITICAL COOLING RATE88
    4.4 CRYSTALLIZATION KINETICS . 93
    4.8 MAGNETIC PROPERTIESOF THE ALLOY 105
    5. CONCLUSIONS 107
    REFERENCES . 110
    APPENDIX A 120

    CHAPTER 1
    INTRODUCTION
    Bulk metallic glasses have an unusual combination of physical, mechanical,
    magnetic, and chemical properties because of their random, non-crystalline atomic
    arrangements making them superior to their crystalline counterparts [1]. They are
    produced by using different techniques all of which involve the rapid solidification.
    They display high strength, low Young’s modulus and excellent corrosion resistance
    [2].
    The atoms are frozen in their liquid configuration as a result of rapid solidification
    [3]. Metallic glasses are non-equilibrium structures with respect to the crystalline
    state. For this reason, they go through structural changes from the as cast state to the
    metastable structurally relaxed state and finally to the crystalline state when
    moderately heated. Physical, chemical, and mechanical properties of the metallic
    glasses are significantly affected by the structural changes that occur during heating
    at temperature low enough to avoid crystallization [4, 5]. Therefore, the study of
    crystallization behaviour of metallic glasses is very important in the sense that the
    crystallization parameters of an amorphous phase reflect how stableit is against the
    thermal treatments that may present in the practical applications.
    The aim of this study in general was to investigate the solidification and
    crystallization behaviour of Fe60Co8Mo5Zr10W2B15bulk glass forming alloy system.
    This alloy was chosen since it was confirmed to have a high glass forming ability by
    the previous studies [6, 7]. The ternary Fe-Zr-B alloys were studied by Pehlivanoğlu
    [7] by adding minor alloying elements systematically and the Mo and W elements
    were found to increase the glass forming ability. The theoretical studies using the
    2
    simulation models also showed that the alloy was a good glass former. However, the
    crystallization kinetics of the alloy has not been studied indetail so far. Therefore,
    this study aims at investigating the crystallization kinetics by means of the
    experimental and analytical methods.
    In addition, for the first time in this study, amorphous phase formation was
    attempted to be obtained by quenching the alloy from the semi-solid state. The
    existence of the semi-solid region between the eutectic and peritectic temperatures in
    Fe60Co8Zr10Mo5W2B15was considered to be utilized for obtaining amorphous phase
    without complete melting of the alloy. The ability to process the bulk amorphous
    alloys in the semi-solid region is expectedto open new perspectives to the study of
    bulk metallic glass formation.
    The literature review on the subject and somebasic concepts of the glass formation
    is given in Chapter Two. The analytical methods employed in the experimental
    studies are explained in this chapter. In the next chapter, the experimental methods
    used and the experiments carried out are presented. In Chapter Four, the results of
    the experiments are given together with the simultaneous discussion. The
    conclusions drawn are given in the Fifth Chapter.
    3
    CHAPTER 2
    THEORY
    2.1HISTORY OF METALLIC GLASSES
    The first amorphous metallic alloys were claimed to have been made by Kramer [8]
    using vapor deposition. Then, it was proposed by Brenner et al. [9] that amorphous
    nickel-phosphorus alloys had beenproduced via electrodepositing.
    Metallic amorphous alloys are comparatively new in the amorphous materials group.
    The first metallic glass was Au75Si25reported by Duwez [10] at Caltech, USA, in
    1960. They showed that the nucleation and growth of crystalline phase could be
    kinetically bypassed in some liquefiedalloys to produce a frozen liquid
    configuration called the metallic glass. The cooling rate used to obtain this structure
    was on the order of 10
    6
    K/s which put a restriction inthe specimen geometry. Only
    thin ribbons, foils and powders were producedwith at least one dimension is small
    enough, on the order of microns, to allow such a high cooling rate [11].
    The fundamental scientific significance and potential engineering applications of
    bulk metallic glasses have increased the attention to studies on their formation,
    structure and properties [12]. The work ofTurnbull group found similarities between
    metallic glasses and other non-metallic glasses such as silicates, ceramic glasses and
    polymers. They pointed out that glass transition seen in conventional glass-forming
    melts could also be observed in metallicglasses produced by rapid quenching [13-15].

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