4,496 research outputs found

    Improving Mechanical Properties of Bulk Metallic Glasses by Approaches of In-situ Composites and Thin Films

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    Bulk-metallic glasses (BMGs) exhibits lots of unique properties, such as, high strengths, high hardness, high specific strengths, superior elastic limits, high corrosion resistance, etc. However, the applications of BMGs are still quite limited due to their intrinsic brittleness and low ductility at room temperature. Many efforts have been conducted to improve the plasticity of BMGs, in which metallic-glass-matrix composites (MGMCs) and thin-film metallic glasses (TFMGs) are two popular and effective approaches. Nevertheless, the deformation mechanisms for the improved plasticity of MGMCs and TFMGs are still far from satisfactory understanding, which will be investigated using both experimental and simulation methods in the present work. For the MGMCs, in situ high-energy synchrotron X-ray diffraction experiments and micromechanics-based finite element simulations have been conducted to examine their lattice strain evolution. The entire lattice-strain evolution curves can be divided into elastic-elastic (denoting deformation behavior of matrix and inclusion, respectively), elastic-plastic, and plastic-plastic stages. Characteristics of these three stages are governed by the constitutive laws of the two phases (modeled by free-volume theory and crystal plasticity) and geometric information (crystalline phase morphology and distribution). The deformation behavior, especially the fatigue behavior, of TFMG materials has been investigated on the some substrates, including 316L stainless steel, BMG, etc. The results show that the four-point-bending fatigue life of the substrates is greatly improved by Zr- and Cu-based TFMGs, while Fe-based TFMG, TiN, and pure-Cu films are not so beneficial in extending the fatigue life of 316L stainless steel. However, quite limited work is reported on the fatigue behavior of TFMG coated on the BMG substrate, which can be a very interesting topic. Moreover, a synergistic experimental/theoretical study are conducted to investigate the micro-mechanisms of the fatigue behavior of TFMGs adhered to BMG substrates. Furthermore, shear-band initiation and propagation under deformation are investigated using the Rudnicki-Rice instability theory and the free-volume models employing finite-element simulations

    Processability of Bulk Metallic Glasses

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    Microfluidic Shear Flow Instabilities in Injection Molded Glassy Metal are investigated. The formation of microfluidic shear flows instabilities involving the presence of different viscosities fluids has been observed in injection molded Bulk Metallic Glasses. The complex rheology of injection molded metastable glassy metal, which has been hypothesized to induce selective clustering of atoms of different steric hindrance, is discussed. Smaller Be, Cu and Ni atomsmay differently rearrange themselves in the bulk metal glassy super cooled liquids forming flow streams of lower viscosity. Segregation of atoms of different size could activate a variety of viscous flow instabilities such as folding and swirling. FEI Scios Dual-Beam Electron scanning and optical microscopy observations of a commercial liquid metal alloy (Zr44Ti11Cu10Ni10Be25) have been carried out. We discussed the influence of short-range order clusters distribution and its effect in locally induced shear flow instability and corrosion resistance

    Deformation behaviour of a Zr-Cu-based bulk metallic glass

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    While inelastic mechanical behaviour of crystalline materials is well-understood in terms of lattice defects, bulk metallic glasses (BMGs) pose significant challenges in this respect due to their disordered structure. They can be produced by rapid cooling from the liquid state (among other technique) and, thus can be frozen as vitreous solids. Due to the absence of a long-range order in atomic structure and a lack of defects such as dislocations, BMGs generally show unique mechanical properties such as high strength and elastic limit, as well as good fracture toughness and corrosion resistance. Typically, inorganic glasses are brittle at room temperature, showing a smooth fracture surface as a results of mode-I brittle fracture. At small scale, it was well documented that inelastic deformation of bulk metallic glasses is localised in thin shear bands. So, in order to understand deformation mechanisms of BMGs comprehensively, it is necessary to investigate formation of shear bands and related deformation process. In this thesis, a history of development of BMGs is presented, followed by a review of fundamental mechanisms of their deformation. [Continues.

    Selective laser melting of glass-forming alloys

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    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 16

    DUCTILITY ENHANCEMENT AND STRUCTURAL ORIGIN OF DUCTILE-BRITTLE TRANSITION IN BULK METALLIC GLASSES

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    Brittleness largely restricts promising applications of the metallic glasses as a new engineering material. Understanding fundamental amorphous structure, deformation mechanisms and search for ways to enhance its ductility is imperative. Among these, establishing a valid structure-property relationship is particularly important. Following these thoughts, a series of research works are conducted. Both the finite element simulation and ins-situ transmission electron microscopy were conducted to investigate the size effect in amorphous ZrCu nanopillars. Studies demonstrate that the deformation is localized near the top of the metallic glass pillars, which looks absent from outside, but form inside. By assigning the free volume constitutive relation to the metallic glass, the radial shear bands were observed when indenting directly a bulk metallic glass, while extra semi-circular shear bands were found when a bonded-interface is introduced as in experiments. Ductility enhancement mechanisms in the titanium thin film coated bulk metallic glasses were investigated with both the Rudnicki-Rice instability theory and free volume model. Reflection of the shear band at the film/substrate interface and shear band branching were observed. On top of that, the effect of adhesion between the film and substrate and the film thickness were also investigated. Shear bands in the BMG composites are found initiate from the second phase/matrix at an angle or ~45 o, forming a blocking mechanism to the shear bands propagation, contributing to ductility improvement. Finally, statistical nanoindentation experiments were employed to study the structure-mechanical property relationship of the metallic glass. The statistical nanoindentation technique finds that the pop-in load and the corresponding maximum shear stress increases gradually with increasing degree of structural relaxation, accompanied with a decrease in the statistical variation. A quantitative model incorporating both thermally-activated and defect-assisted processes is developed to understand the pop-in statistics, in which the pre-existing defects, or soft zones, are distributed randomly in the hard amorphous matrix. Before performing nanoindentation tests, for reliability of the results, the spherical indenter tip radii were calibrated by taking the machine stiffness into the classic Hertzian solution rather than assuming a constant machine stiffness. By this method, the machine stiffness of the nanoindentation system was also explicitly evaluated

    Modeling the mechanics of amorphous solids at different length and time scales

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    We review the recent literature on the simulation of the structure and deformation of amorphous glasses, including oxide and metallic glasses. We consider simulations at different length and time scales. At the nanometer scale, we review studies based on atomistic simulations, with a particular emphasis on the role of the potential energy landscape and of the temperature. At the micrometer scale, we present the different mesoscopic models of amorphous plasticity and show the relation between shear banding and the type of disorder and correlations (e.g. elastic) included in the models. At the macroscopic range, we review the different constitutive laws used in finite element simulations. We end the review by a critical discussion on the opportunities and challenges offered by multiscale modeling and transfer of information between scales to study amorphous plasticity.Comment: 58 pages, 14 figure

    Surface tension-driven shape-recovery of micro/nanometer-scale surface features in a Pt(57.5)Ni(5.3)Cu(14.7)P(22.5) metallic glass in the supercooled liquid region: A numerical modeling capability

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    Recent experiments in the literature show that micro/nano-scale features imprinted in a Pt-based metallic glass, Pt57.5Ni5.3Cu14.7P22.5 [Pt subscript 57.5 Ni subscript 5.3 Cu subscript 14.7 P subscript 22.5], using thermoplastic forming at a temperature above its glass transition temperature, may be erased by subsequent annealing at a slightly higher temperature in the supercooled liquid region (Kumar and Schroers, 2008). The mechanism of shape-recovery is believed to be surface tension-driven viscous flow of the metallic glass. We have developed an elastic–viscoplastic constitutive theory for metallic glasses in the supercooled liquid temperature range at low strain rates, and we have used existing experimental data in the literature for Pt57.5Ni5.3Cu14.7P22.5 [Pt subscript 57.5 Ni subscript 5.3 Cu subscript 14.7 P subscript 22.5] (Harmon et al., 2007) to estimate the material parameters appearing in our constitutive equations. We have implemented our constitutive model for the bulk response of the glass in a finite element program, and we have also developed a numerical scheme for calculating surface curvatures and incorporating surface tension effects in finite element simulations. By carrying out full three-dimensional finite-element simulations of the shape-recovery experiments of Kumar and Schroers (2008), and using the independently determined material parameters for the bulk glass, we estimate the surface tension of Pt57.5Ni5.3Cu14.7P22.5 [Pt subscript 57.5 Ni subscript 5.3 Cu subscript 14.7 P subscript 22.5] at the temperature at which the shape-recovery experiments were conducted. Finally, with the material parameters for the underlying elastic–viscoplastic bulk response as well as a value for the surface tension of the Pt-based metallic glass fixed, we validate our simulation capability by comparing predictions from our numerical simulations of shape-recovery experiments of Berkovich nanoindents, against corresponding recent experimental results of Packard et al. (2009) who reported shape-recovery data of nanoindents on the same Pt-based metallic glass.National Science Foundation (U.S.) (Grant CMS-0555614)Singapore-MIT Allianc

    The Experimental and Theoretical Study of Plasticity Improvement of Zr-Based Bulk Metallic Glasses

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    Bulk metallic glasses (BMGs) attract more and more attention for their great mechanical properties, such as high strength, good corrosion resistance, etc. However, even though extensive studies have been made, their deformation mechanisms are still not well understood. Their limited plasticity and catastrophic failure after yielding severely prevent their broad applications in industry and daily life. To improve their plasticity, some work has been done through miscellaneous processing methods, e.g., thin-film coating, surface treatment, and ion irradiation. The present work also focuses on the plastic deformation of BMGs, and is expected to deepen the fundamental understanding of the deformation mechanisms through the study of several methods, which could improve their plasticity, namely, geometrical constraint, pre-fatigue, and laser-induced constraint. To characterize the improvement, compression, four-point bending fatigue, and nanoindentation experiments were conducted. Moreover, recent work suggests that BMGs also show serrated flows in certain regimes of temperatures and strain rates, which is similar to the Portevin–Le Chatelier effect (PLC) in traditional alloys. Thus, the serrated flow can be used as a probe for studying the deformation process. Through the investigations of serration behavior, it\u27s expected that the details of deformation dynamics can be extracted. The thermograph study and synchrotron X-ray diffraction were also utilized to investigate the shear-band dynamics and structural changes. Serration characteristics were analyzed statistically and a slip avalanche model was successfully applied on BMGs

    Deformation Localization in Constrained Layers of Metallic Glasses: A Parametric Modeling Analysis

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    Localized plastic deformation known as shear banding is a prominent feature in metallic glasses. In this study we perform parametric three-dimensional finite ele- ment analyses, using primarily a thin layer of metallic glass on top of a cylindrical base, to study how physical constraint can affect this localized form of deformation and the corresponding macroscopic stress-strain response. Random perturbation points are added to the metallic glass model to facilitate the formation of shear bands. The modeling result suggests that the mechanical behavior of metallic glasses can be significantly influenced by the geometrical confinement. Under nominally uniaxial compressive loading, a lower thickness-to-diameter ratio results in higher plastic flow stresses. Shear bands tend to concentrate in regions away from the interface with the base material. The findings provide a mechanistic rationale for experimental ob- servations based on the micropillar compression test. The deformation pattern in a multilayered metallic glass structure as well as the deformation pattern in a metallic glas beam subjected to four point bending are also examined
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