In this study, the time-dependent deformation behavior of several model bulk metallic glasses (BMGs) was studied. The BMGs were obtained in different structural states by thermal relaxation below their glass transition temperature, cryogenic thermal cycling, and chemical rejuvenation by micro-alloying. The creep behavior of Zr52.5Ti5Cu17.9Ni14.6Al10 BMG in different structural states was investigated as a function of peak load and temperature. The creep strain rate sensitivity (SRS) indicated a transition from shear transformation zone (STZ) mediated deformation at room temperature to diffusion dominated mechanisms at high temperatures. The relaxation enthalpy of Zr47Cu46Al7 BMG was found to increase significantly with the addition of 1 at% Ti, namely for Zr47Cu45Al7Ti1. Comparison of their respective free volumes indicated that chemical rejuvenation had a more pronounced effect compared to cryogenic thermal rejuvenation. Micro-pillar compression tests supported the improved plasticity with increase in free volume from the rejuvenation effect. Effect of chemistry change on mechanical response and time-dependent deformation was investigated for topologically equivalent Pt-Pd BMGs, where the Pt atoms were systematically replaced with Pd atoms (Pt42.5-xPdx)Cu27Ni9.5P21: x=0, 7.5, 20, 22.5, 35, 42.5). The hardness and reduced modulus increased while the degree of plasticity decreased with increase in Pd-content, which was attributed to the increase in stiffer 3-atom cluster connections. STZ volume was calculated for all the BMGs using cooperative shear model (CSM) for fundamental understanding of the underlying deformation mechanisms.

Metallic glasses are amorphous alloys that do not possess long-range structural order in contrast to crystalline alloys. Ni-free Ti-based bulk metallic glasses (BMGs) have potential for biomedical applications due to their attractive properties such as high strength, good corrosion resistance and excellent micro-formability, which cannot be obtained for conventional crystalline alloys. In this PhD thesis, Ni-free Ti-based BMGs, i.e. Ti40Zr10Cu34Pd14Sn2 and Ti40Zr10Cu36-xPd14Inx (x = 0, 2, 4, 6, 8), were prepared in the shape of rods by suction casting. Both alloy classes were systematically characterized in terms of glass forming ability, thermal stability, phase formation and mechanical properties. The largest diameter obtained in the fully glassy state for Ti40Zr10Cu34Pd14Sn2 alloy is 3 mm and for Ti40Zr10Cu36-xPd14Inx (x = 2, 4, 6, 8) alloys is 2 mm. Base alloy (Ti40Zr10Cu36Pd14) contains some crystalline phase(s) in the glassy matrix for a 2 mm diameter rod. The structural transformations of Ti40Zr10Cu34Pd14Sn2 BMG upon heating were thoroughly analyzed by utilizing different combination of methods. Firstly, we used differential scanning calorimetry (DSC), X-Ray diffraction (XRD) and transmission electron microscopy (TEM) to investigate the first crystallization event. The main products of the first crystallization are possibly -(Ti, Zr) and Cu3Ti (orthorhombic) phases. Secondly, we employed in situ x-ray diffraction in transmission mode using synchrotron beam to deeply study the thermally-induced structural changes like relaxation, glass transition and crystallization. Since the first peak in the diffraction patterns reflects the structure of the glassy phase on the medium-range scale, the position, width and intensity of this peak in diffraction patterns are fitted through Voigt function below 800 K. All the peak position, width and intensity values show a nearly linear increase with increasing temperature to the onset temperature of structural relaxation, Tr = 510 K. However, these values start to deviate from the linear behavior between Tr and glass transition temperature Tg. The changes in the free volume, which was arrested during rapid quenching of the BMG, and the coefficient of volumetric thermal expansion prove that the aforementioned phenomenon is closely related to the structural relaxation. Above 800 K, three crystallization events are detected and the first exothermic event is due to the formation of metastable nanocrystals. For the Ti40Zr10Cu34Pd14Sn2 alloy, 2 mm diameter rods exhibit the best combination of mechanical properties (e.g. large plastic strain and high yield strength) among all the diameters (ø2, ø3 and ø4 mm) under the room-temperature compression tests. With the aim to improve its room-temperature mechanical properties, the processes of pre-annealing and cold rolling have been applied for the 2 mm diameter rods. Annealed and quenched specimens below Tg and in the supercooled liquid region (between Tg and onset crystallization temperature Tx) do not lead to the enhancement of the plasticity compared to as-cast alloys due to annihilation of excess free volume and crystallization. Cold rolling can effectively improve the plasticity of this BMG by inducing structural heterogeneities. Rolled samples up to a thickness reduction of 15% result in the largest plasticity of 5.7%. Low yield strength and visible work hardening ability are observed in the both 10%-rolled and 15%-rolled samples. The deformation behavior of Ti40Zr10Cu34Pd14Sn2 BMG at the elevated temperatures slightly below Tg and in the supercooled liquid region has been investigated. The stress-strain relations for this BMG over a broad range of temperatures (298 ~716 K) and strain rates (10-5 to 10-3 s-1) were established in uniaxial compression. Under compression tests at the highest test temperature of 716 K, the Ti-based BMG partially crystallizes and low strain rates can lead to the formation of larger volume fractions of crystals. In order to further improve the plasticity of Ti-Zr-Cu-Pd BMGs and simultaneously reduce the content of Cu (considering harmful element for the human body), the Ti40Zr10Cu36-xPd14Inx (x = 2, 4, 6, 8) BMGs have been newly developed with different short- or medium-range order in the structure. The compressive global strain of Ti40Zr10Cu36-xPd14Inx (x = 0, 2, 4, 6, 8) can be significantly improved from 4.5% for the In-free alloy to 10.2% for x = 4. However, a further increase of the indium content to 8 at.% results in a decrease of the plasticity. Among all the monolithic Ni-free Ti-based BMGs reported so far, the novel Ti40Zr10Cu32Pd14In4 BMG shows the largest plasticity. Inspired by the dislocation concept in crystalline materials, we propose a strategy for the design of ductile BMGs through minor substitution using relatively large atoms, which make the bonding nature become more metallic and with it less shear resistant. Such a locally modified structure results in topological heterogeneity, which appears to be crucial for achieving enhanced plasticity. This strategy is verified for Ti-Zr-Cu-Pd glassy alloys, in which Cu was replaced by In, and seems to be extendable to other BMG systems. The atomic-scale heterogeneity in BMGs is somewhat analog to defects in crystalline alloys and helps to improve the overall plasticity of BMGs.

No / A novel process chain for serial production of polymer-based devices incorporating both micro- and nano-scale features is proposed. The process chain is enabled by the use of Zr-based bulk metallic glasses (BMG) to achieve the necessary level of compatibility and complementarity between its component technologies. It integrates two different technologies, namely laser ablation and focused ion beam (FIB) milling for micro-structuring and sub-micron patterning, respectively, thus to fabricate inserts incorporating different length scale functional features. Two alternative laser sources, namely nano-second (NS) and pico-second (PS) lasers, were considered as potential candidates for the first step in this master-making process chain. The capabilities of the component technologies together with some issues associated with their integration were studied. To validate the replication performance of the produced masters, a Zr-based BMG insert was used to produce a small batch of micro-fluidic devices by micro-injection moulding. Furthermore, an experimental study was also carried out to determine whether it would be possible by NS laser ablation to structure the Zr-based BMG workpieces with a high surface integrity whilst retaining the BMG’s non-crystalline morphology. Collectively, it was demonstrated that the proposed process chain could be a viable fabrication route for mass production of polymer devices incorporating different length scale features.

Pour prémunir les structures spatiales d'impacts hyper-véloces, le secteur aérospatial est continuellement à la recherche de matériaux toujours plus performants. Dans cette optique, les verres métalliques massifs se présentent comme de potentiels éléments de blindages spatiaux. De récentes études ont mis en exergue une meilleure résistance à la pénétration de ces matériaux comparativement aux blindages actuels. Les impacts par lanceurs permettent d'étudier et caractériser le comportement sous chocs des matériaux. Cependant, les vitesses des projectiles se retrouvent actuellement limitées à 10 km/s, correspondant aux niveaux d'impacts hyper-véloces les plus modérés. Pour s'affranchir de cette limitation, il est possible de se tourner vers les lasers de puissance. Ces dispositifs permettent de générer des ondes de choc dont les niveaux de pression et de vitesse de déformation sont supérieurs aux lanceurs. Les travaux menés ont permis d'étudier et de caractériser le comportement et l'endommagement de plusieurs verres métalliques ternaires ZrCuAl sous choc laser. Plusieurs campagnes expérimentales ont été réalisées sur les installations du Laboratoire pour l'Utilisation des Lasers Intenses (LULI2000 et ELFIE). Une partie de l'équation d'état des nuances étudiées a été obtenue à la fois par choc laser et compression isentropique. Les processus d'endommagement, l'influence des vitesses de déformation et de composition sur la rupture ont été étudiés. Pour des régimes de vitesse de déformation supérieurs de trois ordres de grandeur à ceux disponibles dans la littérature, il a été mis en évidence que les verres métalliques étudiés présentaient une limite à la rupture cinq à dix fois supérieure. / Space industry is always searching for efficient materials to protect space structures from high-velocity impacts. In this context, bulk metallic glasses appear as suitable elements of space debris shielding assemblies. Recent studies revealed a higher tolerance to impact of metallic glasses compared to materials currently used in shield assemblies. Gas-gun and powder launchers are usually used to study and characterize the dynamic and shock behaviour of materials. However, projectiles velocities are currently limited to 10 km/s, corresponding to the lowest high-velocity impacts levels. To overcome this limitation, one may consider shock waves induced by high-power laser facilities, whose pressure and strain rate levels can exceed those induced by canons. Hence, this work enabled to study and to characterize the dynamic and damage behaviours of several compositions of ternary ZrCuAl bulk metallic glasses subjected to shock waves induced by laser irradiation. Several experimental campaigns have been conducted on various laser facilities of the Laboratoire pour l'Utilisation des Lasers Intenses (LULI2000 and ELFIE). A part of the equation of state of the studied compositions was established using both shock waves and quasi-isentropic compressions. Damage processes and the composition and strain rate effects on fracture were also studied. For strain rate levels higher of three or more orders of magnitude than those available in the literature, it was shown that studied bulk metallic glasses displayed a five to ten times higher dynamic tensile limit.

The objective of this thesis is to study the fracture behavior of bulk metallic glasses. For this purpose, detailed finite element investigation of the mode I and mixed mode (I and II) stationary crack tip fields under plane strain, small scale yielding conditions is carried out. An implicit backward Euler finite element implementation of the Anand and Su constitutive model [Anand, L. and Su, C., 2005, J. Mech. Phys. Solids 53, 1362] is used in the simulations. The effects of internal friction (μ), strain softening, Poisson's ratio (ν) and elastic mode mixity (Me) on the near-tip stress and deformation fields are examined. The results show that under mode I loading, a higher μ leads to a larger normalized plastic zone size and higher plastic strain level near the notch tip, but causes a substantial decrease in the opening stress. The brittle crack trajectories and shear band patterns around the notch are also simulated. An increase in ν reduces the extent of plastic zone and plastic strain levels in front of the notch tip. The results from mixed mode simulations show that increase in the mode II component of loading dramatically increases the maximum plastic zone extent, lowers the stresses and significantly enhances the plastic strain levels near the notch tip. Higher μ causes the peak magnitudes of tensile tangential stress to decrease. The implications of the above results on the fracture response of bulk metallic glasses are discussed. The possible variations of fracture toughness with mode mixity predicted by employing two simple fracture criteria are examined. Finally, mixed mode (I and II) fracture experiments on a Zr-based bulk metallic glass are performed. It is found that the fracture toughness increases with Me and Jc under mode I is higher than that under mode II loading by a factor of 4. The operative failure mechanism and fracture process zone size are discerned based on observations of incipient crack growth and fractographs. Lastly, a fracture criterion is proposed which predicts the experimentally observed variation of fracture toughness with mode mixity.

The ferromagnetic bulk metallic glasses (BMGs) started to be investigated only in the last 10 years.They are difficult to cast, but their properties are uniques. The work deals with casting, mechanical and soft magnetic properties of new Fe-based BMGs. Such alloys can be cast directly in samples with various geometries and they can be use as magnetic parts in different devices.

The aim of this study, which was carried out in two main parts, is to investigate the glass forming ability of Fe-based systems. The first part involves the theoretical modeling to cover the requirement of a predictive model to identify the Fe-based alloy families that have high glass forming ability in the frame of atomistic and thermodynamic approach. The second part involves the experimental investigations to prove the results of the conducted theoretical modeling studies. For this purpose, in the first part, theoretical investigations were performed to identify the third alloying elements that will lead to an increase in the glass forming ability on the base of electronic theory of alloys in pseudopotential approximation for selected Fe- based systems, Fe - (B, Zr, Nb, C, W). In the experimental part, in the frame of the theoretical investigation results, one of the theoretically modeled binary system, and the third alloying elements that were predicted to lead an increase in the glass forming ability of the selected binary system, were determined. As a first step, designated compositions were synthesized by using low grade conventional Fe-B alloy as a raw material by using centrifugal casting technique and copper mold casting method. To compare the results, same compositions were also cast from the high purity elements by using the same technique and method. For the characterization of these cast specimens, DSC, XRD, SEM, EDS and metallographic examination techniques were used. Amorphous structure was successfully obtained in the thin sections of the wedge-cast samples for Fe-B-Nb and Fe-B-W ternary systems.

No description available.

Indentation hardness, H, has been widely used to characterize the mechanical properties of materials for more than a century because of the following advantages of this technique; (1) it requires small sample and (2) the test is non destructive in nature. Recent technological advances helped in the development of instrumented indentation machines which can record the load, P, vs. displacement, h, data continuously during indentation with excellent load and displacement resolutions. From these, H and the elastic modulus, E, of the indented material can be obtained on the basis of the ‘contact area’ of the indentation at the maximum load. The estimation of true contact area becomes difficult during ‘pile-up’ and ‘sink-in’, commonly observed phenomena while indentation of a low and high strain hardened materials. In order for the better understanding of these phenomena it is important to understand the plastic flow distribution under indenters. It is also important for the prediction of elastic-plastic properties from the P-h data. Recently, there have been considerable theoretical and simulation efforts on this front with a combination of dimensional analysis and finite element simulations. One of the important input parameter for the dimensional analysis is the ‘representative strain’ under the indenter, which is a strong function of the indenter geometry. However there is no comprehensive understanding of the representative strain under the indenter despite several studies till date. One objective of the present thesis is to conduct an experimental analysis of the plastic flow during the sharp indentation. The plastic zone size and shape under conical indenters of different apex angles in a pure and annealed copper were examined by employing the subsurface indentation technique to generate the hardness map. From these isostrain contours are constructed joining the data having similar strain values. The following are the key observations. (1) The plastic strain contours are elliptical in nature, spreading more along the direction of the indenter axis than the lateral direction. (2) The magnitude of the plastic strain in the contact region decreases with increasing the indenter angle. (3) The strain decay in the indentation direction follow a power-law relation with the distance. The estimated representative strains under the indenters, computed as the volume average strain within the elastic-plastic boundary, decreases with increasing indenter angle. We also performed finite element simulations to generate plastic flow distribution under the indenter geometries and compared with the experimental results. The results suggest that the experimental and computed average strains match well. However, the plastic strain contours do not, suggesting that further detailed understanding of the elasto-plastic deformation underneath the sharp indenter is essential before reliable estimates of plastic properties from the P-h curves can be made routinely. The second objective of this thesis is to understand plastic flow in amorphous alloys. It is now well established that plastic deformation in metallic glasses is pressure sensitive, owing to the fundamentally different mechanisms vis-à-vis the dislocation mediated plastic flow in crystalline metals alloys. Early work has shown that the pressure sensitivity of amorphous alloys gets reflected as high constraint factor, C (hardness to yield stress ratio), which sometimes exceed 3.0. In this thesis, we study the temperature dependence of pressure sensitive plastic flow in bulk metallic glasses (BMGs) using C as the proxy for the pressure sensitivity. Experiments on three different BMGs show that C increases with temperature hence the pressure sensitivity. In addition we have carried out finite element simulations to generate P-h curves for different levels of pressure sensitivities and match them with the experimental curves that are obtained at different temperatures. Simulations predict that higher pressure sensitivity index values are required to match the experimental curves at high temperatures confirming that the pressure sensitivity increases with increasing temperature. The fundamental mechanisms responsible for the increase in pressure sensitivity are discussed in detail. Finally we pose a question, is the increase in pressure sensitivity with temperature is common to other amorphous materials such as strong amorphous polymers? In order to answer this question we have chosen PMMA, a strong amorphous polymer. In this study also we have taken C as a proxy to index the pressure sensitivity. Indentation stress-strain curves are constructed at different temperature using spherical indentation experiments. The C values corresponding to different temperatures are determined and plotted as a function of temperature. It is found that C increases with temperature implying that the pressure sensitivity of amorphous polymers also increases with temperature. The micro-mechanisms responsible for the increase in pressure sensitivity are sought.

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