Macroscopic ductility is promoted in bulk metallic glasses by both composite reinforcements (at low temperatures) and by the activation of viscous flow mechanisms (at high temperatures). It is of fundamental interest to understand deformation physics when both of these strategies are employed at the same time. Despite the quickly growing literature around the room-temperature mechanical properties of metallic glass matrix composites (MGMCs), the deformation behavior of MGMCs over a wide range of temperatures and strain rates has yet to be systematically investigated, especially at high temperatures close to Tg. Here the high temperature compressive behavior of Zr-based MGMCs with in-situ reinforcements is explored systematically over a series of strain rates. Additionally, the volume fraction of second-phase reinforcements was tailored to explore its effect on both inhomogeneous and homogeneous deformation modes. / Singapore-MIT Alliance (SMA)

The microstructure, and phase selections of La₆₆Al₁₄(Cu, Ni)₂₀ alloy were studied by Bridgman solidifications, and composite materials of dendrites in amorphous matrix or micro- and nano- sized eutectic matrix were formed with different cooling rates. The volume fraction of the dendrite phase reaches a maximum at the cooling rate of about 15 K/s, the secondary dendrite arm spacing λ₂ decreases from 4.3 µm to 0.6 µm with the increasing of cooling rate R, and obeys the equation of λ₂R⁰.⁵&#x2077=1.74µm(K/s)R⁰.⁵&#x2077. The compression strength, as well as the elastic strain limit of the dendrite/amorphous matrix composite are 600 MPa, and 2.3%, respectively. Improved ductility was observed for the dendrite amorphous matrix composites with more dendrite phase by slow cooling rate. / Singapore-MIT Alliance (SMA)

Advancements in technologies such as microelectromechanical systems (MEMS), display devices, biomedical products have created an increasing requirement for miniature components on the scale of micrometers to nanometers. Currently, a commonly used fabrication for miniaturization is LIGA (Lithographie, Galvanoformung, and Abformung). It is a reliably manufacturing method for high-aspect-ratio microstructures with a precision of less than one micrometer. The use of electroplating within LIGA techniques, however, limits the range of materials that can be used. But the main disadvantage of LIGA is its cost: high-energy X-rays generated by synchrotron equipment. The homogeneous and isotropic characteristics of amorphous bulk metallic glasses (BMGs) due to the absence of crystallites, grain boundaries and dislocations lead to the scale of the metallic-glass structures can be miniaturized down to the atomic scale, which presents very high strength, hardness, elastic strain limit and corrosion resistance. In addition, the excellent workability and surface printability in the supercooled liquid state (the region defined from the glass transition temperature (Tg) to the crystallization temperature (Tx) of BMG) has been considered to be one of the most attractive properties of BMGs. The lighter Mg-based metallic glasses exhibit their superior glass forming ability (GFA). Consequently, the using of Mg-based BMGs can gain the goals of light devices and simplify manufacturing process. In this study, therefore, besides the study of LIGA process, a new process utilize the thermoplastic properties of BMGs is presented. First, UV (ultraviolet) -LIGA, a more economical process than LIGA, is used to fabricate the master mold with nickle-cobalt (Ni-Co) alloy. Then, this mold is applied to hot emboss on Mg58Cu31Y11 amorphous alloy to form a secondary mold. The hot embossing temperature is set at 423 K (150 oC) according to the Tg of the BMG around 413 K (140 oC). This embossing process shows that the thermoplastic forming ability of the BMG material is better than Polymethylmethacrylate (PMMA) which requires high hot embossing pressure. BMG is not only a good material for hot embossing process to fabricate microstructure directly, but also a fast-forming material for mold (or die) fabrication. On the other hand, other replicated-able moulds are presented to demonstrate the multifunctional ability of BMGs. First, a mold of oxygen free copper (OFC) with a very low hardness of 1.606 GPa, which is a popular material for machining due to its good machinability, is used to hot emboss on Mg58Cu31Y11 BMG with a higher hardness of 3.445 GPa. Second, micro triangular-pyramidal array (MTPA) on a tungsten (W) steel mold is transferred on Mg58Cu31Y11 BMG using this modified multi-step hot-embossing method to reduce the pattern size. In addition, scratch test with the Nano Indenter® XP system is used to study the mechanical behavior of the Mg58Cu31Y11 BMG for the application such as surface printability.

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Plastic flow in bulk metallic glasses (BMGs) localizes into narrow bands, which, in the absence of a microstructure that could obstruct them, propagate unhindered under tensile loading. In constrained deformation conditions such as indentation and at notch roots, extensive shear band formation can occur. A key issue in the context of fracture of BMGs that is yet to be understood comprehensively is how their toughness is controlled by various state parameters. Towards this end, the change in fracture toughness and plasticity with short term annealing above and below the glass transition temperature, Tg, is studied in a Zr-based BMG. Elastic properties like shear modulus, Poisson's ratio as well as parameters defining the internal state like the fictive temperature, Tf, density, and free volume are measured and correlation with the toughness was attempted at. While the elastic properties may help in distinguishing between tough and brittle glasses, they fail to reveal the reasons behind the toughness variations. Spherical-tip nanoindentation and microindentation tests were employed to probe the size, distributions and activation energies of the microscopic plastic carriers with the former and shear band densities with the latter. Results indicate that specimens annealed at a higher temperature, Ta, exhibit profuse shear banding with negligible changes in the local yield strengths. Statistical analysis of the nanoindentation data by incorporating the nucleation rate theory and the results of the cooperative shear model (CSM), reveals that short term annealing doesn't alter the shear transformation zone (STZ) size much. However, density estimates indicate changes in the free volume content across specimens. A model combining STZ activation and free volume accumulation predicts a higher rate in the reduction of the cumulative STZ activation barrier in specimens with a higher initial free volume content. Of the macroscopic physical properties, the specimen density is revealed to be a useful qualitative measure of enhancement in fracture toughness and plasticity in BMGs. We turn our attention next to the brittle fracture in BMGs, with the specific objective of understanding the mechanisms of failure. For this purpose, mode I fracture experiments were conducted on embrittled BMG samples and the fracture surface features were analyzed in detail. Wallner lines, which result from the interaction between the propagating crack front and shear waves emanating from a secondary source, were observed on the fracture surface and geometric analysis of them indicates that the maximum crack velocity to be ~800 m/s, which corresponds to ~0.32 times the shear wave speed. Fractography reveals that the sharp crack nucleation at the notch tip occurs at the mid-section of the specimens with the observation of flat and half-penny shaped cracks. On this basis, we conclude that the crack initiation in brittle BMGs occurs through hydrostatic stress assisted cavity nucleation ahead of the notch tip. High magnification scanning electron and atomic force microscopies of the dynamic crack growth regions reveal highly organized, nanoscale periodic patterns with a spacing of ~79 nm. Juxtaposition of the crack velocity with this spacing suggests that that the crack takes ~10-10 s for peak-to-peak propagation. This, and the estimated adiabatic temperature rise ahead of the propagating crack tip that suggests local softening, are utilized to critically discuss possible causes for the nanocorrugation formation. The Taylor’s fluid meniscus instability is unequivocally ruled out. Then, two other possible mechanisms, viz. (a) crack tip blunting and resharpening through nanovoid nucleation and growth ahead of the crack tip and eventual coalescence, and (b) dynamic oscillation of the crack in a thin slab of softened zone ahead of the crack-tip, are critically discussed. One way of alleviating the fracture-related issues in BMGs is to impart a microstructure to it, which would either impede the growth of shear bands or promote the multiplication of them. One such approach is through the BMG composites (BMGCs) route, wherein a crystalline second phase incorporated in the BMG matrix. There is a need to study the effects of reinforcement content, size and distribution on the mechanical behavior of the BMGC so as to achieve an optimum combination of strength and ductility. For this purpose, an investigation into the microstructure and tensile properties of Zr/Ti-based BMG composites of the same composition, but produced by different routes, was conducted so as to identify “structure–property” connections in these materials. This was accomplished by employing four different processing methods—arc melting, suction casting, semi-solid forging and induction melting on a water-cooled copper boat—on composites with two different dendrite volume fractions, Vd. The change in processing parameters only affects microstructural length scales such as the interdendritic spacing, λ, and dendrite size, δ, whereas compositions of the matrix and dendrite are unaffected. Broadly, the composite’s properties are insensitive to the microstructural length scales when Vd is high (∼75%), whereas they become process dependent for relatively lower Vd (∼55%). Larger δ in arc-melted and forged specimens result in higher ductility (7–9%) and lower hardening rates, whereas smaller dendrites increase the hardening rate. A bimodal distribution of dendrites offers excellent ductility at a marginal cost of yield strength. Finer λ result in marked improvements in both ductility and yield strength, due to the confinement of shear band nucleation sites in smaller volumes of the glassy phase. Forging in the semi-solid state imparts such a microstructure.

This dissertation addresses two critical issues in the mechanical behaviors and structures of bulk-metallic glasses (BMGs): (1) the effect of composition, fabrication method, and pretreatment of plastic deformation on mechanical properties and structures of BMGs; (2) the mechanical response and structural evolution of BMGs in the elastic and plastic region. (Cu50Zr50)94Al6 and (Cu50Zr50)92Al8 amorphous alloys were used to study the effect of composition on mechanical properties and structures of BMGs. The (Cu50Zr50)94Al6 alloy exhibits lower yield stress and Young’s modulus, higher Poisson’s ratio, worse thermal stability, and better plasticity than (Cu50Zr50)92Al8. Both the topological and chemical effects of Al addition account for the differences of mechanical and physical properties between them. A Zr55Ni5Al10Cu30 glass-forming alloy with injection casting (the melting temperatures are 1,550 K and 1,250 K, respectively) and with suction casting was fabricated. The results indicate that despite their amorphous structures, the suction-casting samples exhibit a lower yield stress, lower Young’s modulus, and larger plastic strain than the injection-casting samples (the melting temperature is 1,550 K) due to more quenched-in free volumes in suction casting, which results from the higher cooling rate. The inhomogeneous plastic deformation in Zr50Cu40Al10 BMG samples was introduced by four-point-bend fatigue. There is almost no difference of the stress-strain behaviors between the deformed and undeformed samples. Elastostatic compression was used to introduce homogeneous deformation in Zr70Cu6Ni16Al8 BMG samples. The preloaded samples are softer with decreases of yield strength and Young’s moduli. Anisotropy was observed in the preloaded samples despite their small magnitudes, which even occurred at a relatively low temperature and applied stress level. The structural evolution of Zr70Cu6Ni16Al8 BMG in the elastic region was analyzed with anisotropic pair density function. The analysis of the first shell of Zr70Cu6Ni16Al8 glass confirms the structural changes in the elastic region. The bond reorientation leads to direction dependent changes in the chemical short-range order. The structural evolution in the plastic region of Zr70Cu6Ni16Al8 BMG is investigated as well. The serrations were observed for both the stress-displacement and full width at half maximum-displacement curves. The excess free volume was measured, which increases with increasing the displacement.

Metallic glasses can be considered for many commercial applications because of the higher mechanical strength, corrosion and wear resistance when compared to crystalline materials. To consider them for novel applications, the challenge of preparing metallic glasses from the liquid melt phase and how the properties of metallic glasses change due to relaxation need to be understood better. The glass forming ability (GFA) with variation in composition and inclusion of different alloying elements was studied by using thermal techniques to determine important GFA indicators for Zr-based bulk metallic glasses (BMG). The effect of alloying elements, annealing temperature and annealing time on the thermal and structural relaxation of the BMGs was studied by using an annealing induced relaxation approach. The thermal relaxation was studied by measuring specific heat of the samples using differential scanning calorimeter (DSC) and calculating the enthalpy recovery on reheating in the BMG samples. The structural relaxation was also studied by using extended X-ray absorption fine structure (EXAFS) technique on the as-obtained and relaxed samples. The effects of alloying elements and annealing on electrical resistance were studied by using a two point probe. From the study, it was found that the currently used GFA indicators are inadequate to fully capture and identify the best GFA BMGs. The fragility (beta) of the melt is a new criterion that has been proposed to measure and analyze GFA. The enthalpy relaxation of Zrbased BMGs was found to follow a stretched exponential function, and the parameters obtained showed the BMGs used in the current study are strong glass formers. EXAFS studies showed variations in the structure of BMGs with changes in alloying elements. Furthermore, alloying elements were found to have an effect on the structure of the relaxed BMGs. The resistance of BMGs was found to decrease with relaxation which can be attributed to short range order on annealing.

The thermal and mechanical properties of the Mg-based bulk metallic glasses are reported in this thesis. The original ingots were prepared by arc melting and induction melting. The thermal and mechanical properties of the Mg-based bulk metallic glasses are reported in this thesis. The original ingots were prepared by arc melting and induction melting. The Mg65Cu25Gd10 and Mg65Cu15Ag10Gd10 bulk metallic glasses with different diameters from 3 to 6 mm were successfully fabricated by conventional copper mold casting in an inert atmosphere. The Mg65Cu25Gd10 bulk metallic glass shows the high glass forming ability and good thermal stability. However, the addition of Ag in the Mg65Cu15Ag10Gd10 alloy degrades the thermal stability. Based on the DSC results, the supercooled liquid region

Bulk metallic glasses (BMGs) have displayed impressive mechanical properties, but the use and dimensions of material have been limited due to critical cooling rate requirements and low ductility. The application of severe plastic deformation by equal channel angular extrusion (ECAE) for consolidation of bulk amorphous metals (BAM) and amorphous metal matrix composites (AMMC) is investigated in this dissertation. The objectives of this research are a) to better understand processing parameters which promote bonding between particles and b) to determine by what mechanisms the plasticity is enhanced in bulk amorphous metal matrix composites consolidated by ECAE. To accomplish the objectives BAM and AMMCs were produced via ECAE consolidation of Vitreloy 106a (Zr58.5Nb2.8Cu15.6Ni12.8Al10.3-wt%), ARLloy #1 (Hf71.3Cu16.2Ni7.6Ti2.2Al2.6 -wt%), and both of these amorphous alloys blended with crystalline phases of W, Cu and Ni. Novel instrumented extrusions and a host of postprocessing material characterizations were used to evaluate processing conditions and material properties. The results show that ECAE consolidation at temperatures within the supercooled liquid region gives near fully dense (>99%) and well bonded millimeter scale BAM and AMMCs. The mechanical properties of the ECAE processed BMG are comparable to cast material: σf = 1640 MPa, εf = 2.3%, E = 80 GPa for consolidated Vitreloy 106a as compared to σf = 1800 MPa, εf = 2.5%, E = 85 GPa for cast Vitreloy 106, and σf = 1660 MPa, εf = 2.0%, E = 97 GPa for ARLloy #1 as compared to σf = 2150 MPa, εf < 2.5%, E = 102 GPa for Hf52Cu17.9Ni14.6Ti5Al10. The mechanical properties of AMMCs are substandard compared to those obtained from melt-infiltrated composites due to non-ideal particle bonding conditions such as surface oxides and crystalline phase morphology and chemistry. It is demonstrated that the addition of a dispersed crystalline phase to an amorphous matrix by ECAE powder consolidation increases the plasticity of the amorphous matrix by providing locations for generation and/or arrest of adiabatic shear bands. The ability of ECAE to consolidated BAM and AMMCs with improved plasticity opens the possibility of overcoming the size and plasticity limitations of the monolithic bulk metallic glasses.

Brightness enhancement film (BEF) has been manufactured in foreign factories for backlight module of liquid crystal display (LCD), then it only have some interior factories to put in exploitation. Because of this, the study presents a precision machining and new step-imprint hot embossing process to fabricate pyramid-shaped microlens array. First, a tungsten (W) steel material is manufactured by precision machining. The dimension of a pyramid-shaped microlens on the W steel are about 300 £gm in the base line of three side, 222 £gm in bevel edge of three side, 139 £gm in height of bottom to top, 180 £gm in pitch of the left and right sides between two pyramid-shaped microlens tips, and 85 degree in top angle of three bevel. The W steel mold is used as the first mold. Second, the pyramid peaks of first mold pattern are transferred on bulk metallic glass (BMG) using step-imprint hot embossing method with position adjustable mechanism to form a smaller concave pyramid-shaped microlens array, it can avoid arc radius of cutting tools which is used as the second mold. Another the pyramid peaks are transferred on PMMA (Polymethylmethacrylate) for concave pyramid-shaped microlens array of optical film in the hot embossing system. Finally, the second mold is fabricated to emboss convex pyramid-shaped microlens array of optical film on PMMA. The foregoing method is provided for backlight module of optical films process.