Top PDF Thermoplastic forming and related studies of the supercooled liquid region of metallic glasses

Metallic glasses have some amazing properties that captured my imagination and determined my path for grad school. In APh 110, Winter term 2003-2004, Dr. William L. Johnson gave a seminar lecture describing his research with an enthusiasm that was intoxicating. He described amorphous metals, materials having a random arrangement of atoms that were frozen in a liquid configuration because of a clever choice of alloying elements. These elements were chosen to have large negative heats of mixing and near eutectic compositions meaning the elements were much happier mixed than separate. The atomic sizes of alloying elements were also chosen so that many different sized spheres can log jam efforts of the mixture to crystallize upon cooling from the molten state.

In the present work, optimization of ZrTi based Be bearing glasses for large thermal stability is attempted. Measurement of the liquid fragility of these alloys will be the subject of future investigation. Several quaternary ZrTi based Be bearing glasses with high ΔT and good GFA will be presented. The alloy optimization was approached by examining the ternary ZrTiBe system for compositions exhibiting large ΔT and modest GFA. Some of the ternary compositions investigated here for bulk glass forming ability include those investigated previously by Tanner [13], who focused primarily on their amorphous ribbon forming ability. An appropriate fourth “solute element” at an optimum fraction was added to the ternary compositions, and in most cases, both ΔT and glass forming were seen to increase until too high a concentration of solute precipitated additional phases. Late transition metals were found to be the optimum solute element. Ni was not considered a viable solute atom as ample evidence in the literature suggests that the NiTi rich quasicrystal is the first phase to nucleate in Vitreloy glasses, providing nucleation sites for other crystals that eventually crystallize the alloy [14-15]. This is the approach that led to the development of the recently reported Zr 35 Ti 30 Cu 8.25 Be 26.75 alloy

samples are shown in Fig. 1. During heating, all the samples exhibit a distinct endothermic peak due to glass transition, followed by a supercooled liquid region, and then a sharp exothermic peak due to crystallization. With further increas- ing temperature, the alloys finally melt after several steps of endothermic events. When x increases from 10 to 35, T x

While metallic glasses are generally known for their attractive mechanical properties[52, 53], perhaps their most promising attribute is their potential for “thermoplastic” processing[12, 16, 54, 18, 14, 44, 13]. By virtue of being glasses, they can be softened to viscous liquid states above the glass transition where viscoplastic shaping can be carried out in a manner similar to that applied to process conventional thermoplastics. Unfortunately, this potential for thermoplastic forming is practically limited by the rapidly intervening crystallization of the relaxed “supercooled” liquid. Electrical discharge heating has recently emerged as an effective means to overcome this limitation[1, 51, 2]. It enables rapid and spatially uniform heating to low viscosity states considered to be optimal for thermoplastic shaping, over time scales sufficiently short to bypass crystallization. In this work we demonstrate that subjecting the metallic glass to an intense electric current pulse directed normal to an applied magnetic field can generate Laplace forces sufficiently large to perform thermoplastic shaping operations, thereby producing high quality net-shaped metallic glass articles.

Bulk metallic glasses (BMGs) have been considered as the promising structural and/or functional materials because of their many unique physical and chemical properties. 1) For instance, the BMGs are suitable to produce the separators of the proton exchange membrane fuel cells (PEMFC) owing to their high corrosion resistance, high strength, good toughness and viscous flow workability in the supercooled liquid region, etc. 2) The Ni-based metallic glasses have been used to

In situ ultrasonic methods utilizing a novel notched sample geometry are discussed in Section 2.4 and are applied to a variety of metallic glass forming systems with a range of Angell fragilities in Chapter 5. Two of the systems studied were also studied via the ex situ annealing methods. The results from both types of measurement com- pared favorably; thus supporting the claim that we are truly measuring the properties of the equilibrium supercooled liquid. Additionally, a cooperative shear model for the viscosity model and the corresponding “Johnson indices” are presented in Chapter 3. Chapter 6 discusses two experiments. In the first the possibility of controlling material properties of metallic glasses by varying the cooling rate is examined. It was determined that many other factors come into play. The second experiment was originally designed to measure the in situ ultrasonic properties of the organic glass, glycerol. This proved to be beyond our capabilities, however led to an attempt to explore cavitation behavior in glycerol.

The well controlled microforming process can be achieved by utilizing the superplastic deformation of BMGs in a supercooled liquid region with the help of a computer aided modelling technique for the deformation behaviour of BMGs. In order to investigate the local deformation states in the samples, numerical analyses, e.g. the finite element method (FEM), incorporated with adequate constitutive models are necessary. In this respect, a correct constitutive model in conjunction with well defined viscosity behaviour model in the high formability region are essential for good predicting capability during the microforming process. FEM incorporated with adequate yield models or constitutive equations has been successfully applied to analyzing me- chanical behaviour of not only conventional solid materials but also porous materials, composites and nanostructured materials. 3,8–10)

The purpose of this research is to elaborate a mathematical model that describes the visco- elastic bending behavior of a thermoplastic composite specimen during thermoforming processes. The mathematical model was derived from a simplified physical model which represents a layered composite material of alternating elastic and viscous layers. In a previous research the viscous layers are assumed to be from Newtonian fluid, having a constant shear viscosity, while the elastic layers are linear elastic. This resulted in a mathematical model in form of the diffusion equation, describing a diffusive-like behavior of the deflection angles in the beam. Because polymeric melts exhibit in general a shear thinning behavior, the diffusion equation in this research is improved by including the shear rate dependency of the melt viscosity.

Gardon [34] reviewed the thermal tempering of silicate glasses and terms such as tem- perature equalization and solidification stresses are taken from this work. Unlike the trivial temperature equalization stresses, the analysis of solidification stresses and the stress evo- lution in the glass transition region is a complicated problem and necessitate various levels of viscoelastic phenomenology. Gardon classifies the varius models developed for silicate glasses as instant-freezing theories (e.g., Aggarwala and Saibel [3]), the viscoelastic theory (Lee et al. [50]) and the structural theory (Narayanaswamy [63, 64]). The first is a sim- plistic approach to estimate residual stresses, the second is the linear thermoviscoelastic treatment with time-temperature-superposition principle and the last one is nonlinear vis- coelastic accounting for the temperature history dependence of the glass structure. The latter theory is accepted to be the definitive theory of thermal tempering in silicate glasses since it achieved reasonable agreement with experiments for both the evolution and mag- nitude of temper stresses. In silicate glasses, photoelasticity could be used to monitor both the evolution and final value of temper stresses whereas for BMGs no known method will avail to monitor the stress evolution in situ.

ever, these analyses were limited to binary systems because of the definition of the Miedema’s model which is developed for binary systems. Accordingly, no analyses have been con- ducted for the multicomponent systems, though nearly 350 ternary metallic glasses had been found until 1997. 10) There- fore, there exist great needs to develop an applicable model for calculating T x of multicomponent systems.

These results add to the findings of Haji-Akbari and Debenedetti (11), who showed that the nucleating ice nucleus exhibits a similar rings distribution, and Pirzadeh et al. (34), who highlight the presence of 6 ± 1-membered rings near growing ice surfaces. We stress here, however, that in the liquid snapshots we investigated for the rings analysis, we find only negligible amounts of actual ice (according to different criteria; for details see SI Appendix). Since the majority of 6-membered rings in the MI domains can be seen as ice-like if regarded in isola- tion, this means that it is the relative orientation between rings that is different from the crystal and thus the missing ingredi- ent in forming ice. Because this happens in the MI region, which has a reduced diffusivity compared with other regions, we can speculate that the mechanism giving rise to the initial formation of ice-like clusters in the liquid is collective in nature [a simi- lar argument based on density changes was made by Errington et al. (18)]. This would be consistent with a picture of reorient- ing rings rather than a picture of single-particle attachments via diffusive motion.

500nm- diameters and ∼650nm- and 2µm− heights. This synthesis was carried out by electroplating the metallic glass into a Polymethylmethacrylate (PMMA) template, which was spin-coated onto a pre-sputtered Au seed layer on a Si chip and patterned via e-beam lithography (Fig. 2.1d). In addition, a 2µm−thick film was separately electroplated directly onto another substrate by following the same procedure, and nano-tensile samples with identical geometries were FIB-carved into the film. The two sets of samples - electroplated and FIB-carved - were virtually identical to each other, both in composition and geometry (see Table 2.1 & 2.2 for chemical composition analysis and Figure 2.3 for the SEM images of representative samples). Experiments on these samples allowed for a direct comparison between the mechanical responses of otherwise the same metallic glass nano structures with irradiated vs. as-fabricated surface states. A Ni-P system was chosen because it lends itself well to electroplating. NiP metallic glasses may have different short range order (SRO) compared with the more common binary glass, CuZr, which has more metallic-like bonding (see Appendix discussion).[22] Tensile results revealed catastrophic failure in 500nm-diameter samples and post-elastic deformability in 100nm-diameter samples fabricated by both techniques. The extent of tensile duc- tility was nearly a factor of three greater in FIB samples, which suggests that a less relaxed (irradiated) surface state may facilitate homogeneous plastic flow via activation of numerous diffuse shear bands.

exceptional strengths [1], however poor ductility has limited their industrial applications. Metallic glasses fail by shear localisation along a shear band resulting in catastrophic failure [2]. A leading method for increasing ductility is the formation of a composite containing a volume fraction of crystalline material [2]. This tends to lead to nucleation of multiple shear bands, and the particles inhibit propagation, leading to distribution of shear through multiple shear bands across the material and hence plastic flow.

Recently, a number of multi-component alloy systems, which makes the formation of glass phase possible at relatively low cooling rates (1–100 K/s), were discovered due to their higher thermal stability against crystallization. 1–3) Bulk metallic glasses (BMGs) have been developed for structural applications utilizing their high strength, large elastic deformation limit, and superior corrosion and wear resistance at room temperature. Although the bulk of a cm- order dimension can be produced, the size to be used for engineering and structural application fields is still not enough. Therefore, the efforts to relieve the size limit and to improve the workability of BMGs are needed to fabricate the components made of BMGs. 1,2)

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To circumvent the limited ductility of bulk metallic glasses (BMGs), heterogeneous materials with glassy matrix and different type and length-scale of heterogeneities (micrometer-sized second phase particles or fibers, nanocrystals in a glassy matrix, phase separated regions, variations in short-range order by clustering) have been developed in order to control the mechanical properties. As example, recent results obtained for Cu- and Ti-base structurally imhomogeneous bulk metallic glasses will be presented. This type of clustered glasses is able to achieve high strength together with pronounced work hardening and large ductility by controlling the instabilities otherwise responsible for early failure. We emphasize the possibilities to manipulate such spatially inhomogeneous glassy structures based on martensitic alloys in favor of either strength and ductility, or a combination of both and also discuss the acquired ability to synthesize such M-glasses in bulk form through inexpensive processing routes. [doi:10.2320/matertrans.MJ200725]

The aim of this paper is to study the structural behavior of Cu-based metallic glasses in a wide composition range. The analysis of the concentration dependence of structural and thermodynamic properties should give evidence whether or not phase separation in the Cu-Zr glasses exists, which is of importance for understanding the mechanical properties of BMG’s.

It is well known that Zr-based metallic glasses have a high glass-forming ability (GFA), which enables us to produce the glassy alloy with bulky shape. To clarify the mechanism of such high GFA, a number of studies have been performed in the aspects of structural analysis, transformation behavior and so on. 1,2) Recently, the icosahedral QC phase (I-phase)

Bulk amorphous metals are attractive materials for several reasons. An absence of microstructural features such as crystal planes, dislocations, grain and phase boundaries contribute to appealing mechanical properties such as high hardness and high specific strength. The amorphous structure can also result in attractive magnetic properties such as high magnetic permeability. Corrosion studies have also estab- lished amorphous metals as a group of materials with corrosion properties much more desirable than their crystal- line counterparts. As a result, metallic glasses have found their way into common applications including golf club heads, magnetic security strips, step-down transformers and cell phone cases. Within the last 15 years, many successful steps have been made in both understanding the properties of amorphous metals and in processing bulk quantities effi- ciently.

Strong, robust joining of metal components is a key consideration in the fabrication of complex assemblies and consumer products. Developing optimal welding and joining practices for each class of materials is critical for their integration into products. Bulk metallic glasses (BMGs) are a relatively new class of materials with exceptional mechanical properties. However, they lack an obvious joining process, owing to their unique amorphous microstructure. BMGs are known for their combination of ultra-high strength, high hardness, large elastic limit, and low melting temperatures that allow them to be cast or molded like polymers. These properties have been widely exploited in applications such as electronic cases and golf clubs, but have been mostly relegated to small, near-net shape parts that are components of larger structures. This is due to a number of factors, including the difficulty of casting larger parts, material cost, and the low toughness of BMGs. Recently, a new class of BMG composites has emerged with soft crystalline dendrites. These dendrites allow the material to deform plastically (up to 10% elongation prior to failure), resulting in a material with ultra-high toughness that still preserves the strength of the metallic glass. 1 These alloys have been semi-solidly processed into panels