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While plastics have revolutionized industrial design due to their versatile processability, their relatively low strength has hampered their use in structural components. On the other hand, while metals are the basis for strong structural components, the geometries into which they can be processed are rather limited. The “ideal” material would offer a desirable combination of superior structural properties and the ability to be precision (net) shaped into complex geometries. Here we show that bulk metallic glasses (BMGs), which have superior mechanical properties, can be blow molded like plastics. The key to the enhanced processability of BMG formers is their amenability to thermoplastic forming. This allows complex BMG structures, some of which cannot be produced using any other metal process, to be net shaped precisely. Metals are the most widely used structural material, spanning length scales from ~100 nm to ~100 m in applications where a combination of strength and ductility is required. Compared to plastics, however, metals exhibit limited processability (Fig. 1). The origin of the superb processability of (thermo)plastics is the gradual softening from a solid-like material (glass) below the glass transition temperature, Tg, to a liquid-like material (supercooled liquid) when heated above Tg. From a processing point of view, an ideal material would flow under a forming pressure, which is low, yet sufficiently large that turbulent flow is avoided and gravity and wetting effects can be neglected on the time scale of the process (Fig. 1). Such a desired processing window of forming pressure lies between 10-5 and 1 MPa and can be readily accessed with (thermo)plastics. Plastic processing is typically carried out at viscosities of 103–106 Pa•s and strain rates of 10-2–101 sec-1.33087
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In the simplest case of a Newtonian flow, viscosity, η, translates into the flow stress (or forming pressure), σ, according to σ = η⋅3ε ⋅, where ε ⋅ is the strain rate. Most conventional metals cannot be processed within the ranges described above. They are either processed in their crystalline state, where even at elevated temperatures they possess high strength, or in a highly fluid liquid state above their melting temperature. This results in turbulent flow, wetting effects, and possible segregation, as well as undesirable microstructures on subsequent solidification. Thus, compared to plastic processing, most metal processing methods yield inferior results in terms of versatility of shapes, precision, and economics. These limitations have spurred research focused on processing metals in a softer state1-3.Superplastic formable alloys and semi-solid processingSuperplastically formable (SPF) metallic alloys were developed to withstand plastic deformations of several hundred percent before failure2,3, far beyond the plasticity range of 25 % and less normally associated with metals. However, the flow stresses of SPF alloys, though significantly lower than those observed in conventional metals, are still significantly higher than those of plastics and do not fall into the ideal processing region (Fig. 1) even under strain rates as low as 10-3 sec-1. In a separate development, alloys were developed which can be processed in a semi-solid state, at temperatures where they are partially liquid and partially solid1. As a result, the processing temperature can be reduced, yielding benefits such as lower energy consumption, reduced oxidation, and elimination of the necessity Fig. 1 Properties vs. processability compared via the temperature-dependent strength for conventional steel, SPF alloys, plastics, and BMGs. Conventional metals are represented by a 1045 steel, SPF alloys by aluminum based 2004 SPF, BMGs by Zr44Ti11Cu10Ni10Be25 and Pt57.5Cu14.7Ni5.3P22.5, and PET (polyethylene terephthalate) was chosen as an example system for plastics. The temperature dependent strength (flow stress) is calculated for the fluids from σ = η⋅3ε ⋅ for a strain rate of 10-1 sec-1.