What is metallic glass? - Ashwini Bharathula
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Is metallic glass poised to come of age? Offers a great discussion about the future of metallic glass, take a look and see what you think.
In addition to their ultra-high strengths, metallic glasses are ~10 times springier than most metals. They have an incredible capacity to store and release elastic energy. Check out this fun bouncy ball experiment that talks more about metallic glass’s very high coefficient of restitution (Note: “Liquidmetal” is a commercial name for a class of bulk metallic glasses).
At room temperature, metallic glasses may be superheroes in strength, but at higher temperatures (close to what we call the glass transition temperature), they soften a lot and have the potential to be shaped easily into complicated shapes just like plastic. You can learn more about thermoplastic forming of metallic glasses here: Thermoplastic blow molding of metals, Bulk Metallic Glasses: Constructing the Future, and Processing of BMGs.
Unlike metals that shrink while solidifying from molten liquid, metallic glasses don’t. This allows for them to be cast directly into precise shapes in a single molding step. This combined with excellent surface finish right out of the mold helps cut overall fabrication costs. Here’s how one company called LiquidmetalR Technologies Inc. uses injection molding to produce commercial metallic glass products with complex geometries.
Most metallic materials are polycrystalline, meaning they are made of many crystallites or grains. In each of these grains are atoms that are arranged in an orderly manner but the areas where the grains meet are called ‘grain boundaries’ that have atoms with stretched or broken bonds. Grain boundaries are weak spots that can get preferentially attacked by chemicals. A metallic glass, thanks to its amorphous structure, has neither grains nor grain boundaries and so is more resistant to chemical attack or corrosion. Absence of grain boundaries is also what leads to atomistically smooth, mirror-like surfaces thereby reducing wear considerably.
It was mentioned in the movie that we are unable to build large structures out of metallic glasses yet because they do not exhibit adequate toughness. So what exactly is ‘toughness’? In Materials Science, one definition of toughness is the amount of energy per unit volume that a material can absorb before rupturing. It is a measure of the combination of material’s strength and ductility. Strength is determined by the force it takes to get a material to yield or deform plastically. And ductility is the ability of the material to plastically deform i.e., undergo a permanent change of shape (elongate, bend, dent etc.) and NOT break catastrophically under load. For example, if you hit a bar of metal with a hammer it bends or gets dented, i.e., it absorbs energy and deforms plastically before it ruptures or fails eventually. This bar of metal has decent (but not super high) strength and is very tough due to all the plastic deformation it can undergo before breaking. But if you hit a ceramic bowl or your window glass with a hammer, it breaks or shatters immediately with no plastic deformation whatsoever. These materials have very high strengths but no ductility and therefore have very low toughness values. Many engineering polymers have very low strength and toughness values. Metallic glasses fall somewhere in between. They have ultra-high strengths and are capable of small amount of plastic deformation (attributed to their inherent metallic bonding). So, they are much tougher than your window glass but not nearly as tough as metals. This means that it takes a lot of force for them to yield or plastically deform, but once they yield they have limited ductility. Rather, they break or fracture without warning. So, they are not tough enough for load bearing applications yet. Toughness can be depicted graphically using what we call a stress-strain curve. It is defined as the area under the curve which basically is a measure of the total energy absorbed by the material until failure. The larger the area, the tougher the material.
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