New metallic glass with elastic properties: future replacement for metals?

Our smart phones appear to be one of our most valued possessions today. One aspect that determines the quality of the physical device itself are the characteristics of the casing: the thickness, durability, flexibility, and strength. Without these properties, it is very likely that breakages, deformations, and other unfortunate faults will occur more frequently. Researchers consider these characteristics vital, and take them into account when it comes to the manufacture of new materials. Partcularly with phones and the ever-demanding electronics market, the ultimate aim is to achieve a stylish yet durable device made from a material that is stronger, thinner and smaller.

Researchers at the University of Southern California (USC) have recently developed an incredible new material that exhibits both hard and elastic properties. The metallic-glass material, known as SAM2X5-630, has shown promising characteristics and record-breaking strength (the impact that the material is able to withstand makes it stronger than titanium). SAM2X5-630 has remarkable dual abilities, making it adaptable to numerous applications, ranging from body armour to protective-ware for various devices. Even when subject to heavy impacts and being pushed far beyond its elastic limits (i.e., the maximum force per unit area a solid material can experience before permanent deformation), the material does not deform or fracture — instead, it manages to maintain most of its original strength.

The group of researchers from USC published their work in the journal Scientific Reports, where they announced the creation of the material. The research is funded by the Defense Advanced Research Projects Agency (DARPA) as a three-year program for the development of structural amorphous metals (SAM). The aim of the program is to demonstrate the viability of bulk metallic glass (BMG) in structural applications. BMGs are a type of artificially generated material that consist of disproportionate strength, resilience, and elasticity, due to their unconventional chemical structure.

SAM2X5-630 is one of these bulk metallic glasses, and appears to have the highest impact resistance of any BMG developed to this day. What makes BMGs so desirable is the fact that they combine two previously mutually exclusive properties (metal properties and glass properties) and create a sort of ‘super metal’ that exhibits flexibility sufficient for moulding into complex shapes. Standing alone, metals are crystalline solids, with high density, conductivity, and strength. Their internal structure consists of molecules ordered in a lattice structure, where a disruption within this lattice structure—known as a dislocation—defines the properties of a metal. In comparison, glasses are non-crystalline solids that are very brittle when solid but, at high temperatures, fluid properties become apparent. It is this property that enables glass to be moulded into complex shapes. Physical properties such as these affect the processing potential of glass and metals. Metals display plasticity (permanent deformation of the material) and glass displays elasticity (reversible deformation of the material under a force). Both of these deformations are important when shaping complex parts and, by combining metal with glass, both characteristics are achievable in one material.

BMGs, and the newly developed SAM2X5-630 in particular, require very specific conditions for fabrication. This involves heating a powdered metallic composite to exactly 630 degrees centigrade and then instantaneously cooling it. The success of any bulk metallic glass comes from its behaviour at varying temperatures. For example, if the initial metal composite (which, in this case, was an iron composite) is heated to low-to-normal temperatures of around 200 degrees or lower, the final product would act as a metal with high durability and strength. Heating above 200 degrees, on the other hand, would produce the same composite with the potential to be shaped and moulded, just like a glass. The difficult part, however, is in finding the optimal heating temperature to combine the behaviours of both metal and glass in an advantageous way. For example, the exact same iron composite heated and cooled slightly differently would generate a completely different atomic arrangement that does not exhibit the same properties. Current methods of heating the metal composite are quite slow. However, in order to create SAM2X5-630, a new method of applying a large jolt of electricity was used, which made reaching the high temperature of 630 degrees a lot more achievable.

These high temperatures are required due to their effect on the molecular pattern of the material. The random arrangement of molecules within a material is what gives rise to its characteristics. High heat disarranges the atoms inside the material, while instantaneously cooling the liquid material freezes the atoms in position. This process prevents crystallisation, as the atoms do not have time to arrange themselves, leaving them tightly-packed. The random arrangement means that there are no planes of atoms in the solid material, meaning that movement of atoms becomes very difficult. This gridlocked atomic structure results in the material being extremely hard and strong, as well as giving it the properties of glass and metal.

The applications of SAM2X5-630 range from medical and electronic applications to commercial ones. Given that the electronic casings for cell phones, laptops, and other precision technologies usually need to be 1mm thick, metallic glass is optimal for these applications as it can be easily shaped whilst maintaining its strength. Although the new material isn’t transparent enough to be utilised for strong glass screens, it has the potential to be used in the creation of protective casings for mobile devices that would bounce when dropped. iPhones for example currently have aluminium sides — these could be replaced by the new material to introduce a more impact-resistant casing that is significantly more resilient and less likely to smash.

The material’s ability to be shaped precisely gives it a unique advantage when it comes to medical applications, too. Currently, the main challenge in the field of biomedical materials is biocompatibility (i.e., the behaviour and interaction between the body and a material). Due to the adaptable capabilities of BMGs, it is very easy to shape them in such a way that this interaction could potentially be optimised. Optimisation could be achieved by moulding the material such that air bubbles are intentionally placed to match the metal density to tissue density, and metal elasticity to tissue elasticity. In comparison to current medical technologies, matching the characteristics of the material to surrounding tissues in this way has the potential to lead to developments in biocompatibility. Applications for this technology could range from stents to tissue implants. Given the strength of SAM2X5-630, these applications could be achieved with higher standards than current approaches due to the material’s increased durability.

Other large-scale applications are also feasible for the material. Professor Veronica Eliasson, one of the lead authors of the research paper, states, “This material would be a great candidate for any type of application that is subjected to high dynamic loading rates (e.g. shock or impact)”. In comparison to other BMGs that have been subjected to the same shock experiments, the new material displays clear advantages. The researchers are not aware of any general disadvantages of the new material when compared to already existing BMGs, suggesting that further research will be required to determine any apparent downfalls of the material.

Reaching a stage where this material is fit for commercialisation will take many years. However, given that the material is both energy- and cost-efficient, large-scale applications of the material could eventually lead to it replacing metal entirely.

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