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‘Meta-Crystals’ Make Materials Tougher and Lighter

A group of UK-based researchers has found a way to create more damage-tolerant architected materials by mimicking the more irregular microscale crystalline structure in strong metal alloys. The breakthrough could give way to materials with entirely new properties such as greater strength, lighter weight, and resistance to damage from material stress.

In materials science, mimicking nature in the form of architected materials can often yield improved results such as more strength and lighter weights. Typical architected materials are constructed from “unit cells” that are arranged so they all have the same orientation, like a grid with repeating nodes and struts (similar to the arrangement of a metallic single crystal: the nodes in the lattice are equivalent to the atoms in the single crystal, and the struts are equivalent to the atomic bonds).

Crystal architecture, materials, damage-tolerant materials, crystal structure

Lightweight and damage-tolerant materials inspired by crystal structures for a low-carbon future. (Source: Dr. Minh-Son Pham, Imperial College London)

While this type of single-crystal material is ideal for high-temperature applications, it’s not so good for resisting mechanical stress. When the material is overloaded, localized bands of high stress can occur (“shear bands”) that result in highly localized deformations of the material in the form of cracks, ultimately leading to a collapse.

The team’s research report, “Damage-Tolerant Architected Materials Inspired by Crystal Microstructure,” was published in the January 7, 2019 edition of the journal Nature.

Mimicking the Crystal Microstructures of Metal

The crystal lattices of metal alloys are unique structures. At the atomic level, they consist of unit cells of the same type and orientation, but housed in many domains, each of which contains a lattice orientation that’s different from the orientation of nearby domains (unlike in single crystal arrangements). Researchers from the Department of Materials at Imperial College London and the University of Sheffield have found a way to mimic the crystal microstructure of metals and alloys on a macroscopic scale by constructing a lattice unit cell that consists of an ordered arrangement of nodes connected by struts.

Using a computer-aided design package, the team was then able to 3D print the “meta-crystal” material, resulting in samples more resistant to cracking and bending than typical materials, but also stronger and lighter. The team found that it could increase the strength of the meta-crystals by reducing the size of each grain-like lattice region within the structure. The materials could also be created in a way that directs damage along specific, predetermined structural paths to minimize and halt damage.

“We’re aiming for high strength and lightweight materials and an ability to control damage in a desired manner, and even direct damage to a specific location we want it to be, then arrest the damage by some mechanical mechanisms,” Dr. Minh-Son Pham, assistant professor in the department of materials at Imperial College London, told Design News.

The technique could be used to create parts and components of multi-functional materials with desired properties that could, for example, decrease the weight and increase the fuel efficiency of vehicles without sacrificing safety, or orthopedic devices that eliminate the stress-shielding problem and enable better rehealing of bones. Other applications could include artificial hips, sports helmets, better crumple zone in vehicles, fan blades in aeroengines or turbine blades.

Crystal Lattice Architecture Strengthens Materials Many Times Over

For the purpose of the research, the team created samples using fused deposition modelling, vat-polymerization and powder-bed fusion. To date, the researchers have printed three types of polymers, as well as metals including 316L steel and Ti6Al4V. In principle, printed lattices can be created with a wide variety of materials, including tungsten, nickel alloys and even bio-materials. Essentially, if it’s printable, the crystal lattice approach can be used on it to increase the strength of architected materials many times over. Dr. Pham told Design News that it’s not only about an increase in strength, but also to control any damage to a part by tailoring the lattice orientation.

“The “orderness” (lattices) varies from domain to domain; but within a domain, the lattice orientation is uniform, or poly-oriented. We are developing a computational platform to achieve the optimal strength for poly-oriented lattices,” said Dr. Pham.

For now, the technique is a promising way to create complex and customized components that don’t need to be produced in mass, but have high added value, such as medical devices or expensive parts for aircraft. The team also believes the new approach to creating crystalline metallic alloys will lead to new experimental and computational research to improve understanding of the possibilities afforded by varying both the intrinsic microstructure and the designed mesostructure of meta-crystals.

“This approach will offer a unique way to realize the full potential of 3D printing,” said Dr. Pham.

Going forward, the researchers hope to use the technique to develop a new class of materials that are lightweight, mechanically robust, and smart. 

Tracey Schelmetic graduated from Fairfield University in Fairfield, Conn. and began her long career as a technology and science writer and editor at Appleton & Lange. Later, as the editorial director of telecom trade journal Customer Interaction Solutions (today Customer magazine), she became a well-recognized voice in the contact center industry. Today, she is a freelance writer specializing in manufacturing and technology, telecommunications, and enterprise software.

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