A printed component made up of ‘meta-crystals’ with varying orientations. Photo: University of Sheffield.
A printed component made up of ‘meta-crystals’ with varying orientations. Photo: University of Sheffield.

Working in collaboration with colleagues at Imperial College London, Iain Todd from the Department of Materials Science and Engineering at the University of Sheffield in the UK has been taking a novel approach to producing engineering components using additive manufacturing.

Additive manufacturing (AM), also known as 3D printing, is often used to produce engineering components. By replacing solid materials with lattice structures, the printed components can be much lighter that their solid counterparts, and can be engineered in such a way that they also exhibit property combinations that are inaccessible to conventional solids. These structures are known as architected materials.

The lattice structures typically have a uniform layout, with the nodes all conforming to a regular array and the struts between the nodes all following common planes, and this can create a problem. In a paper in Nature, Todd and his colleagues explain how these uniform lattices replicate the structure of a metallic single crystal: the nodes in the AM lattice are equivalent to the atoms in the single crystal and the struts are equivalent to the atomic bonds. In each of these structures, the atomic planes, or nodes, are all perfectly aligned.

While in some applications, such as the high temperature end of a jet engine, single crystal materials are ideal because of their ability to withstand deformation at extreme temperatures, they have limitations relating to their mechanical performance. This limitation is also observed in AM parts with a uniform lattice structure.

When the structure is compressed with a force sufficient to cause permanent deformation, the lattice shears along one or more of the planes of nodes. With nothing to inhibit this shearing, the collapse becomes catastrophic.

In polycrystalline materials – those with many crystals – the alignment of the atomic planes is random. When a shear force is applied in a particular direction in a polycrystalline material, the resulting crack will slow down or stop when it meets a new crystal, because the atoms in the new crystal will be aligned differently to those in the crystal where the crack initiated. Moreover, it is also possible to introduce different materials in the form of phases, precipitates or inclusions to strengthen the components; these materials can also help to inhibit crack propagation.

It is this fundamental metallurgical understanding that inspired scientists at Imperial College London and the University of Sheffield to mimic polycrystalline microstructures in AM lattices, with the aim of developing robust, damage-tolerant architected materials.

By developing computer models of atomic structures, scaling them up and creating meso-structures based on polycrystalline materials, the engineers are transforming the way that materials are designed, producing what they call 'meta-crystals'.

Experimental testing of components made from these meta-crystals has demonstrated that they are highly energy-absorbant, with the polycrystal-like material able to withstand almost seven times the energy before failure of materials that mimic a single-crystal structure. And while basic metallurgical concepts inspired the development of these architected materials, the researchers are also using the creation of architected materials as an alternative approach to studying complex metallurgical phenomena.

"This approach to materials development has potentially far-reaching implications for the additive manufacturing sector," said Todd. "The fusion of physical metallurgy with architected meta-materials will allow engineers to create damage-tolerant architectured materials with desired strength and toughness, while also improving the performance of architectured materials in response to external loads.

"And while these materials can be used as standalone structures, they can also be infiltrated with other materials in order to create composites for a wide variety of applications."

"This meta-crystal approach could be combined with recent advances in multi-material 3D printing to open up a new frontier of research in developing new advanced materials that are lightweight and mechanically robust, with the potential to advance future low carbon technologies," added Minh-Son Pham of Imperial College London.

This story is adapted from material from the University of Sheffield, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.