To strengthen alloys, metallurgists often tweak the size and orientation of microscopic crystal grains within the material. Now researchers have applied the same principles at a much larger scale to create robust structural materials that could be used to build engine parts or prosthetics (Nature 2018, DOI: 10.1038/s41586-018-0850-3).
3-D printing involves building a 3-D object layer by layer. When combined with computer-aided design, the technique can craft highly intricate shapes. For example, 3-D printers can produce lightweight lattice structures that contain a repeating series of nodes and struts, similar to the cross-hatched trusses found in certain bridges and tower cranes.
But these repetitive, latticed structures are only strong up to a point. When one part of a 3-D printed lattice gives way under an extreme load, it triggers a domino effect that leads to catastrophic failure of the whole lattice—just like a single crystal shearing along a plane.
Minh-Son Pham at Imperial College London has now borrowed some core concepts from metallurgy to solve this problem. Most crystalline materials are made of many small crystal grains fused together. The orientation of the crystal lattice in one grain may not line up with its neighbors, and it might even have a different crystal structure altogether, with its atoms adopting a different spatial arrangement. This configuration can actually improve the material’s response to strain because the boundaries between these grains can act as buffers that prevent atoms from slipping too far out of position, halting the spread of fractures.
Pham’s team has mimicked these microscopic features, 3-D printing polymer or stainless-steel lattices with nodes and struts that are analogous to the atoms and bonds of a crystal. Within each grain, the unit cells—the smallest repeating geometrical units—range in size from 5–40 mm across. “The only limit is the size of the printer,” says Pham. The approach allowed Pham’s team to create hierarchical structures that contain the same patterns at many different scales, from atomic level to the macroscale.
The researchers found that decreasing the size of each grain region strengthened the lattice considerably, as did angling the grains relative to one another, just like in polycrystalline materials. The team printed a lattice with these features using a brittle polymer, which was able to absorb more than six times as much energy as a continuous, single-crystal lattice of the same material.
“In my opinion, this is one of the most creative recent works related to metallurgical research,” says María Teresa Pérez-Prado, a metallurgist at the IMDEA Materials Institute, who was not involved in the research. “It opens up new avenues to engineer lattice structures with unprecedented properties.”
Pham’s team could also tailor the unit cells within a lattice so that they resembled different types of crystal structures. In some grains, the unit cells had an extra node at the center; in others, the unit cells had extra nodes on each facet. In a lattice printed from a polymer called polylactic acid, the grains with nodes on the facets were stronger because they contained more struts, while the other cells could bend more easily and absorb strain energy. This mixture of properties could be especially useful in the leading edge of a fan blade in an airplane engine, which should give way rather than shatter if hit by a rogue object, Pham says.
Thanks to their similarity to polycrystalline materials, the 3-D printed lattices could also be used to study fundamental metallurgy problems. Pérez-Prado says that it’s difficult to understand the effects of individual features of microscopic crystal grains because reducing their size will also affect the grain boundaries, for example. The 3-D printed lattices, in contrast, should be much easier to fine tune. “We have total control over the structure,” Pham says, “so we can change just one factor and get really clean data.”