A team of researchers has observed a new mechanism for metallic bending. The discovery not only upends previous notions about how metals deform, but could help guide the creation of stronger, more durable materials. AN open-access paper on the study is published in the journal Nature Communications.
This creates new opportunities for materials design. It adds another parameter we can control to enable strength and ductility.—Izabela Szlufarska, a professor of materials science and engineering at UW–Madison and co-corresponding author
Ductility is the ability of a metal to bend. Most approaches to increase a metal’s strength do so at the expense of flexibility; as metals become more resistant to bending, they’re more likely to crack under pressure. However, the researchers’ new mechanism for bending might allow engineers to strengthen a material without running the risk of fractures.
Engineers typically manipulate the strength of a metal through techniques such as cold working or annealing, which exert their effects through small, yet important, structural irregularities called dislocations.
It’s a truism that’s held since 1934, when three researchers independently realized that dislocation explained an ages-old paradox: Metals are much easier to bend than their molecular structures—which typically take the form of regularly repeating three-dimensional grids—would suggest.
Dislocations are tiny irregularities in the otherwise well-ordered crystal lattice of a metal. They arise from slight mismatches. Normal metals bend because dislocations are able to move, allowing a material to deform without ripping apart every single bond inside its crystal lattice at once.
Strengthening techniques typically restrict the motion of dislocations. However, Szlufarska and colleagues discovered that the material samarium cobalt—known as an intermetallic—bent easily, even though its dislocations were locked in place.
It was believed that metallic materials would be intrinsically brittle if dislocation slip is rare. However, our recent study shows that an intermetallic can be deformed plastically by a significant amount even when the dislocation slip is absent.—Hubin Luo, Ningbo Institute of Industrial Technology in China
Instead, bending samarium cobalt caused narrow bands to form inside the crystal lattice, where molecules assumed a free-form “amorphous” configuration instead of the regular, grid-like structure in the rest of the metal. Those amorphous bands allowed the metal to bend.
It’s almost like lubrication. We predicted this in simulations, and we also saw the amorphous shear bands in our deformation studies and transmission electron microscopy experiments.—Izabela Szlufarska
A combination of computational simulations and experimental studies was critical to explaining the result.
It is often easier to carry out theoretical simulations to explain existing experimental results. Here, we first theoretically predicted the existence of shear bands and their role in plasticity in samarium cobalt; these were entirely surprising phenomena. We then confirmed these results experimentally with many different types of experiments to test our theory and to be sure that the predicted phenomenon can be indeed observed in nature.—Hongliang Zhang, a UW–Madison postdoc
The researchers plan to search for other materials that might also bend in this peculiar manner. Eventually, they hope to use the phenomenon to tune a material’s properties for strength and flexibility.
This might change the way you look for optimization of material properties. We know it’s different, we know it’s new, and we think we can use it.—Izabela Szlufarska
Hubin Luo, Hongwei Sheng, Hongliang Zhang, Fengqing Wang, Jinkui Fan, Juan Du, J. Ping Liu & Izabela Szlufarska (2019) “Plasticity without dislocations in a polycrystalline intermetallic” Nature Communications doi: 10.1038/s41467-019-11505-1