Ductility is the ability of a metal to bend. Most approaches to increase a metal's strength do so at the expense of flexibility - and as metals become more resistant to bending, they are 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. "Everybody in the metals community knows that dislocations are critical," says Szlufarska. Dislocations are tiny irregularities in the otherwise well-ordered crystal lattice of a metal. They arise from slight mismatches - picture the pages of a book as rows of atoms, and imagine how the neat stack of paper becomes ever-so-slightly distorted at the spot where someone inserts a bookmark.
Strengthening techniques typically restrict the motion of dislocations. So it was quite a shock when Szlufarska and colleagues discovered that the material samarium cobalt - known as an intermetallic - bent easily, even though its dislocations were locked in place. 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 is almost like lubrication," says Szlufarska. "We predicted this in simulations, and we also saw the amorphous shear bands in our deformation studies and transmission electron microscopy experiments." A combination of computational simulations and experimental studies was critical to explaining the perplexing result, which is why Szlufarska and her group were exceptionally suited to crack open the mystery.
"This might change the way you look for optimization of material properties," says Szlufarska. "We know it's different, we know it's new, and we think we can use it."
COMPAMED-tradefair.com; Source: University of Wisconsin-Madison