Steel is one of the most common structural materials, truly one of the foundations of modern civilization. An alloy of iron and carbon, steel has been made since biblical times. With two thousand years of experience in steelmaking, it would seem all there is to know about steel would already be known. But for Allison Beese, McFarlane Assistant Professor in Materials Science and Engineering, the secret world of steel is still unfolding.
"There is a lot known about traditional steels, but advanced, high-strength steels are a different story," says Beese. "Traditional fracture models don't accurately predict the properties of these advanced steels. We're still trying to understand the mechanisms for damage accumulation and fracture, and steel companies are very interested in developing other designs for steel."
The fine structure of fine steel
The best steels have high strength in conjunction with ductility, the ability to be stretched. Alloying small amounts of materials such as manganese can add to a steel's strength, while phase transformations during processing or deformation can change the steel from soft to extremely hard and strong.
For most of the history of steelmaking, metalworkers found out through trial and error how to produce steel with the qualities they wanted. They had no knowledge of "microstructure;" they just knew that certain ingredients and procedures worked.
One of the most famous steels ever made, called Damascus steel, was highly valued in ancient and medieval times for its strength, flexibility, and ability to hold an extremely sharp edge -- traits that made it the material of choice for the production of swords and knives. (The vaunted Valyrian steel in the novel series and TV show Game of Thrones reportedly was modeled on Damascus steel.) The source of its unique qualities was a mystery until 2006, when a German research team found that a centuries-old blade of Damascus steel contains carbon nanotubes and cementite (iron carbide) nanowires, which must have been created during the forging process -- a process whose details were lost in the late 1700s and have yet to be re-discovered.