New Advanced Steel Is Stronger and Lighter Than Titanium Alloy

Many automobile owners might not have noticed one of the fundamental changes that has happened to their vehicles over the last several years. The amount of iron and steel going into new automobiles decreased by 8 percent between 1995 and 2011, with just over 60 percent of the average car being made of the two metals that final year.

The increasing use of lighter aluminum and composite materials, along with iron and steel’s relatively low strength-to-weight ratio, has been pushing engineers away from the former material workhorses of automotive manufacturing. But the desire to improve steel’s mechanical properties by lowering its density while keeping it just as strong has kept metallurgists working hard. They’ve been able to lower density by doping steel with aluminum, but the alloy suffers from brittleness wherever the two metals meet.

Now, researchers at South Korea’s Pohang University of Science and Technology say they have figured out a way to turn those brittle boundaries into elements that strengthen the steel-aluminum alloy. The image above shows the result of their work, a high-aluminum-content low-density steel. The metal alloy has a strength-to-weight ratio higher than titanium alloy, the lightest and strongest metallic material previously known to humanity.

“There is increasing demand for a broad range of structural materials for environmentally benign, energy-efficient, lightweight engineering systems,” the researchers write in a study published today in the journal Nature. “The balance of lightness, strength and ductility in metallic alloys has been explored since the Bronze Age. Unfortunately, strength and ductility are mutually exclusive.”

They were able to push the well-studied steel-aluminum alloy further by introducing a bit of nickel to the process, which they discovered catalyzed the creation of nanometer-sized iron and aluminum particles that they couldn’t shear apart in tests. Their fabrication method allowed them to control the size and distribution of these particles within the alloy to get the characteristics they were looking for.

“These findings provide a new alloy-design route to lightweight steels, demonstrating that the combination of specific strength and ductility accessible to steels is greater than previously thought, and increasing the density-compensated tensile damage tolerance of structural metal for terrestrial applications,” they write. “Furthermore, the attractive combination of physical and mechanical properties in the low-density steel described here is obtainable by simple thermal treatments which are compatible with existing commercial processes of the steel industry.”

Top Image: Annealed microstructure of high-specific-strength steel. Fine FeAl-type B2 precipitates form during annealing in between the B2 stringer bands in steel matrix. The specimen was annealed for 15 min at 900 C. Photo courtesy of Hansoo Kim/Nature.