Can a Hummer H2 weigh significantly less without giving up strength, or resorting to aluminum or composites? Yes—if University of Virginia researchers can make amorphous steel as tough as it is strong.
According to professor Gary Shiflet of the University of Virginia's School of Materials Sciences, "Most metals are crystalline, having atoms arranged periodically in three dimensions." Except, that is, for those metals that are amorphous. Their atomic structure is random, not crystallized, so no crystallization defects occur in its structure. It's highly non-corrosive, non-magnetic, and many times stronger than conventional metals.
Three times stronger than high-strength steel at the same weight (its density is 8 g/cm3, the same as conventional steel), amorphous steel could challenge aluminum, plastics and composites for domination in the coming years as the light-weight material of choice for transportation applications. That is, once researchers work their way past one little problem with the current batch of amorphous steel: it's brittle. It shatters like glass when shocked, just like the evil terminator when he was frozen in Terminator 2: Judgment Day. "There are various ways of toughening this material so that it is as tough or tougher than conventional steels," says Shiflet.
The Defense Advanced Research Projects Agency (DARPA) began sponsorship of the University of Virginia's amorphous steel studies in 2001 as part of a larger amorphous metals program. One of DARPA's customers, the U.S. Navy, was looking for a non-metallic steel that was both stronger and lighter than conventional steel. Amorphous steel held promise, but researchers only were able to create the material in small batches by super-cooling molten metal at the rate of one-million degrees per second. This preserved the random, glass-like structure and prevented crystallization. Working with modelers from DARPA, Shiflet and his colleagues deduced that adding a small dose of lanthanides—large-radius rare earth elements—would keep the crystals from forming by creating enormous local stresses in their structures. Yttrium, it turns out, worked best for steel and allowed large batches of the material to be produced in a process that can be scaled to production levels.
"If you divide the strength by the density," says Shiflet, "you get this tripling of strength because you use less material for the same strength." According to Shiflet's calculations, a plate of amorphous steel 1/4-in. thick has the same strength as a conventional steel plate that is 1.0-in. thick; this ratio remains constant no matter the thickness of the material. Which means a vehicle utilizing this material in structural applications would be three times as strong, or the weight of the amorphous steel structure would be 1/3 that of a high-strength steel design.
The biggest change would come in how that structure was formed. Shiflet says it's possible to heat the material up to its glass transition temperature, and form it. "You can injection mold or cast it into any shape that you want, then cool it back below that temperature to make it a solid again," he says. It's similar to what a glass blower does as he heats the glass up, molds it, then cools it down. Automakers may one day form entire structural panels in a single dimensionally correct piece, then attach the rest of the vehicle structure to it.
For now, the greatest challenge is adding toughness to what is an already strong material. DARPA and the U.S. Navy have expressed interest in solving this problem, and Shiflet says the ultimate answer might come from the glass industry. "We could laminate it with a conventional steel," he says, "or temper it so that the outside layer is in compression, just like tempered glass." Another method would use a modified heat treatment process like that used for conventional steel which grows crystals on the surface in non-random orientations. "The result would be a composite of amorphous and crystalline steel that would be both tough and strong."
Lest you think that's the limit of this technology, Shiflet says experiments with melt-spinning—squirting the molten material onto a spinning copper wheel—has produce fibers that are 20-microns wide, which means the steel can be made into fibers and threads, then woven to produce a flexible steel cloth. And while shoe maker Nike passed on producing brightly anodized amorphous aluminum sport shoes, the steel mesh could be used for clothing or as a replacement for more expensive fibers in advanced composites. "People talk about steel as being a mature industry," says Shiflet, "and that's just not true. Steel has a way of fighting back because there are always things there to be discovered."