A New Time For Titanium (1)

Among metals, titanium's strength and lightness, corrosion resistance, and ability to withstand extreme temperatures have long distinguished its value, particularly for weight- and environment-sensitive applications. When it was first described in the late 18th century, a co-discoverer named the metal for the Titans - gods born of Earth and sky in ancient Greek mythology.

Time has only burnished titanium's luster. "I'm a materials scientist, and so people sometimes ask me, 'What's your favorite element?'" says Andrew Minor, professor of materials science and engineering. For buildings, airplanes, missiles, spaceships, and more, he says, "If you want the strongest material for the least amount of weight, it's titanium. If we could, we would make everything out of titanium."

Indeed, for industrial designers, the prospect of strong, lightweight, highly fuel-efficient cars, trucks, and airplanes, for example, or super corrosion-resistant cargo ships, titanium must be the stuff of dreams.

The problem? "It's too expensive," Minor says of industrial-grade titanium or titanium alloys that might otherwise replace steel when only the strongest, most durable materials will suffice. The cost of making titanium is about six times greater than that of stainless steel. As a result, its uses have remained limited to specialty parts for aerospace, high-end items like jewelry, or other niche applications.

What's more, pure titanium has only moderate strength, Minor explains. It can be strengthened with elements like oxygen, aluminum, molybdenum, vanadium, and zirconium; however, that is often at the expense of ductility - a metal's ability to be drawn or deformed without fracturing.

Now, after a decade of research, a new era for titanium, including greatly expanded engineering applications, may be approaching, thanks to Minor and his Berkeley colleagues, including Mark Asta, Daryl Chrzan, and J.W. Morris Jr., also professors in the Department of Materials Science and Engineering. They've been probing and prodding titanium in any number of ways in hopes of expanding its practical use for a variety of structural or engineering applications.

Instead, what drives the excessive cost of commercial-grade titanium, Minor explains, is the complex Kroll process most often used to make titanium bars, ingots, and other forms of metal that can be fabricated into useable parts and other products. The process includes the use of expensive materials like argon gas, and it is energy-intensive, requiring multiple melts at extremely high temperatures, especially to control oxygen impurities.

Indeed, titanium and oxygen have a puzzling relationship, one that Minor, Asta, Chrzan, Morris, and colleagues have wanted to understand better. The team knew that an oxygen impurity is often used for titanium alloys to harness a potent strengthening effect. Titanium made with just a tiny increase in the amount of atomic oxygen can result in a metal with a several-fold increase in strength.

Unfortunately, the oxygen can also yield an even larger decrease in the metal's ductility. It becomes brittle and will fracture and break.

But "oxygen is everywhere," Minor says of the difficulty in maneuvering around titanium's high responsiveness to oxygen. "It's not some impurity coming from the source material that you can just avoid."

He characterizes titanium's sensitivity to oxygen as extreme. "It's truly strange how powerful it is," Minor says. It exerts effects on the metal, both good and bad, whereas the presence of similar amounts of oxygen is insignificant for metals like aluminum and steel because it can be dealt with in processing much more easily.

To learn more, the team turned to high-performance computing to model the deformation process in titanium under stress and with differing amounts of oxygen. Computer models, Asta says, are a "powerful set of tools that let us investigate this outstanding challenge in titanium metallurgy."

Of the team's major discoveries, a shuffling of oxygen atoms in titanium's crystal structure when the metal is under stress became key to understanding the loss of ductility. In a non-stressed state, oxygen molecules reside without incident in natural gaps between atoms of titanium. But under mechanical forces, the oxygen atoms can shuffle to adjacent spaces where they provide less resistance to dislocations that, if they spread, weaken the metal.

"The oxygen promotes a structural weakness," says Minor. As mechanical forces deform the metal, the displaced oxygen atoms, rather than blocking the spread of structural defects, can facilitate a so-called planar slip.

A planar slip, Asta says, is like a ripple of defects in the metal's crystal structure that build one on the other, eventually leading to fractures, cracks, and a brittle piece of metal.

To understand how a dislocation can form and spread in titanium, Chrzan suggests visualizing trying to move a large, heavy rug.

"A very large rug can be picked up at one end and dragged across the floor to a new position," he says. But another way to move the rug is to create a ripple at one end and then, by shuffling your feet across the top of the carpet, you can "walk" the ripple to the other end. Provided nothing blocks its movement, the entire rug will have been displaced by a distance equal to the width of the ripple.

Such "ripples" in titanium can be seen with electron microscopy. "You can see all the dislocations are lined up, in rows," Minor says. "And that's bad for ductility because if they line up and only follow each other, they don't get tangled up [and thus stopped] such that the metal doesn't work harden. You get a stress concentration, and that's where you get a crack."

(To be continued)