Researchers Develop Stronger Titanium Alloy

Researchers at the Department of Energy's Pacific Northwest National Laboratory (PNNL) have created a stronger titanium alloy—in a development that could lead both to the production of lighter vehicle parts as well as new high-strength alloys.

The researchers knew that a titanium alloy previously developed from a low-cost process had very good mechanical properties, but wanted to know how to make it even stronger. Using powerful electron microscopes and a unique atom probe imaging approach, they were able to peer deep inside the alloy's nanostructure to understand its composition better in order to improve on its qualities.

At 45% the weight of low carbon steel, titanium is lightweight but not particularly strong unless it is alloyed with other metals. Fifty years ago, metallurgists tried blending it with iron, along with vanadium and aluminum. The resulting alloy, Ti185, was very strong—but only in places. The mixture tended to clump: iron clustered in certain areas, creating defects known as beta flecks that made it difficult to commercially produce the alloy reliably.

Six years ago, PNNL and collaborators found a way around that problem and also developed a low-cost process to produce the material at an industrial scale. Instead of starting with molten titanium, the team substituted titanium hydride powder, which reduced the processing time by half and dramatically reduced the energy requirements.

"We [also] found that if you heat treat it first with a higher temperature before a low-temperature heat treatment step, you could create a titanium alloy 10-15% stronger than any commercial titanium alloy currently on the market and that it has roughly double the strength of steel," says Arun Devaraj, a material scientist at PNNL.

Using an atom probe tomography system, Devaraj and the team examined the alloy to see how the individual atoms are arranged in 3D. The atom probe dislodges one atom at a time and sends it to a detector. Lighter atoms "fly" to the detector faster, while heavier items arrive later, each atom type identified depending on the time it takes to reach the detector and its position identified by the detector.

The researchers discovered that, via the optimized heat-treating process, they had created micron-sized and nanosized precipitate regions—each with high concentrations of certain elements. Treating the regions at a higher temperature of 1,450 degrees Fahrenheit achieved a unique hierarchical nanostructure.

When the strength was measured by pulling or applying tension and stretching it until it failed, the treated material achieved a 10-15% increase in strength, which is significant, given the low cost of the production process. Steel used to produce vehicles has a tensile strength of 800-900 megapascals, whereas the 10-15% increase achieved at PNNL puts Ti185 at nearly 1,700 megapascals, or roughly double the strength of automotive steel at half the weight.

The team collaborated with Ankit Srivastava, an assistant professor in Texas A&M's Material Science and Engineering Department, to develop a simple mathematical model for explaining how the hierarchical nanostructure can result in the exceptionally high strength. The model, when compared with the microscopy results and processing, led to the discovery of this strongest titanium alloy ever made.

"This pushes the boundary of what we can do with titanium alloys," says Devaraj. "Now that we understand what's happening and why this alloy has such high strength, researchers believe they may be able to modify other alloys by intentionally creating microstructures that look like the ones in Ti185."

For example, if the nanostructures of alloys of aluminum—a less-expensive metal than titanium—can be seen and hierarchically arranged in a similar manner, it could help the auto industry build lighter, more fuel-efficient vehicles that emit less carbon dioxide.