Metallurgists Develop Low-Cost, Hierarchically Nanostructured Titanium Alloy with Superior Strength

Stronger than any commercial titanium alloy currently on the market, an improved titanium alloy gets it strength from the innovative way atoms are arranged to form a special nanostructure. Researchers from the Department of Energy's Pacific Northwest National Laboratory (PNNL) have succeeded in observing this arrangement and manipulating it to create the strongest ever titanium alloy with a lower process cost.

Using Atom Probe Tomography, researchers are able to create an 'atomic map' of the arrangement of various atoms in this titanium alloy. (credit: Pacific Northwest National Laboraotry)

The researchers have described their work in a paper reported in the Nature Communications journal. According to them the material is ideal to fabricate lighter vehicle components, and this advancement achieved by the researchers could help to develop other high strength alloys.

In their earlier study, the researchers had already demonstrated the superior mechanical properties of the titanium alloy created from their low-cost process. In the new study, they explored how to make even stronger titanium alloys. They were able to gain insights into the nanostructure of the alloy, using a unique atom probe imaging approach and powerful electron microscopes. With this knowledge, they developed the strongest ever titanium alloy.

Titanium is lightweight, at 45% of the weight of low carbon steel, and it does not possess superior strength. To obtain a stronger alloy it is generally blended with other materials. Five decades ago, metallurgists created an alloy named Ti185 by blending titanium with low-cost iron, aluminum and vanadium. However, superior strength was only observed for this alloy in places, and the alloy had a tendency to clump. Iron clustered in some areas, leading to beta fleck defects, making the alloy unsuitable for reliable commercial production.

In an earlier study carried out around six years ago, the PNNL researchers and its collaborators devised a low-cost process for the industrial-scale production of the alloy. They used titanium hydride powder in place of the molten titanium as the starting material, reducing the energy requirements significantly and shortening the processing time by one-half. This led to a low-cost process currently being used by Advance Materials Inc, who co-developed the method with PNNL metallurgist Curt Lavender. The company sells advanced materials such as the titanium hydride powder to industries, including aerospace.

More like a medieval blacksmith, the PNNL team knew that using the heat treatment process could produce a much stronger alloy. The elements can be rearranged in different ways at the atomic level by heating the material in a furnace at various temperatures, and subsequently cooling it in cold water in order to produce a stronger material.

Blacksmithing has now moved from an art form to be a more scientific approach. Even though the basic principles remain the same, metallurgists can refine the properties based on the requirements of a specific application. The PNNL researchers knew that if it is possible to observe the nanostructure of the alloys, they could improve the heat-treating process to obtain the tailored nanostructure and make the strongest ever titanium alloy.

We 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 percent stronger than any commercial titanium alloy currently on the market and that it has roughly double the strength of steel.

Arun Devaraj, Materials Scientist, PNNL

"This alloy is still more expensive than steel but with its strength-to-cost ratio, it becomes much more affordable with greater potential for lightweight automotive applications," added Vineet Joshi, who serves as a metallurgist at PNNL.

Using electron microscopy, Devaraj and the collaborators observed the microstructure of the alloy at a resolution of hundreds of the nanometer scale. The atom probe tomography system at the Environmental Molecular Sciences Laboratory (EMSL), a DOE Office of Science User Facility located at PNNL, was used to observe the arrangement of individual atoms in 3D.

The atom probe displaces the atoms individually and sends them to a detector. The arrival time is lower for lighter atoms, compared to heavier atoms. The identification of each atom is based on the time taken by the atom to reach the detector. The detector is able to identify the position of each atom, enabling the researchers to generate an atomic map of the sample to observe the position of each individual atom within it.

The researchers found that their optimized heat-treating process produced an alloy with an alpha phase, consisting of micron-sized and nanosized precipitate regions in a matrix referred as the beta phase. Each region has higher concentrations of certain elements.

The aluminum and titanium atoms liked to be inside the nano-sized alpha phase precipitates, whereas vanadium and iron preferred to move to the beta matrix phase.

Arun Devaraj, Materials Scientist, PNNL

The atomic arrangement was different in these two regions. A unique hierarchical nanostructure was obtained when the regions were heat treated at 1,450°F.

The treated material showed a 10% to 15% increase in tensile strength, which is an important achievement while considering the low cost of the process. The tensile strength of automotive steel is roughly 800-900 MPa, whereas the 10% to 15% increase attained by the PNNL researchers resulted in Ti185 with nearly double the tensile strength (1700 MPa) of the automotive steel, with a weight of nearly one-half.

The PNNL researchers gained insights into the exceptionally high strength obtained as a result of the hierarchical nanostructure, using a simple mathematical model developed in cooperation with Ankit Srivastava, an assistant professor at Texas A&M's material science and engineering department. Using the simulation results, microscopy results and the process, the PNNL team discovered the strongest ever titanium alloy.

This pushes the boundary of what we can do with titanium alloys. 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.

Arun Devaraj, Materials Scientist, PNNL

For example, if it is possible to observe the nanostructure of the alloys of inexpensive aluminum and hierarchically arrange it in a similar manner, the automakers can produce even lighter vehicles, which consume less fuel and release less carbon dioxide, a greenhouse gas contributing to global warming.

DOE's Vehicle Technologies Office — Propulsion Materials Program provided support to this research work utilizing capabilities developed under PNNL's internally funded Chemical Imaging Initiative.

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