PNNL team develops higher-strength, lower-cost titanium alloy aimed at improving vehicle fuel economy and reducing CO2 emissions
An improved titanium alloy—stronger than any commercial titanium alloy currently on the market—gets its strength from the novel way atoms are arranged to form a special nanostructure. For the first time, a team led by researchers at Pacific Northwest National Laboratory (PNNL) have been able to see this alignment and then manipulate it to make the strongest titanium alloy (hierarchical nanostructured Ti-185, or HNS Ti-185) yet developed. On top of the gains in strength, the new alloy benefits from a lower cost process.
In an open access paper published in the journal Nature Communications, the researchers note that that material is an excellent candidate for producing lighter vehicle parts, and that this newfound understanding may lead to creation of other high strength alloys.
The primary approach to lightweighting is through increased use of structural materials with high specific strength (strength to weight ratio). In recent years, titanium alloys, more specifically β-titanium alloys, have been widely explored as candidate materials owing to their attractive specific strength, toughness and corrosion resistance. For example, the Boeing 787 Dreamliner, which is one of the most fuel efficient airplanes in its class, is utilizing about 15% titanium alloys, the largest percentage of titanium used in a passenger airplane [to] date. However, wide scale adoption of β-titanium alloys in transportation applications has been limited due to its high cost.
The cost of β-titanium alloys can be lowered by replacing the expensive β stabilizers such as Mo, Cr and V (in full or in part) by low cost Fe such as the case in Ti–1Al–8V–5Fe (Ti-185) alloy introduced in 1960s. Ti-185 alloy demonstrated high tensile and shear strength proving to be an excellent candidate for fastener applications. Despite the advantages, the production of Ti-185 alloy in bulk by conventional ingot processing remained a challenge since addition of Fe in excess of 2.5 wt% in titanium alloys lead to segregation of Fe. The segregation of Fe results in the formation of inhomogeneous β structures also known as β flecks, which is detrimental to the mechanical performance of the alloy.—Devaraj et al.
The researchers had earlier demonstrated a novel route to develop engineering components of β-titanium alloys using a low-cost TiH2 powder feedstock. The resulting mechanical properties of the Ti-185 alloy developed through this low-cost process were at par with ones developed via the conventional route.
By using the titanium hydride powder, they also reduced the processing time by half and they drastically reduced the energy requirements. A company called Advance Materials (ADMA) co-developed the process with PNNL metallurgist Curt Lavender and sells the titanium hydride powder and other advanced materials to the aerospace industry and others.
In this latest work, they enabled the low-cost powder processed Ti-185 alloy to achieve a strength level higher than all current commercial titanium alloys by achieving a hierarchical nanostructure.
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
|Using Atom Probe Tomography, researchers are able to create an “atomic map” of the arrangement of various atoms in this titanium alloy. Source: PNNL. Click to enlarge.|
The team used electron microscopy to zoom in to the alloy at the hundreds of nanometers scale. Then they zoomed in even further to see how the individual atoms are arranged in 3-D using an atom probe tomography system at EMSL, the Environmental Molecular Sciences Laboratory, a DOE Office of Science User Facility located at PNNL.
The atom probe dislodges just 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 is identified depending on the time each atom takes to reach the detector and each atom’s position is identified by the detector.
Scientists thus are able to construct an atomic map of the sample to see where each individual atom is located within the sample.
By using such extensive microscopy methods, researchers discovered that by the optimized heat treating process, they created micron-sized and nanosized precipitate regions—known as the alpha phase, in a matrix called the beta phase—each with high 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
The atoms are arranged differently in these two areas. Treating the regions at higher temperature of a 1,450 degrees Fahrenheit achieved a unique hierarchical nano structure.
When the strength was measured by pulling or applying tension and stretching it until it failed, the treated material achieved a 10-15 percent increase in strength which is significant, especially considering the low cost of the production process.
Steel used to produce vehicles has a tensile strength of 800-900 megapascals, whereas the 10-15 percent increase achieved at PNNL puts Ti-185 at nearly 1,700 megapascals, or roughly double the strength of automotive steel while being almost half as light.
The resulting 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, said Vineet Joshi, a metallurgist at PNNL.
The team collaborated with Ankit Srivastava, an assistant professor at 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. 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 Ti-185.—Arun Devaraj
For example, aluminum is a less expensive metal and if the nanostructure of aluminum alloys can be seen and hierarchically arranged in a similar manner, that would also help the auto industry build lighter vehicles that use less fuel and put out less carbon dioxide that contributes to climate warming.
DOE’s Vehicle Technologies Office — Propulsion Materials Program supported this research using capabilities developed under PNNL’s internally funded Chemical Imaging Initiative.
Arun Devaraj, Vineet V. Joshi, Ankit Srivastava, Sandeep Manandhar, Vladimir Moxson, Volodymyr A Duz, Curt Lavender (2016) “A Low Cost Hierarchical Nanostructured β-Titanium Alloy for Lightweighting Applications,” Nature Communications, April 1, 2016, DOI: 10.1038/NCOMMS11176.