Titanium (Ti) is the tenth most prevalent metal in the Earth's crust. Although minor quantities of titanium are present in practically every rock, it is seldom found in considerable concentrations. It is a lustrous grey metal with a high strength and low corrosion rate, resulting in its utilization in various applications.
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Titanium is a lightweight and strong structural metal with the highest strength-to-weight ratio of any metal. However, it is difficult and costly to process it while keeping an appropriate mix of strength and ductility — a metal's ability to be stretched out without breaking. As a result, Ti has been consigned to specialized applications in a limited number of sectors.
Modifications of Pure Titanium
Researchers from Berkeley Lab's Molecular Foundry have found a new and feasible future for Ti.
By modifying pure titanium at nanoscale under ultra-low temperatures to make extra-strong 'nanotwinned' titanium, its ductility was not compromised. This method is referred to as cryo-forging.
Metals' mechanical characteristics are influenced by their grains, which are small crystalline sections with repeating atomic rules that make up the material's inner structure. Metals are strengthened at grain boundaries where the pattern changes by preventing flaws known as dislocations from spreading across and degrading the material's structure. Consider the grains to be streets and the grain boundaries to be stoplights that prevent atomic "cars" from passing.
Simply shrinking the size of a metal's granules to create more limits by forging it – squeezing the substance at extreme temperatures or even room temperature by rolling or hammering it – is one way to reinforce it.
However, ductility is sometimes sacrificed as a result of this type of processing; the internal structure is split up, making it more prone to fracture. The increase in "stoplights" and smaller grain "streets" causes an atomic traffic jam, which fractures the material.
Nanotwins are a sort of atomic arrangement in which the crystal structure's microscopic boundaries line up perfectly, like mirror reflections of each other. The grain "streets" stoplights on the atomic highways transform into nanotwinned road bumps, making it possible for atoms to travel around without accumulating stress while preserving improved strength.
Creating Nanotwinned Materials
Nanotwinned materials are not a brand-new concept. However, creating them usually necessitates the use of specialist procedures, which can be costly.
These methods have only worked with a few metals, such as copper, and are typically only employed to generate thin films. Furthermore, thin-film features do not always translate to bulk counterparts.
Cryo-forging – changing the structure of the metal at ultra-low temperatures – was employed by the study team to make nanotwinned titanium.
To begin the process, a cube of very pure (more than 99.95 percent) titanium is inserted in liquid nitrogen at minus 321 degrees Fahrenheit. Compression is given to each axis of the cube while it is immersed. The material's microstructure begins to develop nanotwin boundaries under these conditions. The cube is next heated to 750°F to eliminate any structural faults that have developed between the twin boundaries.
The researchers placed the newly produced substance through a series of tests, performing electron microscopy to figure out what gave it its unique features. It was discovered that nanotwinned titanium had higher formability during these tests because it can form new nanotwin barriers as well as undo already formed boundaries, both of which aid deformation.
The material was exposed to temperatures as high as 1,112 degrees Fahrenheit, which is hot enough to melt lava, and discovered that it retained its structure and capabilities, indicating the material's flexibility.
Nanotwinned titanium can sustain more strain at super-low temperatures than regular titanium, which is the polar opposite of most metals at low temperatures - conventional materials become much more brittle. The number and size of these nanotwin structures can alter the metal's properties.
In the study, researchers found that nanotwinning increased titanium’s strength and enhanced its ductility by 30% at ambient temperature.
Ideal qualities of nanotwinned titanium were also preserved at relatively high temperatures, demonstrating that they would persist in the temperate San Francisco Bay Area climate and in the extreme cold of outer space and near the strong heat of the heat a jet engine.
Future Outlooks for Cryo-Forged Nanotwinned Titanium
Cryo-forging nanotwinned titanium is potentially cost-effective, scalable for commercial production, and provides a product that can be easily recycled.
Furthermore, although the nanotwinned method was conducted in titanium, it could be applied to other metals. The researchers intend to expand on the titanium-based approach and see if it can be applied to other metals in the future.
Continue reading: Titanium Dioxide Nanoparticles: Industrial Applications And Developments.
References and Further Reading
Zhao, S., Zhang, R., Yu, Q., Ell, J., Ritchie, R. and Minor, A., (2021) Cryoforged nanotwinned titanium with ultrahigh strength and ductility. Science, 373(6561), pp.1363-1368. Available at: https://doi.org/10.1126/science.abe7252
Nanowerk news. (2021) Nanotwinned titanium forges path to sustainable manufacturing. [online]. Available at: https://www.nanowerk.com/nanotechnology-news2/newsid=58959.php