Nano-twinned materials are not new. However, making them typically requires specialized techniques that can be costly. These techniques work for some metals, such as copper, and are typically only used to make thin films. Thin-film properties are rarely found in bulk materials.
The mechanical properties of metals depend in part on their grains, small areas of crystals with repeating atomic patterns that form the material’s internal structure. Boundaries between grains, where the pattern changes, strengthen the metal by preventing defects (dislocations) from moving across and weakening the material’s structure.
One way to strengthen a metal is to forge it and shrink the size of its grains to create more boundaries. This forging compresses the metal at high temperatures or uses rolling and hammering at room temperature to compress it. However, forging to increase strength often comes at the expense of ductility; forging breaks up the internal structure, making it brittle and prone to fracturing.
To create nano-twinned titanium, the researchers used cryo-forging to manipulating the metal’s structure at ultra-low temperatures. They started with a cube of 99.95% pure titanium which they submerged in liquid nitrogen at −321°F. Force applied to all sides of the cube compresses it and nanotwin grain boundaries begin to form in the titanium. The cube is then heated to 750°F to remove any structural defects that may have formed between the grain boundaries.
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