“This study is the first time someone has produced a pure nanotwinned structure in bulk material,” says Andrew Minor, the study’s project lead and director of the National Center for Electron at the Molecular Foundry, a nanoscience user facility at Berkeley Lab. “With nanotwinned titanium, we no longer have to choose between strength and ductility but instead can achieve both.”
The mechanical properties of metals depend in part on their grains – the individual crystalline areas of repeating atomic patterns that form the material’s internal structure. Shrinking the size of the grains increase a material’s strength but at the expense of other qualities like ductility.
“The strength of a material is normally correlated with the size of the interior grains – the smaller the better,” says Minor, “but high strength and ductility are generally mutually exclusive properties.”
Nanotwins are an atomic arrangement where the boundaries in the crystal structure line up symmetrically like mirror images. To create nanotwinned titanium, the research team used cryo-forging.
The technique starts with a cube of 99.95%+ pure titanium placed into liquid nitrogen at minus 321° F. While the cube is submerged, compression is applied to each axis of the cube. Under these conditions, the structure of the material begins to form nanotwin boundaries. The cube is later heated to 750° F to remove any structural defects that formed in between the twin boundaries.
The researchers found that nanotwinning doubled the metal’s strength and increased its ductility by 30% at room temperature. At super-low temperatures, the improvement was even more dramatic – the nanotwinned titanium was able to double in length before fracturing.