A New Time For Titanium (3)
Nanotwinned titanium
(Continued)
Most recently, Minor and Robert Ritchie, professors of materials science and mechanical engineering, developed a pioneering bulk processing method to make pure titanium that is less expensive and yields a metal with greater tensile strength and ductility.
professors Daryl Chrzan, Mark Asta, and Andrew Minor standing by the TEAM I electron microscope
Materials science and engineering professors (from left) Daryl Chrzan, Mark Asta, and Andrew Minor with the TEAM I (Transmission Electron Aberration-corrected Microscope) project at Berkeley Lab's National Center for Electron Microscopy. (Photo by Adam Lau / Berkeley Engineering)
Aside from alloys, another way to strengthen structural metals is to tailor the size of crystals - also known as grain - that make up the metal by using heat and mechanical processing, such as rolling or pressing. By reducing the grain size to sub-micrometers or nanometers, researchers can introduce so-called nanotwinned structures, or defects in the metal caused by aligned crystal structures. The nanotwinned structures improve strength and lower the risk of fracture by acting as a barrier to planar slips. By tailoring the spacing and orientation of the nanotwinned structures, Minor says, the mechanical properties can be optimized even further. But traditional methods of doing so are neither trivial nor cheap.
Instead, Minor, Ritchie, and colleagues introduced multiple nanotwinned structures in pure titanium by means of a cryo-mechanical process. They used cube-shaped pieces of titanium that were pressed along three sides in liquid nitrogen. The gentle compression, Minor says, controls the density of nanotwinned structures that strengthen the metal while preserving its initial grain structure. Best of all, the process does not rely on intense heat and maybe a more sustainable way to make titanium for a much wider range of applications than today.
The mechanical properties of the cryo-forged material, specifically strength and ductility, hold at extremely high as well as cryogenic temperatures. Minor says the performance of the nanotwinned titanium makes it ideal for things like extremely hot jet engines as well as very cold operating environments that would suggest uses like retaining rings for superconducting magnets, structural parts of liquefied natural gas tanks, as well as materials to be exposed to deep ocean or deep space environments.
Asked if the new commercial-grade titanium fabrication process might be brought to scale one day soon, Minor says, why not? It's harder to do things like the Kroll process that's used today, where the material has to be isolated electrically and the entire process takes massive amounts of power. "And this cryo-forging, you know, we would just be putting things in a bath."
