A New Time For Titanium (2)
Design strategies that interrupt the oxygen-atom shuffling process or promote nanostructures to stop planar slips from piling up could lead to better alloys. These alloys would have applications, especially in the automotive and aerospace industries, Minor says.
Cryo-forging nanotwinned titanium
Professor Andrew Minor pours liquid nitrogen on a titanium sample, demonstrating the cryo-forging process used to create nanotwinned titanium in his lab. (Photo by Adam Lau / Berkeley Engineering)
To address these and other issues, the team relies on a mix of computer modeling, transmission electron microscopy (TEM), and other imaging modalities, and experiments.
"One of the things that's been nice about this project is that sometimes the computationalists and theorists are out a little bit ahead, and other times it's the experimentalists," Asta says. "We meet frequently and talk about our findings and our new ideas."
The team's study of titanium's oxygen sensitivity, for example, led to a study of titanium alloyed with aluminum and oxygen. They found that oxygen embrittlement could be eliminated by adding small amounts of aluminum, especially at cryogenic temperatures, which are below -150 degrees Celsius.
With just the right amounts of aluminum and oxygen, the team says, a new ordering of the titanium crystal structure prevented a shuffling of oxygen atoms that would lead to a damaging pileup of dislocations and ultimately fractures. What's more, because the introduction of aluminum reduced the oxygen sensitivity of titanium overall, processing costs to create a usable metal would also be reduced.
In yet another study, the team looked at research going back to the 1960s showing that many metals and alloys display dramatic increases in ductility when subjected to periodic electrical pulses during deformation of the metal. However, the underlying mechanisms of why this so-called electroplasticity might be true are not clear.
"Electroplasticity can lead to reduced costs for metallurgical processing since it takes less energy to form metal with electrical pulses than heating the entire metal up to a high temperature to achieve the same formability," Minor says. "Interestingly, this effect of electroplasticity is universal in that it has been shown to work for essentially every metal, not just titanium."
The team performed tensile tests of the metal under three different conditions: room temperature with no electric current, with a periodic electrical pulse of 100 milliseconds duration, and with a constant current. Because applying electric current heats the metal, the team was worried about distinguishing the effects caused solely by electricity from those caused by heat.
Their results showed that, despite using a smaller periodic pulse than previous studies, the pulsed-current method improved the tensile elongation of the titanium alloy as well as its maximum strength. They note that this effect was specific only to the pulsed-current experiment.
With the aid of TEM to see changes in the metal's crystal structure, their results suggest that the pulsed-current treatment suppresses planar slip dislocations. The researchers found that the electrical pulse hardens the material and frustrates the development of planar slip by maintaining a diffuse, 3D dislocation pattern that ultimately delivers high strength and ductility.
(To be continued)
