Two experimental metal alloys have been found to resist cracking at 20 kelvin, or -253C, a temperature at which comparable metals normally become dangerously brittle.
The findings were published in Nature on 14 October by an international team led from the University of California, Berkeley, Lawrence Berkeley National Laboratory and the University of Oxford.
The alloys, made from niobium and titanium, belong to a family known as body-centred cubic metals, which also includes iron, tungsten and chromium. Their atoms sit in a crystal pattern that is usually vulnerable to brittle failure in extreme cold, where a small crack that a warmer metal would absorb by changing shape instead spreads.
Neither alloy showed a measurable shift towards brittle behaviour at the lowest temperature tested.
Metals of this kind normally change shape through the movement of tiny defects in their crystal structures, called dislocations, which let layers of atoms shift relative to one another and absorb stress without breaking. As the temperature falls, those dislocations become increasingly difficult to move.
The researchers observed something else happening instead: nanoscale twinning, in which small sections of the crystal structure rearrange into mirror-image patterns, alongside a localised change in the crystal structure itself. Where the newly formed regions meet the boundaries separating neighbouring crystals, they concentrate stress, and those concentrations generate additional defects, giving the metal new opportunities to deform rather than crack.
The team compared the effect to a road system finding alternative routes when its usual roads are blocked by ice.
The samples were cooled with gaseous helium and pulled apart from opposite ends on the Engin-X instrument at the ISIS Neutron and Muon Source in Oxfordshire, with a neutron beam directed through them during the test. Electron microscopes and atom-probe analysis were used afterwards, alongside computer modelling, to establish how the processes worked together.
"For decades, the ductile-to-brittle transition has been viewed as a fundamental limitation of body-centered cubic metals. What surprised us is that these alloys remain remarkably damage tolerant even at 20 kelvin," said Professor Dong Liu, of the University of Oxford's Department of Engineering Science.
The lead author, Dr David Cook, said the results suggest a change of emphasis. "Traditionally, the focus was on how readily existing dislocations can move through a metal, but now it appears that the ability to create new dislocations at interfaces may also be critical," he said, adding that the insights may allow engineers to "design underlying microstructural conditions that allow a material to create its own routes to remain ductile at extreme temperatures".
This is the point at which the vocabulary deserves attention. The word for the thing that is helping is "defect". The alloys do not survive the cold by having fewer flaws than other metals. They survive it by producing more of them, at speed, at the exact moment they are being pulled apart.
This newspaper accepts that the work is excellent engineering. It is nonetheless a reversal. A metal is now tough in proportion to how much damage it can do to itself while being damaged, and a sample that leaves the rig with more defects than it arrived with has not failed. It has coped.
Our analysts have followed the logic through to the manufacturing standards that will one day cover these alloys. A cryogenic component will pass inspection on the strength of faults it produced on the way in, and the Department of Unnecessary Research asks only that the certificate record them, as the researchers do, as routes.