Two experimental alloys resisted cracking at −424°F by forming new crystal defects, offering a possible route to tougher metals for extreme cold.

Most metals become stronger as they cool, but some also become more vulnerable to breaking. Iron, tungsten, and chromium belong to a class called body-centered cubic metals, whose atomic arrangement can make them increasingly brittle as temperatures fall. A crack that the metal might otherwise accommodate by bending or stretching can instead cause a fracture.

That ability to deform depends heavily on the movement of dislocations, line-like defects in the metal’s crystal structure. At low temperatures, these defects become harder to move, limiting how much the material can deform to relieve the forces building up around a crack.

Alloys resist cracking at −424°F

Two experimental niobium-titanium-based alloys avoided this problem even at 20 kelvin, about −253°C (−424°F), by changing how they deform. An international team led by the University of California, Berkeley, Lawrence Berkeley National Laboratory, and the University of Oxford found that the alloys remained remarkably resistant to cracking, with no measurable transition to brittle behavior at the lowest temperature tested. Their findings, published in Nature, reveal how the alloys create new opportunities for deformation when the usual mechanism becomes difficult.

To test the alloys under these conditions, the researchers used the Engin-X instrument at the ISIS Neutron and Muon Source in Oxfordshire. They cooled samples with gaseous helium to around −253°C, then exposed them to a neutron beam while a testing machine pulled them from both ends. The tests precisely measured how much force the samples could withstand and how far they stretched before breaking.

Crystal defects that keep alloys deforming

Electron microscopes allowed the team to examine structural changes around cracks and inside the tested metal. The researchers observed nanoscale twinning, a process in which tiny regions of the crystal reorganize into mirror-image orientations, coupled with a local transformation of the crystal structure. They also used atom-probe analysis to map the distribution of individual elements and computer modeling to investigate how the different mechanisms worked together to prevent brittle failure.

When the newly formed structures reached boundaries between neighboring crystals, they created concentrated points of stress that triggered new dislocations. These fresh defects allowed the material to keep deforming and absorb the forces around a crack. Even as existing dislocations became harder to move, the alloys could generate new ones at these boundaries.

Lead author Dr. David Cook, who was at the University of California, Berkeley, during the study and is now at the University of Cambridge, said: “Our results suggest we should change how we think about what governs the transition to brittleness for these metals. 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. Using these insights, we may be able to design underlying microstructural conditions that allow a material to create its own routes to remain ductile at extreme temperatures.”

Potential applications include components exposed to exceptionally cold conditions in the energy, transportation, and aerospace sectors.

Oxford corresponding author Professor Dong Liu, of the University of Oxford’s Department of Engineering Science, said: “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. By uncovering how twinning and phase transformation work together to generate plasticity before conventional dislocation motion, we identified a new pathway for designing structural body-centered cubic metals that can operate in extreme cryogenic environments. This class of materials will be extremely useful for components experiencing extreme hot and cold temperatures in one application.”