Researchers from Berkeley, Oxford and partner institutions report in Nature that two niobium-titanium alloys resisted cracking at 20 kelvin by activating nanoscale twinning, a finding that could guide materials for extreme cold.
Researchers from the University of California, Berkeley, Lawrence Berkeley National Laboratory and the University of Oxford report that two experimental metal alloys stayed remarkably damage tolerant at 20 kelvin, or -253°C (-424°F), a temperature at which comparable metals normally turn brittle. The study, published on 14 October in Nature, concerns body-centered cubic metals, a family that includes iron, tungsten and chromium and is known for a ductile-to-brittle transition in extreme cold. According to the team, neither of the two alloys, both made from niobium and titanium, showed a measurable shift toward brittle behavior at the lowest temperature tested.
The researchers say the explanation lies in a different way of coping with stress. Ordinarily these metals deform through the movement of crystal defects called dislocations, but dislocations become harder to move as temperatures fall, so a small crack can spread and cause fracture. In the new alloys the team observed nanoscale twinning, in which small regions of the crystal rearrange into mirror-image patterns alongside a localized change in crystal structure. Where these regions meet the boundaries between neighboring crystals they concentrate stress and generate fresh dislocations, giving the metal new ways to deform instead of cracking. The experiments used helium cooling and a neutron beam at the ISIS Neutron and Muon Source in Oxfordshire while samples were stretched, followed by electron microscopy, atom-probe analysis and computer modeling.
The authors argue that the result changes how brittleness in these metals should be understood: the ability to create new dislocations at interfaces may matter as much as how easily existing ones move. They suggest it offers a route to designing structural metals for cryogenic conditions, with potential relevance to energy systems, transportation and aerospace. The findings come from laboratory tests on two experimental alloys, and the account here relies on a university press release distributed through ScienceDaily rather than on an independent reading of the paper, so it is not yet clear how readily the approach can be turned into commercial materials.
