Metals That Keep Their Nerve in the Cold
AMost people assume that cold makes materials harder, and in one sense this is true: cooling a metal generally makes it stronger. Strength, however, is not the same as toughness. As temperatures fall, many metals also become more brittle, which means that instead of bending under a heavy load, they snap. A component that performs well at room temperature may therefore behave very differently when it is exposed to intense cold. This change, known as the ductile-to-brittle transition, is a serious concern for engineers who design parts that must work in extremely cold conditions.
BThe problem is especially severe for metals with what is called a body-centred cubic (BCC) crystal structure, a group that includes iron, tungsten and chromium. Close to absolute zero, a tiny crack that such a metal would normally absorb by changing shape can instead spread rapidly through the material and cause it to fracture. According to Professor Dong Liu of the Department of Engineering Science at the University of Oxford, this transition has for decades been regarded as a fundamental limitation of BCC metals.
CTo understand why, it helps to know how metals normally deform. Inside a metal, the atoms are arranged in regular layers, but the arrangement is never perfect. It contains defects known as dislocations, which allow layers of atoms to slide past one another when the metal is put under stress. It is this sliding that lets a metal bend or stretch rather than break. At low temperatures, however, dislocations in BCC metals become harder to move. With fewer ways to relieve the stress that builds up around the tip of a crack, the metal is more likely to fail suddenly.
DA new study published in the journal Nature suggests that this limitation is not as fixed as once believed. An international team, with Liu as the corresponding author and Dr David Cook as lead author, tested two experimental alloys based on niobium and titanium. Cook carried out the research while working at the University of California, Berkeley, and has since moved to the University of Cambridge. The researchers found that both alloys resisted cracking even at 20 kelvin, or minus 253 degrees Celsius, which is only 20 degrees above absolute zero.
EThe explanation lies in an unexpected way of dealing with stress. When the alloys were strained in extreme cold, tiny regions of their crystal structure rearranged themselves into mirror-image patterns, a process called nanoscale twinning. This took place together with a local change in the crystal structure itself. Where these newly formed regions met the boundaries between neighbouring crystals, known as grains, they concentrated stress, and this in turn produced fresh defects. These new defects gave the metal extra ways to deform, so that it changed shape instead of cracking. The team compares the effect to traffic being redirected when the main roads are covered in ice: when the usual route for deformation is blocked, the material creates alternative routes of its own.
FReaching these conclusions required a wide range of techniques. The samples were cooled using gaseous helium and then pulled from both ends in a specialised testing machine, which recorded how much force they could withstand and how far they stretched before breaking. During the tests, beams of neutrons were sent through the materials using the Engin-X instrument at the ISIS Neutron and Muon Source in Oxfordshire, allowing the team to observe what was happening inside the metal as it deformed. Afterwards, electron microscopes were used to examine the samples, including the areas close to cracks. A technique called atom-probe analysis was also used to map how individual elements were distributed in the alloys. Finally, computer simulations helped the researchers understand how the different processes worked together.
GIn the authors' view, the findings challenge some long-held assumptions. Scientists have traditionally concentrated on how easily existing defects can move through a metal. The new results suggest that the ability to create new defects at interfaces may be just as important in deciding whether a metal turns brittle. "Our results suggest we should change how we think about what governs the transition to brittleness for these metals," says Cook.
HThe practical consequences could be considerable. Liu says that the alloys remain remarkably damage tolerant even at 20 kelvin, and that materials of this kind could be useful for components that face both extremely hot and extremely cold temperatures. The researchers point to possible uses in energy systems, transport and aerospace engineering. For the moment, though, the niobium-titanium alloys remain experimental materials rather than commercial products. Looking further ahead, Cook suggests that engineers might one day design materials whose internal structure allows them to create their own paths to remain ductile under extreme conditions.