A team of researchers led by the Massachusetts Institute of Technology (MIT) (Cambridge, Massachusetts) say they have found a way to reduce the effects of metal fatigue by incorporating a laminated nanostructure into steel.
The layered structuring gives the steel a bone-like resilience, MIT says, allowing it to deform without the spread of microcracks that can lead to fatigue failure.
The findings are described in a paper in the journal Science by C. Cem Tasan, a metallurgy professor at MIT; Meimei Wang, an MIT post-doctoral researcher; and six others at Kyushu University (Fukuoka, Japan) and the Max Planck Institute (Dusseldorf, Germany).
“Loads on structural components tend to be cyclic,” Tasan explains. For example, an airplane goes through repeated pressurization changes during every flight, and components of many devices repeatedly expand and contract due to heating and cooling cycles. While such effects typically are far below the kinds of loads that would cause metals to change shape permanently or fail immediately, they can cause the formation of microcracks—which over repeated cycles of stress spread a bit further and wider, ultimately creating enough of a weak area that the whole piece can suddenly fracture.
“A majority of unexpected failures [of structural metal parts] are due to fatigue,” Tasan says. For this reason, large safety factors are used in component design, leading to increased costs during production and component life.
Tasan and his team say they were inspired by the way nature addresses the same kind of problem, by making bones lightweight but very resistant to crack propagation. A major factor in bone’s fracture resistance is its hierarchical mechanical structure, so the team investigated microstructures that would mimic this in a metal alloy.
The question was, he says, “Can we design a material with a microstructure that makes it most difficult for cracks to propagate, even if they nucleate?”
Bone provided a clue of to how to do that, Tasan says, through its hierarchical microstructure—that is, the way its internal structures have different patterns of voids and connections at many different length scales—with a lattice-like internal structure that combines strength with light weight.
From there, the team developed a kind of steel that has three key characteristics, which combine to limit the spread of any cracks that form. In addition to having a layered structure, which tends to keep cracks from spreading beyond the layers where they start, the material has microstructural phases with different degrees of hardness, which complement each other.
“Every time [a crack] wants to propagate further, it needs to follow an energy-intensive path,” Tasan explains, noting that the result is a significant reduction in such spreading.
Finally, the material also includes metastable composition, which incorporates tiny areas that are poised between different stable states—with some more flexible than others. Their phase transitions can help absorb the energy of spreading cracks and even lead the cracks to close back up, Tasan says.
To further understand the roles of these three characteristics, the team compared different steels that each had a combination of two out of the three key properties. None worked as well as the three-way combination.
“This showed us that our modification has better fatigue resistance than any of these,” Tasan says.
The testing of such materials under realistic conditions is difficult to do, Tasan explains, partly because of the extreme sensitivity of the materials to surface defects.
“If you scratch it, it’s going to fail much faster,” Tasan says. As a result, meticulous preparation and inspection processes for the test samples are essential.
This finding is just the first step in the group’s research, Tasan says, adding that it remains to be seen what would be needed to scale up the material to quantities that could be commercialized—as well as what applications would benefit most.
“Economics always comes into it,” he says. “I’m a metallurgist, and this is a new material that has interesting properties. Large industries such as automotive or aerospace are very careful about making changes in materials, as it brings extra effort and costs.”
But he insists there are likely to be several uses where the material would be a significant advantage.
“For critical applications, [the benefits] are so critical that change is worth the extra trouble,” Tasan says. “This is an alloy that would be more expensive than a basic low-carbon steel, but the property benefits have been shown to be quite exceptional, and it is with much lower amounts of alloying metals than other proposed materials.”
The research was supported by the European Research Council (Brussels, Belgium) and MIT’s Department of Materials Science and Engineering. The team included Motomichi Koyama, Zhao Zhang, Kaneaki Tsuzaki, and Hiroshi Noguchi of Kyushu University and Dirk Ponge and Dierk Raabe of the Max Planck Institute.
Source: MIT, news.mit.edu.