The sliding of a perfect twin boundary, with mirrored crystal lattices on both sides, was long considered to be impossible at room temperature in metals. Researchers have now shown that it is possible when a nanoscale twin boundary within a copper nanopillar is compressed along certain orientations, through in-situ transmission electron microscopy (left) and molecular dynamics simulation (right). Image: Zhang-Jie Wang, Qing-Jie Li, Ming Dao, Evan Ma, Subra Suresh, Zhi-Wei Shan.Most metals and semiconductors, from the steel in a knife blade to the silicon in a solar panel, are made up of many tiny crystalline grains. The way these grains meet at their edges can have a major impact on the material's properties, including its mechanical strength, electrical conductivity, thermal properties, flexibility and so on.
When the boundaries between the grains are of a particular type, called a coherent twin boundary (CTB), this adds useful properties to certain materials, especially at the nanoscale. It increases their strength, making the material much stronger while preserving its ability to be deformed, unlike most other processes that add strength. Now, researchers have discovered a new deformation mechanism with these twin crystal boundaries, which could help engineers figure out how to use CTBs to tune the properties of some materials more precisely.
As the researchers report in a paper in Nature Communications, it turns out that, contrary to expectations, a material's crystal grains can sometimes slide along CTBs. The researchers comprise: Ming Dao, a principal research scientist in the Department of Materials Science and Engineering at Massachusetts Institute of Technology (MIT); Subra Suresh, professor of engineering and president-designate of Nanyang Technological University in Singapore; Ju Li, professor in MIT's Department of Nuclear Science and Engineering; and seven others at MIT and elsewhere.
While each crystal grain is made up of an orderly three-dimensional array of atoms in a lattice structure, CTBs are places where, on the two sides of a boundary, the lattice forms a mirror-image of the structure on the other side. Every atom on either side of the coherent twin boundary is exactly matched by an atom in a mirror-symmetrical location on the other side. Much research in recent years has shown that lattices that incorporate nanoscale CTBs can have much greater strength than the same material with random grain boundaries, without losing another useful property called ductility, which describes a material's ability to be stretched.
Some previous research suggested that these twin crystal boundaries are incapable of sliding due to the limited number of defects. Indeed, no experimental observations of such sliding have been reported before at room temperature. Now, a combination of theoretical analysis and experimental work has shown that in fact, under certain kinds of loads, these grains can slide along the boundary. Understanding this property will be important for developing ways to engineer material properties to optimize them for specific applications, Dao says.
"A lot of high-strength nanocrystalline materials [with grains sizes measuring less than 100nm] have low ductility and fatigue properties, and failure grows quite quickly with little stretching," he says. Conversely, in metals that incorporate CTBs, that "enhances the strength and preserves the good ductility".
Understanding how these materials behave when subjected to various mechanical stresses is important for being able to harness them for structural uses. For one thing, it means that the way the material deforms is quite uneven: distortions in the direction of the planes of the CTBs can happen much more readily than in other directions.
The researchers conducted their experiment with copper, but the results should apply to some other metals with similar crystal structures, such as gold, silver and platinum. These materials are widely used in electronic devices, Dao says. "If you design these materials" with structures in the size range explored in this work, which involves features smaller than a few hundred nanometers across, "you need to be aware of these kinds of deformation modes."
The sliding, once understood, can be used to gain significant advantages. For example, researchers could design extremely strong nanostructures based on the known orientation dependence. Alternatively, by knowing the type and direction of force that's required to initiate the sliding, it might be possible to design a device that could be activated, such as an alarm, in response to a specific level of stress.
"This study confirmed CTB sliding, which was previously considered impossible, and its particular driving conditions," says Zhiwei Shan, a senior co-author and dean of the School of Materials Science and Engineering at Xi'an Jiao Tong University in China. "Many things could become possible when previously unknown activation or enabling conditions are discovered."
"This work has identified through both systematic experiments and analysis the occurrence of an important mechanical characteristic which is found only in certain special types of interfaces and at the nanoscale. Given that this phenomenon can potentially be applicable to a broad range of crystalline materials, one can envision new materials design approaches involving nanostructures to optimize a variety of mechanical and functional characteristics," says Suresh.
"This discovery could fundamentally change our understanding of plastic deformation in nanotwinned metals and should be of broad interest to the material research community," comments Huajian Gao, professor of engineering at Brown University. "CTBs are key to engineering novel nanotwinned materials with superior mechanical and physical properties such as strength, ductility, toughness, electrical conductivity and thermal stability. This paper significantly advances our knowledge in this field by revealing large-scale sliding of CTBs."
This story is adapted from material from MIT, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.