New Property Found in Unusual Crystalline Materials

Materials with a special kind of boundary between crystal grains can deform in unexpected ways.


Most of the metals and semiconductors are composed of many tiny crystalline grains. These grains meet at their edges and can significantly impact the solid’s properties.

When boundaries between the grains are called coherent twin boundaries (CTB), this adds valuable properties to specific materials, particularly at the nanoscale. These crystal boundaries tend to increase the strength of the crystalline material while preserving its ability to be deformed, unlike most other processes that add strength.

Now, MIT scientists have discovered a mechanism to deform these twin crystal boundaries. Despite desires, surprisingly, a material’s precious stone grains can sometimes slide along these CTBs.

Each crystal in the boundary comprises a 3D array of atoms in a lattice structure. All atoms are exactly similar to those in a mirror-symmetrical location on the other side. Many scientists suggest that its lattice structure incorporates nanoscale CTBs to have much greater strength than the same crystalline materials with random grain boundaries without losing another useful property called ductility.

The research has shown that these grains can slide along the boundary under certain loads. Understanding its properties could pave the way to better engineering material properties to optimize them for specific applications.

Experimental observation of coherent twin boundary (CTB) sliding in a nanopillar subjected to compression. (Courtesy of the researchers)

Ming Dao, a principal research scientist at MIT, said, “A lot of high-strength nanocrystalline materials [with grains sizes measured in less than 100 nanometers] have low ductility and fatigue properties, and failure grows quite quickly with a little stretching. Conversely, in the 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 to be 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.”

Scientists mainly included metals like copper, gold, silver, and platinum during experiments.

Dao explained, “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.”

Molecular dynamics simulation showing coherent twin boundary (CTB) sliding in a nanopillar under compression. (Courtesy of the researchers)

This CTB sliding can offer several benefits. It could design extremely strong nanostructures based on the known orientation dependence.

Zhiwei Shan, a senior co-author, said, “This study confirmed CTB sliding, which was previously considered impossible, and its particular driving conditions. Many things could become possible when previously unknown activation or enabling conditions are discovered.”

Huajian Gao, the Walter H. Annenberg Professor of Engineering at Brown University, said, “This discovery could fundamentally change our understanding of plastic deformation in nanotwinned metals and should be of broad interest to the material research community.”

“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.”

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