New Property Found in Unusual Crystalline Materials

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

New Property Found in Unusual Crystalline Materials
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. Here, authors show 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 of the metals and semiconductors are composed of many tiny crystalline grains. These grains meet at their edges and can have a major impact on the solid’s properties.

When boundaries between the grains called coherent twin boundary (CTB), this adds valuable properties to specific materials, particularly at the nanoscale. These crystals boundaries tend to increase the crystalline materials strength 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. In spite of desires, surprisingly a material’s precious stone grains can some of the time slide along these CTBs.

Each crystal in the boundary composed of a 3D array of atoms in a lattice structure. All atoms are exactly similar to the atoms in a mirror-symmetrical location on the other side. Many scientists suggest that its lattice structure incorporate 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 under certain kinds of loads these grains can slide along the boundary. Understanding its properties could pave the way to better engineer material properties to optimize them for specific applications.

New Property Found in Unusual Crystalline Materials
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, sand 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 in order 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.”

During experiments, scientists mainly included metals like copper, gold, silver, and platinum.

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.”

New Property Found in Unusual Crystalline Materials
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.”