A group of researchers from the University of Pittsburgh, PA, Drexel University, Philadelphia, PA, and the Georgia Institute of Technology, Atlanta, say that to understand how nanomaterials behave, it’s necessary to understand the atomic-scale deformation mechanisms that determine their structure and, therefore, their strength and function. So they have engineered a new way to study these mechanisms and revealed an interesting phenomenon in tungsten. The group is the first to observe atomic-level deformation twinning in body-centered cubic (BCC) tungsten nanocrystals.

Fig. 1 – The computer model (left) and experimental image reveal atomic-level deformation twinning in a tungsten nanowire under axial compression. The lattice of the deformationinduced twin band (between yellow lines) is a mirror image of that of the parent crystal. (Credit: Ting Zhu)
The team used a high-resolution transmission electron microscope (TEM) and sophisticated computer modeling to make the observation.

Deformation twinning, they explain, is a type of deformation that, in conjunction with dislocation slip, allows materials to permanently deform without breaking. In the process of twinning, the crystal reorients, which creates a region in the crystal that is a mirror image of the original crystal. Twinning has been observed in large-scale BCC metals and alloys during deformation. However, whether twinning occurs in BCC nanomaterials or not remained unknown.

“To gain a deep understanding of deformation in BCC nanomaterials, we combined atomic-scale imaging and simulations to show that twinning activities dominated for most loading conditions, due to the lack of other shear deformation mechanisms in nanoscale BCC lattices.” said Scott Mao, a professor in the Swanson School of Engineering at the University of Pittsburgh.

The team chose tungsten as a typical BCC crystal. Using a TEM, the team observed atomic-scale twinning. Previously, this type of study was not possible, due to difficulties of making BCC samples less than 100 nanometers in size, as required by TEM imaging. Jiangwei Wang, a graduate student at University of Pittsburgh, and Mao developed a way of making the BCC tungsten nanowires. Under a TEM, Wang welded together two small pieces of individual nanoscale tungsten crystals to create a wire about 20 nanometers in diameter. This wire was durable enough to stretch and compress while Wang observed the twinning phenomenon in real time using a high-resolution TEM. (See Figure 1)

To better understand the phenomenon observed by Mao and Wang’s team at the University of Pittsburgh, Christopher Weinberger, an assistant professor in Drexel’s College of Engineering, developed computer models that show the mechanical behavior of the tungsten nanostructure at the atomic level. His modeling allowed the team to see the physical factors at play during twinning. This information will help future researchers theorize why it occurs in nanoscale tungsten and plot a course for examining this behavior in other BCC materials.