Pitt | Swanson Engineering
For years, materials scientists have been perplexed by the question of why alloys don’t meet their full potential strength. A deep look at atomic patterns finds the answer.

Reprinted with the permission of Binghamton University

By Rachael Flores
November 27, 2017

Sometimes calculations don’t match reality. That’s the problem faced by materials scientists for years when trying to determine the strength of alloys, resolving the disconnect between the theoretical strength of alloys and how strong they actually are. So, what has been missing?

New research has found the answer with a collaboration between researchers at Binghamton University, the University of Pittsburgh, the University of Michigan and Brookhaven National Laboratory. The U.S. Department of Energy’s Office of Science also supported the work.

Researchers used advanced technology to look at alloys on an atomic level in order to understand what has been affecting the strength and other properties. 

Binghamton University materials science and engineering professor Guangwen Zhou was one of the scientists working on the project. The Pitt team included Jörg Wiezorek and Guofeng Wang from the Department of Mechanical Engineering and Materials Science, and Judith Yang in Chemical and Petroleum Engineering.

Zhou and his team used a Transmission Electron Microscope (TEM) for the study, a tool that has been around since 1935 and has evolved dramatically in recent years with the incorporation of aberration correction techniques and environmental capabilities. It’s powerful enough to look deep into the structure of atoms.

“We were able to observe that the changes in alloys from surface segregation were accompanied by the formation of dislocations in the subsurface,” explained Zhou. “Atoms typically make patterns, but when there’s a dislocation, that means the pattern has been interrupted.”

Dislocations are what make the alloys weaker than the theoretical calculations predict and Zhou’s research found that surface segregation is what leads to those dislocations.

“By understanding how the dislocation forms, we can start to control it,” said Zhou.

This could lead to strengthening a variety of alloys that are valued specifically for their strength and light weight.

According to Zhou, this groundbreaking research provides insight into what needs to change in order to strengthen the variety of alloys used in airplanes, jewelry, medical tools, bridges, cookware and other common objects.

The study, “Dislocation nucleation facilitated by atomic segregation,” was recently published in Nature Materials.



Jörg Wiezorek, professor of mechanical engineering and materials science
Dr. Wiezorek was involved in the inception stage, the drafting, and writing of the manuscript. He provided continuum elasticity-based dislocation theory calculations. His contributions helped evaluate the energetic feasibility of the apparently observed dislocation nucleation events, which were initiated by solute atom segregation and surface phase formation-related local crystal lattice strain build-up. The calculations also facilitated distinction between the numerous possible scenarios for their mutual strain field interaction to identify the most likely ones that control the dislocation motion after formation. Dr. Wiezorek also contributed to the Burgers vector and dislocation core character determination and interpretation of the atomic resolution transmission electron microscopy images and movies. 

Guofeng Wang, associate professor of mechanical engineering and materials science
Dr. Wang’s group participated in this project right from the beginning when the collaborators at SUNY Binghamton observed some interesting phenomena in CuAu thin films but not in pure Cu thin films. The researchers hypothesized that the Au surface segregation process is responsible for the observed dislocation nucleation. To examine this hypothesis and complement the experimental study, Yinkai Lei and Zhenyu Liu—two PhD students from Dr. Wang’s group who have since graduated—performed extensive atomistic simulations to predict the dislocation core structure, the slip plane of the 1/2[110] dislocation, and the equilibrium structure of the Au segregated CuAu alloy surfaces. The theoretical predictions agreed excellently with the HRTEM images. Hence, these simulations provide much insight into and good explanation of the observed dislocation nucleation process at an atomic scale.

Judith C. Yang, professor chemical and petroleum engineering
Dr. Yang’s group hosted Lianfeng Zou, a PhD student from Dr. Guangwen Zhou’s group at the University of Binghamton, for a few years at the University of Pittsburgh, where he learned transmission electron microscopy (TEM), including in situ environmental TEM, as well as creating the thin films of CuAu alloy. Lianfeng Zou used in situ environmental TEM to visualize the unusual dislocation nucleation and migration of the copper-gold alloy at the atomic scale in real time. Dr. Yang also facilitated the interactions with Drs. Wiezorek and Wang at Pitt. Before becoming a professor at SUNY Binghamton, Dr. Zhou was the first PhD in Dr. Yang’s group.

Matt Cichowicz, Communications Writer, 2/13/2018

Contact: Paul Kovach