Imagine you’re attending a concert with your best friend from high school. The venue is packed and the crowd is roaring, but you both manage to cram yourselves into a small spot near the center of the room. Hundreds of people surround you, but the audience seems to fill the space in an oddly structured way. Everyone has their own view of the stage, and everyone seems happy.
Then, you witness a group of teenagers elbowing their way through the crowd. They’re moving in your direction, so you step aside to let them pass. But instead of moving forward, they stop near your original spot and decide to stay put. Everyone standing in the area has to rearrange themselves to make more room, but it doesn’t matter because they can still see the stage. They are still happy.
Materials Science and Engineering (MSE) alumnus, Paul Chao (’17), says that the way people physically respond to these situations (rearranging themselves in crowded spaces) is akin to how atoms in materials respond to different mechanical stresses like compression and expansion. By measuring the arrangements within deformed materials (post-stress), scientists can better understand the structure and mechanical properties of these materials in different environments.
What we observed and investigated provides another perspective for scientists who are interested in understanding HEAs.Paul Chao, Alumnus, Materials Science and Engineering, Carnegie Mellon University
During his undergraduate career at Carnegie Mellon University, Chao worked with graduate student Shivram Kashyap in MSE Professor Anthony Rollett’s lab to complete his senior research project on the texture of high entropy alloys (HEAs).
“When you think of a high entropy alloy,” says Chao, “think of an element soup in which five or more elements are proportionally equal and randomly arranged in an ordered structure.”
In the lab, Chao and Kashyap analyzed the crystallographic arrangement of atoms within HEAs that endured three different types of mechanical stress: tensile stress, which causes expansion; compressive stress, which causes compression; and rolling stress, which causes in-plane strain and a decrease in thickness.
When scientists expand, compress, or roll a material, they inject large amounts of energy into its atomic structure, forcing some of the atoms to break bonds and find “new homes” with other, neighboring atoms. As the atoms settle into their new homes, they shift and change direction, which alters the crystallographic orientation of the grains in the material. Chao and Kashyap captured the results of these movements in images while conducting their research. They used two different approaches when capturing images of the rearranged atoms: x-ray diffraction (XRD) and electron backscatter diffraction (EBSD).
“The first method requires a diffractometer that can obtain the macroscopic texture of the sample by analyzing how the x-ray interacts with the material’s crystal arrangement,” explains Chao. “The second method requires the use of a scanning electron microscope (SEM) with the add-on accessory EBSD detector. A focused electron beam scans across the surface of the polished sample. The electrons that escape from the sample diffract to form patterns called kikuchi bands, which tell us the crystallographic orientation at the point where the beam hits the surface.”
The crystallographic orientation of the alloys at different locations on the material resulted in a beautifully colored map that represents the data Chao and Kashyap collected using EBSD.
“The color pixels in each image represent the crystal orientation at specific points in the sample,” Chao explains. “When analyzing the data, you can use the colors to identify the atomic arrangement of the material. We found the attributes of the grain boundaries [where two colors meet in the images] to be particularly interesting.”
Researchers can learn more about the mechanical properties of HEAs by analyzing their texture and how they respond to different kinds of mechanical stress. As HEA’s gain scientific interest, Chao and Kashyap’s preliminary work suggests another method scientists can use to inform their understanding of this alloy.
“What we observed and investigated provides another perspective for scientists who are interested in understanding HEAs,” says Chao. “They could potentially use this research to aid their own future research in this area.”