Model governing deformation of materials under stress challenged

In a paper published recently in Nature Communications, researchers in UC Irvine's Samueli School of Engineering report the discovery of extended slip bands -- a finding that challenges the classic model developed in the 1950s by physicists Charles Frank and Thornton Read. While the Frank-Read theory attributes slip band formation to continuous dislocation multiplication at active sources, the UC Irvine team found that extended slip bands emerge from source deactivation followed by the dynamic activation of new dislocation sources.

The process that results in extended slip bands was observed at the atomic scale by the UC Irvine researchers as they performed mechanical compression on micropillars of an alloy of chromium, cobalt and nickel, recently found to be among the toughest materials on Earth. Using scanning transmission electron microscopy made possible by the UC Irvine Materials Research Institute and large-scale atomistic modeling, the team was able to view the confined slip band as a thin glide zone with minimal defects and the extended slip band with a high density of planar defects.

"The full dynamics of slip band formation at the fundamental level had not been understood through the more than 70 years since the Frank-Read theory was developed," said corresponding author Penghui Cao, UC Irvine associate professor of mechanical and aerospace engineering. "Our ability to capture these processes at atomic and nanometer scales provides new insight into collective dislocation motion and microscopic deformation instability in advanced structural materials."

He said that deformation banding, where strain concentrates in local zones, is common in many human-made and natural substances and systems, including crystalline solids, metals, granular media and even geologic faults under compressive stress.

"With the advent of new, advanced 'supermaterials' such as the CrCoNi alloy, a deep understanding of their behaviors is more critical than ever," Cao noted. "This foundational knowledge will accelerate the discovery of materials with tailored and predictable mechanical properties to meet the rising demand for advanced materials resilient to the extreme environments across energy and aerospace sectors."

Joining him in this project were graduate students, research specialists and other professors in UC Irvine's Department of Mechanical and Aerospace Engineering and Department of Materials Science and Engineering. Funding for the research was provided by the U.S. Department of Energy, UC Irvine and the National Science Foundation (through the UC Irvine Center for Complex and Active Materials).