Materials scientists at Purdue University have successfully reshaped the internal structure of certain alloys at extremely small scales, developing a cobalt aluminum intermetallic nanolaminate that shatters the usual tradeoff between high strength and flexibility.
Breakthrough Cobalt Aluminum Intermetallics Shatter Strength-Flexibility Tradeoff
Intermetallics are solid materials made from two or more metallic elements arranged in a highly ordered crystal structure. While bulk CoAl intermetallics are recognized as a high-strength compound, they have historically been especially brittle at room temperature. This brittleness has made the compound difficult to manufacture into complex shapes and limited its ability to withstand sudden mechanical stress, despite its potential suitability for demanding turbine components.
Engineering Crystal Defects and Amorphous Interfaces
To overcome this limitation, the Purdue research team approached the problem by deliberately introducing imperfections into the material at the atomic level. In a crystal, atoms normally follow a repeating geometric pattern, but dislocations represent microscopic irregularities or disruptions where atoms are no longer aligned in a perfectly ordered pattern.
Although the word defect suggests a weakness, dislocations give metals a way to change shape under extreme force by allowing layers of atoms to move rather than break apart. Because CoAl typically lacks enough mobile dislocations to deform substantially at room temperature, previous efforts to improve its plasticity through changing compositions or combining it with other materials yielded only limited results because they failed to create enough high-density dislocations.
The Purdue team solved this by directly introducing dislocations into the CoAl during sputtering deposition. Furthermore, the researchers designed a framework of amorphous interfaces (FAIs), which consist of flexible internal boundaries where atoms lack the ordered arrangement found in a crystal. More importantly, we designed the framework of amorphous interfaces (FAIs) — flexible boundaries in the materials for structural flexibility, which partially crystallize during deformation and promote the nucleation of the dislocations in CoAl intermetallics,
Xinghang Zhang stated.
Exceptional Yield Strength and Plastic Strain Performance
During deformation, parts of these flexible internal boundaries crystallize and help generate additional dislocations, giving the surrounding CoAl layers more ways to absorb force instead of fracturing. Micropillar compression tests conducted on the cobalt aluminum intermetallic nanocomposites at the Purdue Electron Microscopy Center revealed exceptional mechanical performance.
Testing showed that the material reached a yield strength of 6 gigapascals (GPa) alongside sustained work hardening to approximately 8.5 GPa, placing it roughly six to 10 times higher than the yield strength of high-strength structural steel. In addition to its extreme strength, the CoAl material sustained 15% of plastic strain under compression at room temperature.
This combination of ultrahigh mechanical strength and outstanding plasticity make the current CoAl nanolaminate system one of the best intermetallic systems reported to date,
said Ke Xu, a postdoctoral researcher in materials engineering and first author of the study.
Potential Applications in Next-Generation Aerospace Turbines
The research team’s findings point toward significant practical applications in advanced propulsion systems. According to the study’s corresponding author, Xinghang Zhang, a professor in Purdue’s School of Materials Engineering, the material holds distinct promise for aerospace engineering.
Among other applications, they can potentially be used in the next-generation materials of turbine blades for aeroengines, which are gas turbine engines that generate thrust for aircraft propulsion,
Zhang explained. High-strength, plastically deformable CoAl alloys could allow an engine or turbo to spin faster while sustaining higher centrifugal force, improving their performance.
Additional Purdue collaborators on the research paper, titled Plasticity in brittle intermetallics enabled by framework of amorphous interfaces and preexisting dislocations,
include Haiyan Wang, the Basil S. Turner Professor of Engineering in materials engineering and the Elmore Family School of Electrical and Computer Engineering.
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