Nanoscale CoAl design delivers 6 GPa strength with 15% plastic strain at room temperature

Materials engineers have developed the ability to manipulate structure and matter at the nanoscale for solid-state alloys called intermetallics, making it possible to alter their properties for improved performance.

Intermetallics, which consist of two or more metallic elements in an ordered crystal structure, are especially good candidates for these enhanced physical and chemical properties, owing to the critical role they play in demanding applications and products such as jet engines, gas turbines, and energy storage and automotive systems. The remarkable strength, high melting temperatures and superior creep resistance of intermetallics make them indispensable for structural use in extreme environments.

In a paper published in Science Advances, Purdue University engineers have demonstrated a way to achieve simultaneous high strength and plasticity in cobalt aluminum (CoAl) intermetallics.

Xinghang Zhang, a professor in Purdue’s School of Materials Engineering, is the corresponding author of the paper, titled “Plasticity in brittle intermetallics enabled by framework of amorphous interfaces and preexisting dislocations.” Other Purdue collaborators include Haiyan Wang, the Basil S. Turner Professor of Engineering in materials engineering and the Elmore Family School of Electrical and Computer Engineering, and Ke Xu, a postdoctoral researcher in materials engineering and first author.

“Bulk CoAl intermetallics are a high-strength compound,” Zhang said. “Among other applications, they can potentially be used in next-generation turbine-blade materials for aeroengines, which are gas turbine engines that generate thrust for aircraft propulsion. High-strength, plastically deformable CoAl alloys could allow an engine or turbo to spin faster while sustaining higher centrifugal force, improving performance.”

Bulk CoAl alloys, like many intermetallics, are very brittle, especially at room temperature. Improved plasticity in CoAl intermetallics would make them easier to process into industrial products, which would allow engineers to design more complex structures for engine applications.

“In this study, we show that CoAl can exhibit significant plasticity at room temperature, offering a new, alternative approach to improve the plastic deformation capability in CoAl,” Xu said.

Novel approach

Previous attempts to increase plasticity tailored the microstructure of CoAl intermetallics by modifying compositions or producing composites but had little success in producing sufficiently high-density dislocations in CoAl intermetallics, leading to very limited plasticity.

Plasticity in CoAl intermetallics at room temperature requires abundant dislocations—microscopic irregularities where atoms no longer align in an orderly way—which enable the metals to withstand extreme forces without fracturing.

“We directly introduced dislocations in CoAl during sputtering deposition,” Zhang said. “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.”

Strength and plasticity

The high-density dislocations introduced during the growth process, along with the framework of amorphous interfaces, make the CoAl intermetallic ultrastrong. Researchers recorded a yield strength of 6 GPa (gigapascal, a stress measurement), some six to 10 times higher than high-strength structural steel. The resultant intermetallic also demonstrated the ability to sustain 15% plastic strain under compression at room temperature.

“This combination of ultrahigh mechanical strength and outstanding plasticity makes the current CoAl nanolaminate system one of the best intermetallic systems reported to date,” Xu said.

The researchers employed a nonequilibrium fabrication approach—magnetron sputtering deposition, a technique to apply a thin film onto a substrate—to prepare the CoAl intermetallics with a framework of amorphous aluminum-cobalt binary interfaces. This differs from the traditional metal-casting method (from liquid to solid) for intermetallic processing.

“This nonequilibrium fabrication approach enables us to fabricate materials from alloy vapor to a solid, introducing a significant number of dislocations in CoAl,” Zhang said. “We were able to achieve significant strength and plasticity in CoAl, which can’t be realized via traditional casting.”

The researchers used in situ mechanical testing in a scanning electron microscope to probe the mechanical performance (strength and plasticity) of the CoAl intermetallics. This technique allowed the researchers to monitor the deformation behavior with micrometer precision.

Outside collaborators included professor Yashashree Kulkarni and her Ph.D. student Anand Mathew from the University of Houston, who performed molecular dynamics simulations to elucidate the deformation mechanisms at the atomic level for CoAl. Their simulations revealed the crystallization of FAIs and dislocation emission from the layer interfaces into CoAl layers.

Scaling innovation

Now that the researchers have demonstrated that certain types of layer interfaces can significantly enhance the plastic deformability of CoAl, the next step will be to implement the same concept for producing bulk CoAl nanocomposites for industrial-scale applications.

“We will also be testing the concept using other intermetallics, with the goal of establishing the general applicability of FAIs for improving plasticity in this metal class,” Xu said.

The research will be spearheaded by Zhang’s Nanometal Group, which works on integrating synthesis, in situ nanomechanical testing and advanced microstructure characterization at atomic scales to design high-strength deformable metallic materials. Zhang’s overall research includes synthesis of nanomaterials, radiation damage in nanostructured materials, and mechanical behavior of nanostructured metals and functional materials.

The research documented in the journal paper potentially has profound implications for advanced technology fields.

“Ductile intermetallics will significantly boost our capabilities for designing advanced materials for aerospace and outer space, energy and defense applications,” Zhang said.

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