Materials scientists can now reshape the internal structure of certain alloys at extremely small scales, allowing them to enhance properties such as strength, durability, and flexibility. One especially promising group of materials is known as intermetallics.
Intermetallics are solid materials made from two or more metallic elements arranged in a highly ordered crystal structure. Their unusual atomic organization can give them exceptional strength, high melting temperatures, and strong resistance to creep, which is the slow deformation of a material under prolonged heat and stress.
These qualities make intermetallics valuable for demanding technologies, including jet engines, gas turbines, energy storage systems, and automotive components. However, many of these materials have a major weakness. They tend to be extremely brittle.
Making a Strong but Brittle Material More Flexible
In research published in Science Advances, engineers at Purdue University demonstrated a new way to combine very high strength with substantial plasticity in cobalt aluminum (CoAl) intermetallics.
Plasticity refers to a material’s ability to permanently change shape without cracking or breaking. For industrial applications, this is important because a material that cannot deform is difficult to shape into complex components and may fracture suddenly under stress.
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 the next-generation materials of turbine blades 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 their performance.”
Like many intermetallics, bulk CoAl is especially brittle at room temperature. Giving it greater plasticity could make it easier to manufacture and allow engineers to create more sophisticated structures for engines and other high-performance systems.
“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.
Atomic Defects Become an Advantage
Earlier efforts to improve the plasticity of CoAl focused on changing its composition, adjusting its microstructure, or combining it with other materials. These approaches produced limited results because they did not create enough high-density dislocations inside the intermetallic.
Dislocations are microscopic irregularities in a crystal where atoms are no longer aligned in a perfectly ordered pattern. Although the word defect may suggest a weakness, dislocations can help metals deform under extreme force instead of breaking apart.
For CoAl to become plastic at room temperature, the material needs a large number of these dislocations.
“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.”
The framework of amorphous interfaces consists of flexible internal boundaries that do not initially have the same orderly crystal structure as the surrounding material. As the CoAl deforms, parts of these interfaces crystallize and help generate new dislocations.
Up to 10 Times Stronger Than Structural Steel
The combination of dislocations introduced during fabrication and the framework of amorphous interfaces produced an exceptionally strong CoAl intermetallic.
Tests showed that the material reached a yield strength of 6 GPa (gigapascal, a stress measurement). That is approximately six to 10 times higher than the yield strength of high-strength structural steel.
Yield strength describes how much stress a material can withstand before it begins to deform permanently. Despite its extreme strength, the CoAl material also 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,” Xu said.
A Different Way to Manufacture Intermetallics
The researchers produced the material using magnetron sputtering deposition, a process that applies a thin film to a surface. This nonequilibrium fabrication method allowed them to create CoAl with amorphous aluminum cobalt binary interfaces.
The approach is very different from traditional metal casting (from liquid to solid), which begins with molten material and allows it to cool into a solid form.
Instead, magnetron sputtering deposition creates the material from alloy vapor. This process enabled the researchers to introduce many more dislocations into the CoAl than conventional casting normally permits.
“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.”
Watching the Material Deform
To measure the mechanical performance (strength and plasticity) of the CoAl intermetallics, the team conducted in situ mechanical testing inside a scanning electron microscope.
This method allowed the researchers to observe the material as it deformed and track its behavior with micrometer precision.
Professor Yashashree Kulkarni and her PhD student Anand Mathew from the University of Houston also contributed to the project. They used molecular dynamics simulations to examine the processes occurring inside CoAl at the atomic level.
The simulations showed that the frameworks of amorphous interfaces crystallized during deformation. They also revealed dislocations moving from the layer interfaces into the surrounding CoAl layers, helping explain how the material could deform without quickly fracturing.
Scaling the Metal for Industrial Use
The experiments showed that specially designed layer interfaces can substantially improve the plastic deformability of CoAl. The researchers will now try to apply the same concept to bulk CoAl nanocomposites that could be produced 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 next phase of the research will be led by Zhang’s Nanometal Group. The group combines material synthesis, in situ nanomechanical testing, and advanced atomic-scale microstructure analysis to develop metallic materials that are both strong and deformable.
Zhang’s broader research also covers nanomaterial synthesis, radiation damage in nanostructured materials, the mechanical behavior of nanostructured metals, and functional materials.
Potential Applications in Aerospace and Energy
The findings could have important consequences for several advanced technology sectors. Stronger and more ductile intermetallics may support the development of improved turbine blades, aircraft engines, energy systems, defense technologies, and materials designed for use in space.
“Ductile intermetallics will significantly boost our capabilities for designing advanced materials for aerospace and outer space, energy and defense applications,” Zhang said.
Funding for this research was provided primarily by the National Science Foundation’s Metals and Metallic Nanostructures program.