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New research shows that composite metal foam (CMF) is incredibly resilient at high temperatures, able to withstand repeated heavy loads even at temperatures of 400°C and 600°C. Coupled with the material's high strength-to-weight ratio, the finding suggests that CMF could be used in applications ranging from automobile engines to aerospace components to nuclear power technologies.

"CMF has many attractive properties, which make it appealing for a wide range of applications," says Afsaneh Rabiei, corresponding author of a paper on the work and a professor of mechanical and aerospace engineering at North Carolina State University. "But if you want to use a material in engines, airplane parts or any application involving repeated loading and high temperatures, you need to know how the material will perform.

"This is important for any application, but particularly when equipment failure could affect public health and safety—such as jet engine vanes, ducts, and exhaust flaps; turbine blades; hypersonic vehicle airframes and hot trailing edges of wings; gas and steam turbines; automobile brake system components and internal combustion engine parts; nuclear reactor fuel cladding and many more structures that go in service under extreme conditions of heat and load."

CMFs are foams that consist of hollow spheres—made of materials such as stainless steel, nickel, or other metals and alloys—embedded in a metallic matrix. The resulting material is both lightweight and remarkably strong at absorbing compressive forces, with potential applications ranging from aircraft wings to vehicle armor and body armor.

In addition, CMF is better at insulating against high heat than conventional metals and alloys, such as steel. The combination of light weight, strength and thermal insulation means that CMF also holds promise for use in storing and transporting nuclear material, hazardous materials, explosives and other heat-sensitive materials.

To see how CMF would perform under repeated stress at high temperatures, the researchers worked with NC State's Constructed Facilities Laboratory, which is designed to test materials and structures under extreme circumstances.

For this study, the researchers worked with CMFs consisting of steel spheres in a steel matrix. The CMF samples were put through a repeated cycle of loading while exposed to temperatures of 23°C (73°F), 400°C (752°F), and 600°C (1,112°F).

At 400°C, the CMF withstood a cycle of loading that alternated between 6 and 60 megapascals (or between 870 and 8,702 units of pound-force per square inch) for more than 1.3 million cycles without failure before the researchers halted the test due to time constraints.

At 600°C, the CMF withstood a cycle of loading that alternated between 4.6 and 46 megapascals (or between 667 and 6,671 units of pound-force per square inch) for more than 1.2 million cycles without failure before the researchers halted the test due to time constraints.

"Knowing that in a compression-compression fatigue setting, the fatigue life of solid stainless-steel decreases significantly as temperature increases from room temperature to 400°C and 600°C, these results were remarkable," Rabiei says. "Our findings indicate the fatigue life of the steel-steel CMF is not diminished and that this lightweight material performs tremendously well in the extreme environment of high temperature cyclic loading.

"This discovery is exciting, and we're open to working with industry partners who would like to explore potential applications for CMF. This work was done with an eye toward developing a material that could be used to improve safety and efficiency related to the shipping of hazardous materials, so that's one potential application. But these findings are also relevant to any application where equipment may be exposed to high loads and high temperatures."

The paper, "Performance of Composite Metal Foams Under Cyclic Loading at Elevated Temperatures," is open access in the Journal of Materials Science. First author of the paper is Zubin Chacko, a recent Ph.D. graduate from NC State. The paper was co-authored by Gregory Lucier, a research professor at NC State and manager of the Constructed Facilities Laboratory.