Ta-Cu nanocomposite
Past research has demonstrated that smaller is better when it comes to designing and fabricating stronger and tougher metals. One of the major challenges facing this field involves translating the increased strength properties of metallic nanomaterials into bulk metals. To address this challenge, Johns Hopkins engineers sought to develop a bulk bicontinuous metal-metal nanocomposite.
“We decided to work with composite materials because bulk testing of nanoporous metals tends to result in brittle failure. Nanoporous metals have around 70% void space, which is partly what makes them great catalysts, but the bulk structure doesn’t bear load efficiently,” said Ian McCue, first author on the paper and a graduate student in Dr. Jonah Erlebacher’s research group.
The goal was creating a strong material with potential for structural applications, and the team selected tantalum (Ta) and copper (Cu) to achieve this aim. Tantalum is a critical component in a number of products, including capacitors in cell phones and computers, surgical instruments, and jet engine components. Copper is an extremely ductile metal, and is used extensively in wiring because of its electrical conductivity. Liquid metal dealloying (LMD), a novel processing technique, was utilized to fabricate a Ta-Cu composite. LMD uses a molten metal as a medium to selectively dissolve one of the alloying components; another material fills in the empty spaces of the dissolved material, similar to how sand can fill the spaces in a jar filled with rocks.
“A couple years ago a research group in Germany filled nanoporous gold with a polymer and showed that it increased the strength and ductility. It was an amazing result, but gold is not a structural material, and amorphous polymers do not show size-dependent strengthening like metals because they have a different mode of deformation,” said McCue.
Molten copper dissolved titanium from a titanium-tantalum alloy and filled in the porous space. The resulting Ta-Cu nanocomposite showed impressive properties, including a 10-fold increase in yield stress when the feature size was decreased from 10 micrometres (about the width of a cotton fiber) to 70 nanometers (the size of some virus particles) while still retaining ductile behavior.
Going forward, the researchers expect to further study processing conditions to better understand the development of these composites, as well as how each phase contributes to the bulk mechanical properties.
“Size-dependent strengthening has been extensively studied and, while useful, does not bridge the gap from materials science to materials engineering,” said McCue. “I’m hopeful that this research points researchers in the direction of designing engineering-relevant materials.”
This research was funded by the National Science Foundation. Other authors on the paper include Stephen Ryan and Dr. Jonah Erlebacher, JHU Materials Science; Dr. Kevin Hemker, JHU Mechanical Engineering; Xiandong Xu and Mingwei Chen, WPI Advanced Institute for Materials Research, Tohoku University; and Nan Li, Los Alamos National Laboratory. Their findings were reported in Advanced Engineering Materials.