Chinese scientists say they can strengthen pure gold by adding tiny holes
Chinese scientists have developed a method to create pure gold that is both lightweight and strong by forming uniformly small pores within the solid metal. Researchers believe this technique could be pivotal for developing materials urgently needed in the aerospace, automotive and consumer electronics industries where lightweight yet high-strength materials are in demand. In conventional metal forming methods such as casting, welding and 3D printing, bubbles are seen as significant material flaws and engineers take utmost care to avoid them.
Internal bubbles in metals can lead to strength loss, bubbles at metal junctions can reduce durability and surface bubbles can impair the material’s finish. However, a team from the Shenyang National Laboratory for Materials Science at the Chinese Academy of Sciences (CAS) found that instead of eliminating these voids, “refining their size and regulating their shape and distribution could not only mitigate their detrimental effects but also offer additional benefits”.
The research, led by Jin Haijun of CAS, was published in the peer-reviewed journal Science on August 8. “[This] suggests an attractive way to manipulate the properties of solids,” said Brent Grocholski, a senior editor at Science.
The team used gold as a model material and created uniformly structured porous gold through a de-alloying corrosion process. By compressing and then annealing – or heating and cooling – the metal, they produced a new material filled with dispersed nanopores smaller than 100 nanometres. Tests revealed that by adding nanopores at a volume fraction of 5 to 10 per cent, the strength of gold increased by 50 to 100 per cent, meaning it could bear higher loads while maintaining good plasticity. In some samples, the plasticity was even superior to that of fully dense gold of the same size.
“This improvement is due to the dispersed nanopores helping to alleviate stress and strain concentration around the voids, thereby inhibiting the initiation of cracks,” Jin said in an official report by CAS. “The material’s large specific surface area facilitates interactions between the surface and dislocations, which enhances strength and strain hardening rates, the latter contributing to improved plasticity,” Jin said.
The size and density of the pores also matter. “So far, our results indicate that voids with a diameter in the range of 10 to 100nm are beneficial to strengthening, without compromising ductility. Further refining the mean void size to below 10 nm may further harden the material, but will decrease the ductility,” the team said in the paper.
Traditionally, enhancing the strength of materials while reducing weight has been achieved by adding lighter alloy elements, such as aluminium in lightweight steel and lithium in aluminium alloys. In contrast, the Chinese team’s subtractive approach offers an environmentally friendly and cost-effective strategy to enhance a metal with “no extra weight, no pollution”. The dispersed nanovoids reduce the density of pure gold by more than 10 per cent, aiding in material lightness and recyclability.
This approach largely preserves the physical and chemical properties of the material – such as thermal and electrical conductivity – and its corrosion resistance. “For instance, gold with nanovoids can be utilised as connector or contact materials in electronics,” Jin said. “This strengthening strategy might also be applied to other metals and engineering alloys as long as the nanovoids can be effectively integrated into the material, with potential applications across multiple fields.”
Source: https://www.scmp.com/