Rice University researchers (from left) Olawale Lawal, Ramathasan Thevamaran, Edwin Thomas and Sadegh Yazdi hold clay models of deformed cubes that represent the results of their microscale experiments. The researchers smashed silver microcubes at near supersonic speeds to see how deforming their crystalline structures could make them stronger and tougher. Photo: Jeff Fitlow/Rice University.Scientists at Rice University are smashing metallic micro-cubes to make them ultrastrong and tough by rearranging their nanostructures upon impact. In a paper in Science, the Rice team report that firing a tiny, nearly perfect cube of silver onto a hard target turns its single-crystal microstructure into a gradient-nano-grained (GNG) structure.
The purpose of the experiment was to learn how materials deform under overwhelming stress, as might be experienced by a bulletproof vest or a spacecraft that encounters micrometeorites. The researchers believe that creating a gradient nanostructure in materials by way of deformation will make them more ductile and therefore less likely to fail catastrophically when subsequently stressed. Ultimately, they want to develop nano-grained metals that are tougher and stronger than anything available today.
Led by materials scientist Edwin Thomas, dean of Rice's George R. Brown School of Engineering, the team used its advanced laser-induced projectile impact test (LIPIT) rig to shoot microcubes onto a silicon target. The test rig allowed them to be sure the cube hit the target squarely.
The Thomas lab developed the LIPIT technique several years ago to fire microbullets for testing the strength of polymer and graphene film materials. This time the researchers were essentially testing the bullet itself.
"The high-velocity impact generates very high pressure that far exceeds the material's strength," Thomas explained. "This leads to high plasticity at the impact side of the cube while the top region retains its initial structure, ultimately creating a grain-size gradient along its height."
The original projectiles needed to be as perfect as possible, which required a custom fabrication method, Thomas said. The cubes for the study were synthesized via bottom-up seed growth, producing single crystals about 1.4µm per side, around 50 times smaller than the width of a human hair.
LIPIT transforms laser power into the mechanical energy required to propel the cubes toward a target at supersonic velocity. The cubes were placed on top of a thin polymer film that thermally isolated them and prevented the laser itself from deforming them. When a laser pulse hit an absorbing thin-film gold layer underneath the polymer, the laser energy caused it to vaporize. That expanded the polymer film, launching the microcubes.
The distance traveled by the microcubes was small – about 500µm – but the effect was large. While the experiments were carried out at room temperature, the cube's temperature rose by about 350°F upon impact at 400 meters per second, inducing dynamic recrystallization.
The team fired silver cubes at the target at various orientations and measured the results of the impact from every angle, inside and out, and from the nanoscale on up. Controlling the orientation of the crystal's impact gave the researchers enormous ability to influence the resulting structure and potentially its mechanical properties.
Other industrial processes produce materials with grains that can range from the nanocrystalline up to the coarse-grained; according to Thomas, neither structure is ideal. While nanocrystalline structures make metals stronger, they also increase their susceptibility to catastrophic brittle failure due to the way those grains localize strain. Studies have demonstrated that creating a gradient-nano-grained structure from the nanometer to the micron scale may provide high strength yet alleviate such brittle failures by distributing stress more effectively.
The one-step Rice process makes structures with a range of grains from about 10nm to 500nm over a distance of 500nm. That produces a gradient at least 10 times higher than achievable with other techniques, the researchers reported.
They also discovered that the impact stores considerable elastic energy in the material, which leads to slow but continuous recrystallization of the metal at room temperature, even though silver's melting point is more than 1700°F. Electron microscope analysis of samples eight days after impact showed the crystals were still seeking equilibrium, Thomas said.
In addition to promising pathways for creating ultrastrong and tough metals, the researchers believe their work may influence such other modern material processing techniques as cold spray and shot peening.
This story is adapted from material from Rice University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.