Material scientists and engineers have always been looking for ways to create ultrastrong materials that are also tough enough to resist failure. Decreasing the grain size to the nanoscale in metals is one way to do this. There is a downside to this approach, however, in that it makes the material more prone to brittle fracture as strain localizes in nanocrystalline metals and forms cracks when the material is strained.

One way to overcome this drawback is to create gradients in the grain structure. This makes the material become more ductile under applied tensile stresses so it does not crack. The problem is that previous techniques to create GNG structures have proven complicated and produced structures with grain structure gradients that were too low to make a difference to a material’s mechanical properties.

Making an extreme GNG structure

Now, a team led by Edwin Thomas of Rice has found that firing a nearly perfect cube of silver onto a hard target (in this case, made of silicon) turns its single-crystal microstructure into an extreme GNG one. The researchers did their experiments because they wanted to find out how materials deform under extreme stress – like that experienced by a bulletproof vest or a spacecraft that is hit by micrometeorites.

Thomas and colleagues made their microcubes (which measured around 1.4 microns per side) using a bottom-up seed-growth process, which produced defect-free materials that had a nearly-perfect cubic geometry. This synthesis method was carefully chosen since other techniques, such as focused ion beam (FIB) milling, produce surface damage and ion contamination.

Laser-induced projectile impact testing

“We then used a laser-induced projectile impact testing (LIPIT) apparatus built in our laboratory to launch the microcubes at supersonic velocities to directly impact an impenetrable target,” explains Rice team member Ramathasan Thevamaran. “Next, we studied the microstructures using scanning electron and transmission electron microscopy (SEM and TEM). These analyses allow us to find out how the materials deform and how defects are created in them with atomistic resolution.”

The high-velocity impact generates very high pressures that far exceed the material’s strength, adds Thomas. “This leads to high plasticity at the impact side of the cube while the top region retains its initial structure, ultimately creating a grain-sized gradient along its height.”

Gradient at least 10 times higher

The technique produces structures with grains ranging in size from about 100 to 500 nm over a distance of 500 nm. This yields a gradient at least 10 times higher than the previous methods to create GNG structures.

Smashing microcubes for strength and toughness

“Our technique is the first to create extreme GNG structures in metallic microcubes via high-velocity impact deformation,” Thevamaran tells nanotechweb.org. “This result could help improve modern material processing technologies, such as cold spray and shot peening, as well as point to new ways to creating ultrastrong and tough metals.”

The team, reporting its work in Science DOI: 10.1126/science.aag1768, is now busy trying to better control the gradient of grain size in its silver microcubes. “This will allow us to design materials that behave in the way we want,” adds Thevamaran.