Microscopic silver cubes were the bullets in Rice University experiments to show how deformation upon impact can make materials stronger and tougher. (Credit: Thomas Group/Rice University.)
A cross-section composite image of a silver microcube impacted on its side shows decreasing grain size closer to where the deformed cube hit the target. Rice University scientists believe their research will lead to better materials for high-impact applications. (Credit: Thomas Group/Rice University.)Metals that make up the structural components of airplanes and spacecraft need to be strong and tough enough to resist fracture. A promising approach is to reduce the size of the grains in these metals to the nanoscale – but this comes at a price. Nanostructured metals are ultra-strong but susceptible to catastrophic brittle failure.
In the drive to develop metals that are both strong and resistant to failure, engineers have come up with a compromise – metals in which there is a gradual decrease in the size of the grains from the interior to the surface. These so-called gradient nanograined (GNG) structures can alleviate catastrophic failure by allowing ductile behavior to take place to relieve tensile stresses while maintaining overall strength.
Now researchers from Rice University and the University of Massachusetts, Amherst have come up with a simple way of producing ‘extreme’ GNG structures in cubes of Ag by firing them at supersonic speeds onto a solid target [Thevamaran et al., Science 354 (2016) 312].
“This GNG structure – with grain size varying from nanocrystalline to coarse-grained – may result in ultra-strong and tough metals,” says Edwin L. Thomas of Rice University, who led the study. “Our studies show promising pathways to creating GNG-structured metals for improving both strength and toughness of metals, which usually have a tradeoff.”
Previous attempts to produce GNG structures have relied on multistep surface mechanical grinding or surface mechanical attrition treatments. But the approach devised by Thomas and his colleagues is a one-step process able to produce a variation in grain size from 10 nm to 500 nm over a distance of 500 nm.
The researchers produced large quantities of identical, single crystal, defect-free Ag microcubes using a bottom-up seed-growth process. The microcubes were then launched at supersonic velocities of ∼400 m/s towards a rigid, impenetrable target using an advanced laser-induced projectile impact testing (LIPIT) technique developed by the researchers.
Analysis by high-resolution transmission electron microscopy (TEM) and selective area diffraction (SAD) reveals a strong grain size gradient from one side of a cube to the other. The stress at the impact site induces nanograins, while on the other side of the microcube a more coarse-grained structure is produced.
“The GNG structure will provide us new ways of creating ultrastrong and ultra-tough metals,” says Thomas, “[which] will be of great interest for applications in extreme environments such as protecting aircraft turbine blades from microparticle impacts, protecting spacecraft from space debris, and body or vehicle armor.”
Metallic components with GNG structure could also be used to make infrastructure and cars lighter and more fuel-efficient, he suggests. The researchers are now exploring how to tailor the gradient for specific functional properties and will be evaluating the mechanical properties of GNG-structured metals.
Ke Lu of the Institute of Metal Research in China believes the key novelty of this work is the creation of very large grain size gradients in pure Ag.
“The strain gradient obtained is much larger than that in conventional plastic deformation techniques and originates from a very high deformation rate in a very small sample of an easy-to-deform metal (Ag),” he explains. “It is a smart idea.” But he cautions that it could be difficult to produce a large gradient if any one of the three key factors is missing.
This article was originally published in Nano Today (2017), doi: 10.1016/j.nantod.2017.02.005