Tikalon Blog by Dev Gualtieri

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Nano-Grained Structural Materials

May 24, 2011

Chemists are principally concerned with the composition of materials, but materials scientists are concerned with their structure as well. My favorite definition of materials science is as follows:

Materials science is the science of how the macroscopic properties of materials relate to their composition and structure.

When I studied materials science, my coursework was a mixture of physics , chemistry and engineering . The curriculum had evolved from metallurgy , but we were studying statistical mechanics and quantum mechanics in the chemistry building, classical mechanics in the physics building, and fracture mechanics in the engineering building. I enjoy reminding people that the iconic chemist, Linus Pauling , wrote many metallurgy papers; and Sir William Lawrence Bragg , author of the eponymous Bragg's Law and a Nobel Physics Laureate at age 25, conducted experiments on dislocation reactions using bubble rafts .

Nanotechnology has opened a huge new area of study for materials scientists. Now, you can take nearly every material that's been developed since the bronze age and make nanoscale versions with completely different properties . Fortunately, these different properties are often better properties.

Bronze age artifact

A bronze chariot for a vase or vessel used for religious purposes.

A Bronze Age artifact found in 1855 at Ystad in Scandia, southern Sweden.

(Via Wikimedia Commons )

A team of materials scientists at the Shenyang National Laboratory for Materials Science, Institute of Metal Research , Chinese Academy of Sciences , Shenyang, China , has investigated the mechanical properties of what they call gradient nanograined copper .[1-5] This material is designed to take advantage of the high tensile strength of nanograin copper, while simultaneously keeping some of the ductility of natural copper.

One analogy to the utility of ductility is that huge tree that likely sits in your front yard. In high winds (caused by global warming , of course) the tree limbs bend considerably. This ability to bend protects them from breaking off.

Nano-grained metals are very strong, and coarse-grained metals are ductile. Nano-grained metals are somewhat like ceramics . They have high strength and zero ductility. The reason for this is because the small grains suppress the movement of dislocations, which is the primary mechanism for ductility. Nano-grain materials do not work harden , so strain becomes localized and failure occurs shortly after initial yield.

Previous studies found a way to add ductility to nano-grained materials by attaching them to a ductile substrate. The ductile substrate acts to average the local strain among nano-grains. In this way, nano-grained copper films on a polymer substrate had a 10% greater elongation , but at that point the composite failed when the copper delaminated from the polymer.

The solution to the delamination problem is a better match of the nano-grained material and its carrier materials. To accomplish this, the Chinese team created a "dog bone" tensile rod of copper having a 6 mm diameter gauge section with a coarse grain structure at its axis, gradually becoming a nano-grain structure at the surface. This gradient nanograined copper rod was found to have ten times the strength and twice the ductility of normal, coarse-grained copper.

They were able to stretch the rod to twice its length, the gradient structure was maintained, and no cracks formed. The proposed mechanism for such performance is grain boundary migration. This makes sense, since the gradient structure puts very similar grains alongside each other.

The nano-grain surface was created by a process known as "surface mechanical grinding treatment," or SMGT. In this process, the copper rod was first annealed at 723 K for 1.5 h to obtain a coarse-grained polycrystalline structure throughout. At that stage, the average grain size was about 21 μm. The SMGT is performed by placing the rod in liquid nitrogen and working it repeatedly at 600 rev/min rotation by a 3 mm radius hemispherical tool of tungsten carbide and cobalt . Details of the SMGT are in the supplementary material for the team's publication,[2] and in a previous publication.[3]

Although it's somewhat more complex than diamond turning , the surface mechanical grinding treatment should be applicable to most components, since the mechanical work done by the hemispherical tool could be programmed in a milt-axis positioner.

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