Mechanical annealing: why smaller means stronger

It has been known for some 50 years that as metal structures approach the micrometre scale and smaller, they get stronger. But the reason why has until now remained little understood. Various hypotheses have been put forward to explain the phenomenon, but only now with advanced imaging systems such as electron microscopes can we see in detail how materials behave mechanically on such small scales.

Materials scientists Zhiwei Shan and Andrew Minor, from the Lawrence Berkeley National Laboratory in California, US, together with colleagues at Minneapolis-based nanomechanical testing specialists Hysitron, and the General Motors R&D Center in Detroit, have used electron microscopy to study what happens when pillars of nickel with diameters of between 150 and 400 nanometres are compressed under a diamond punch.

The results of the study were published recently in the journal Nature Materials.

When a metal object is deformed, defects in the material known as dislocations move along the planes of the crystal structure. At larger scales, the irreversible change known as plastic deformation is the result of a dislocation slip, in which trillions of dislocations per square centimetre tangle up and multiply as they run into each other and slide along the slip planes.

However, on micrometre scales and smaller, dislocations can be driven out of the material, reducing their density by 15 orders of magnitude, leaving behind a perfect crystal. Stanford University engineering professor William Nix named this process ‘dislocation starvation’.

Compression of a nickel pillar whose free end has a diameter of about 150 nm

In the latest experiments, the effect was most evident in the smaller diameter nickel pillars. In the larger pillars, mechanical annealing was incomplete, and some dislocations remained after compression. Yet even these larger structures displayed enhanced strength, which underlines the point that it is the creation of mobile defects that determines the degree of mechanical strength in such small volumes.

“The beauty of the pillar-testing geometry is that we can straightforwardly define stress,” says Minor. “Then we can correlate the measured stresses with discrete plastic events recorded in-situ, and more clearly interpret the quantitative data from our experiments.”

Study lead author Shan, who works now for Hysitron, adds: “This finding not only for the first time confirms the hypothesis of dislocation starvation by Nix and his group with compelling direct experiment evidence, but also raises the expectation of generating perfect nanostructural materials through mechanical deformation. I expect that this discovery will find application in nano-industries, especially those puzzled by the existence of high density growth-in dislocations.”

Commenting on the latest study, Nix says: “A basic tenet of materials science is that metals can be strengthened by introducing defects into the crystal lattice, through mechanical deformation. Cold worked metals are typically stronger than annealed metals because of this effect. Now, Shan et al. have shown that tiny metal crystals can be made stronger by removing defects from them, by mechanical deformation. This constitutes hardening by dislocation starvation rather than hardening by ordinary dislocation interactions.”

While the process may be counter-intuitive, a number of researchers have long suspected that dislocation exhaustion at free-surfaces would lead to a dramatic strengthening of the material. Shan and his co-workers have shown that the process is more effective than had been imagined.

Kevin Hemker, a professor at Johns Hopkins University, who with Nix co-authored a Nature News & Views article describing the work of Shan et al., comments: “The in-situ observations reported by Shan and colleagues now provide unambiguous evidence of the importance of dislocation starvation, and the discreteness of dislocation processes in governing the strength of nano-pillars. More broadly, these experiments show that direct observations can provide much needed clarity in understanding complex material behaviour at the nanoscale.”

Further reading:

“Mechanical annealing and source-limited deformation in submicrometre-diameter Ni crystals”, Shan et al., Nature Materials 7, 115 (2008).

“Nanoscale deformation: seeing is believing”, Hemker & Nix, Nature Materials 7, 97 (2008).

Figure: Compression of a nickel pillar whose free end has a diameter of about 150 nm. Before compression (left) the pillar has a high density of defects, visible as dark mottling. After compression, all the defects have been driven out, in a process known as ‘mechanical annealing’ (source: National Center for Electron Microscopy/US Department of Energy).

Article first published in Nanomaterials News.