Based on much experimental and theoretical work in the last decade or so, mechanical properties of materials at nano-scale are proving to significantly deviate from their bulk counterparts. This is true not only for nano-structureD materials (i.e. composed of nano-scale components like nanocrystalline materials) but also for nano structureS (surface-dominated structures like carbon nanotubes (CNT’s), nanowires, etc.). Nanoindentation has been a very effective and well-characterized technique for determination of hardness, modulus, and stiffness, and for crystalline materials the indentation hardness has been widely shown to be significantly higher at shallower indentation depths (so-called indentation size effect, or ISE). However, inserting a sharp indenter tip into a material inevitably sets up strong strain gradients in the deforming volume, which is often linked to the origin of the ISE. Moreover, the infinitesimal volumes probed via this technique are coherent with the remaining matrix, rendering the effects of free surfaces on mechanical properties inaccessible.

To significantly reduce the effects of strain gradients and to investigate the effects of free surfaces on plasticity of nano-scale single crystals, metallic glasses, and polycrystals, an increasingly popular experimental technique involves uniaxial compression of nano-pillars made out of these materials. For example, with the help of this technique, it was shown that pure metals and metallic alloys exhibit strong size effects in plastic deformation manifested through “smaller is stronger” phenomenon. There are several theories (both computational and experimental) trying to explain the observed size effect, but at this point there is no unified phenomenological model fully explaining these size effects. One of these theories is coined as Dislocation Starvation, or a condition attained in fcc metals when most mobile dislocations annihilate at a free surface leaving the crystal effectively dislocation-free, and subsequent plasticity is dominated by nucleation events. This concept based on experimental work on nano-pillars was first introduced by Greer and Nix  [Phys Rev B 2007], and recent work of Shan, et al [Nature Materials 2008] on in-situ TEM deformation of Ni nano-pillars clearly shows a phenomenon the authors call “mechanical annealing,” which is very consistent with the dislocation starvation mechanism. A common technique for fabricating these nano-pillars involves the use of the FIB (Focused Ion Beam), which inevitably introduces some Ga+ damage into the surface. This damage layer certainly is expected to have some effect on the strength of these nano-crystals. Moreover, in the recent work of Bei, et al, it was shown that Mo micro-posts made without the use of the FIB all attain the theoretical strength of Mo and don’t  show any significant size effect.

In this Journal Club topic, I’d like to discuss the following 4 articles, which involve nano-pillar compression. The first one (Shan, et al) shows the results of in-situ TEM FIB-machined Ni (fcc) nano-pillar compression and postulates that mechanical annealing, or dislocation escape in response to mechanical deformation, is prevalent in fcc metals at these scales. The 2nd one (Bei, et al) – the only one where the compression specimens were not fabricated by using FIB – does not find any size effects in deformation of Mo posts made through selective etching of NiAl-Mo eutectic. And the final, 3rd one (Shan, et al) discusses enhanced ductility and lack of catastrophic failure in metallic glass nanopillars.

As a natural continuation of this discussion, a Journal Club topic on plasticity at sub-micron scale will be led by Professor Wei Cai of Stanford University on July 15th. 

Articles for Discussion:

1. Z. W. Shan, R. K. Mishra, S. A. Syed Asif, O. L. Warren, and A. M. Minor, ”Mechanical annealing and source-limited deformation in submicrometre-diameter Ni crystals” Nature Mater. 7, 115 (2008)

2. H. Bei, S. Shim,, E.P. George, M.K. Miller, E.G. Herbert, and G.M. Pharr, “Compressive strengths of molybdenum alloy micro-pillars
prepared using a new technique” Scripta Materialia 57 (2007) 397–400

3. Z. W. Shan, J. Li, Y. Q. Cheng, A. M. Minor, S. A. Syed Asif, O. L. Warren, E. Ma
“Plastic flow and failure resistance of metallic glass: Insight from in situ compression of nanopillars” Phys. Rev. B  77, 155419 (2008)