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for Inhomogeniety and size of microstructure - a new paradigm in understanding deformation mechanism of nano crystalline metals

Inhomogeniety and size of microstructure - a new paradigm in understanding deformation mechanism of nano crystalline metals


Sub-micrometer scale material samples differ fundamentally from macroscopic ones by their large surface to volume ratio. These surfaces include interfaces between the grains of polycrystalline metals, interfaces between multilayers, as well as free surfaces. They interfere with the known deformation and failure mechanisms of macroscopic samples and may contribute new mechanisms which are not yet fully understood. In this talk we will describe a novel experimental apparatus to test nano scale free standing metal samples in-situ in Scanning or Transmission Electron Microscope. The apparatus is applied to test Aluminum and Gold films with grain sizes 20-200 nm and with thicknesses 50-200 nm under uniaxial tension. We find, as grain size decreases below 50 nm, elastic modulus decreases, and metals show non-linear elastic response with small plastic deformation. Films with average grain size of 50-100nm recover all of the plastic strain under macroscopic stress-free condition. This recovery shows three distinct activation energies. When the grain size is increased to 100-200nm, the films do not recover plastic strain, but they show strong Bauschinger effect during unloading, while still under tension. We hypothesize, inhomogeneities in the microstructure (grain size and orientation variations) create non uniform local stress fields during loading as relatively larger or favorably oriented grain deform plastically while smaller or unfavorably oriented grains accommodate the strain elastically. Upon unloading, the elastically deformed grains, in order to reduce their strain energy, induce reverse plasticity in the larger grains leading to time dependent strain recovery in films with 50-100 nm grain size, and Bauschinger effect in films with 100-200 nm grain size.


If time permits, we will also briefly discuss a recent experiment on fruit flies which suggests that mechanical tension in axons plays a key role in memory formation.

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