Effective Use of Focused Ion Beam (FIB) in Investigating Fundamental Mechanical Properties of Metals at the Sub-Micron Scale

Abstract

AbstractRecent advances in the 2-beam focused ion beams technology has enabled researchers to not only perform high-precision nanolithography and micro-machining, but also to apply these novel fabrication techniques to investigating a broad range of materials' properties at the sub-micron and nano-scales. In our work, the FIB is utilized in manufacturing of sub-micron cylinders, or nano-pillars, as well as of TEM cross-sections to directly investigate plasticity of metals at these small length scales. Gold nano-pillars, ranging in diameter between 200 nm and several micrometers were fabricated from bulk gold and epitaxial gold films on MgO substrates and subsequently compressed using a Nanoindenter fitted with a custom-fabricated diamond flat punch. We show convincingly that fundamental mechanical properties like flow stress, yield strength, and stiffness strongly depend on the sample size, as some of our smaller specimens were found to plastically deform in uniaxial compression at stresses as high as 800 MPa, a value ~50 times higher than for bulk gold.We believe that these high strengths are hardened by dislocation starvation. In this mechanism, once the sample is small enough, the mobile dislocations have a higher probability of annihilating at a nearby free surface than of multiplying and being pinned by other dislocations. Therefore, plasticity is accommodated by the nucleation and motion of new dislocations rather than by interactions of existing dislocations, as is the case for bulk crystals. To validate this mechanism, direct observation of dislocations was accomplished by utilizing the Omniprobe micromanipulator, coupled with FIB-milling and Pt deposition, for fabrication of site-specific TEM specimens. Preliminary TEM images show the lack of mobile dislocations in deformed pillars, which agrees with the proposed dislocation starvation mechanism, as discussed.