In situ tensile testing of nanoscale freestanding thin films inside a transmission electron microscope

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M.T.A. Saif Department of Mechanical and Industrial Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801 (Received 19 November 2004; accepted 17 February 2005)

The unique capability of rendering opaque specimens transparent with atomic resolution makes transmission electron microscopy (TEM) an indispensable tool for microstructural and crystallographic analysis of materials. Conventional TEM specimens are placed on grids about 3 mm in diameter and 10–100 ␮m thick. Such stringent size restriction has precluded mechanical testing inside the TEM chamber. So far, in situ testing of nanoscale thin foils has been mostly qualitative. Micro-electro-mechanical systems (MEMS) offer an unprecedented level of miniaturization to realize sensors and actuators that can add TEM visualization to nano-mechanical characterization. We present a MEMS-based uniaxial tensile experiment setup that integrates nanoscale freestanding specimens with force and displacement sensors, which can be accommodated by a conventional TEM straining stage. In situ TEM testing on 100-nm-thick freestanding aluminum specimens (with simultaneous stress measurement) show limited dislocation activity in the grain interior and consequent brittle mode of fracture. Plasticity at this size scale is contributed by grain boundary dislocations and partial dislocations.

I. INTRODUCTION

The transmission electron microscope (TEM) is a versatile analytical tool for materials research.1 Its unique capabilities in visualizing microstructural features (defects, phases, precipitates and interfaces), diffraction patterns and in performing microanalysis of materials with atomic resolution have not only advanced the understandings in fundamentals of materials deformation, but also helped in synthesis and characterization of engineering materials. In situ TEM techniques are advanced level applications where the microstructures, defects, and their roles in deformation are directly observed as the experiment progresses. Progress in applying the TEM to realtime investigations2–7 has helped studies on deformation mechanisms in a wide variety of materials systems including defects8–14 and phases15,16 in bulk structures, interfaces,17,18 and nanocrystalline19–21 and nanostructured22,23 materials, all of which have contributed significantly to the state of the art in the structureproperty mechanics of materials.

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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0220 J. Mater. Res., Vol. 20, No. 7, Jul 2005

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The rapid developments in nanostructured materials have caused a resurgence of in situ TEM techniques, where visualization of experimental details becomes as important as measurement resolution. This is because of the modification of length-scale dependence of materials behavior at the submicron scale and even breakdown of established models for the ultrasmall (10 nm and below) grain sizes. As an example, breakdown of the Hall–Petch equation (which predicts

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