A Method for Extracting Quantitative Data During in-situ TEM Nanoindentation
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A method for extracting quantitative data during in-situ TEM nanoindentation A.M. Minor1,2, E.T. Lilleodden2 , E.A. Stach3 , and J.W. Morris, Jr. 1,2 1 Department of Materials Science & Engineering, University of California, Berkeley, CA; 2 Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA, 3 National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, Berkeley, CA. Abstract The development of a novel transmission electron microscope holder has made real time observations of nanoindentation possible. Using a piezo-ceramic loading mechanism, a diamond indenter is pushed into the surface of a sample, while the electron beam images the deforming sample in cross section. In this paper, we present the method for calibrating the forcedisplacement-voltage relation and load-frame compliance associated with this instrument. This allows quantitative force-displacement measurements to be obtained, in the manner of traditional indentation experiments. As an example of the utility of this technique, we present observations of the indentation behavior of an Al thin film on silicon, which have been previously shown [1]. Indentation into a coarse grain shows a displacement excursion corresponding to the nucleation of dislocations, and is compared to force-displacement responses measured with instrumented indentation techniques. Introduction Methods for the characterization of mechanical properties at nanometer length-scales are needed, since the dimensions of practical devices continue to shrink and the reliability of these devices is controlled in part by mechanical behavior. Scaling relations may breakdown as the microstructural length-scale tends toward zero, and this can have important implications on the analysis of material strength. For example, Gane and Bowden observed that “the resistance of a metal crystal to indentation when measured on a very small scale is different to that observed in conventional micro-hardness measurements” [2]. The volume of deformation associated with nanoindentation experiments provides the ability to investigate the fundamental processes associated with the initiation of plasticity in initially dislocation-free material. Discrete displacement bursts have been commonly observed in traditional nanoindentation experiments, where high-resolution force and displacement measurements allow calculations of the mean pressure and associated shear stresses applied to the indented sample. Although analyses of the stress occurring at the excursion point imply that such events are associated with dislocation nucleation, direct evidence of this mechanism is obtainable only in situ. Experimental Technique A sample holder for in situ nanoindentation in an electron microscope was developed previously [1,3,4]. A diamond indenter is rigidly attached to a piezo-ceramic crystal, which will displace when a voltage is applied to the crystal. The indentation is made perpendicular to the direction of the electron beam, allowing real-time imaging of the imposed deformation.
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