Direct observations of incipient plasticity during nanoindentation of Al
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E.A. Stach National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, Berkeley, California 94720
J.W. Morris, Jr. Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, and Department of Materials Science and Engineering, University of California, Berkeley, California 94720 (Received 16 June 2003; accepted 23 September 2003)
The mechanical testing technique for in situ nanoindentation in a transmission electron microscope is described and is shown to provide real-time observations of the mechanisms of plastic deformation that occur during nanoindentation. Here, the importance of this technique was demonstrated on an aluminum thin film deposited on a single-crystalline silicon substrate. Significant results include direct observation of dislocation nucleation, characterization of the dislocation distribution created by indentation, and the observation of indentation-induced grain boundary motion. The observations achieved by this technique provide unique insight into mechanical behavior studied with conventional instrumented nanoindentation techniques and also provide microstructural-level understanding of the mechanics of ultrasmall volumes.
I. INTRODUCTION
Conventional nanoindentation techniques have been developed over the past 20 years to probe the mechanical properties of materials at the submicrometer length scale.1,2 Such experiments have far-reaching importance to the microelectronics industry and to nanotechnology in general. A multitude of behaviors have been observed with conventional indentation techniques, (e.g., Nanoindenter XP, MTS Corp., Minneapolis, MN), but direct observation of the deformation is importantly lacking. As reported below, the direct observation of indentation behavior has now been achieved through in situ nanoindentation in a transmission electron microscope. This technique provides unique insight into the mechanisms associated with indentation-induced deformation. For example, a phenomenon whose source is widely debated in the nanoindentation community is the appearance of a discrete jump in displacement during loading. This so-called pop-in effect is alternately ascribed to the nucleation of dislocations,3–5 the aggressive activation and multiplication of preexisting dislocations,6 or the fracture of native oxide or other surface films.7 While all three mechanisms may be important in particular cases, nanoindentation results are sensitive to the nucleation or motion of individual dislocations; because the sharp indenters typically used in nanoindentation experiments 176
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J. Mater. Res., Vol. 19, No. 1, Jan 2004 Downloaded: 17 Mar 2015
can result in contact radii of the order 100 nm and displacements of the order several nanometers, the resolution of the measured displacements are of the order Burger’s vector. Additionally, the indented volume is often dislocation-free because the dislocation density of well-annealed single crystals can be as low as 103/cm2.8 Therefore, nanoindentation e
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