Extended abstract of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, USA, July 29 – August 2, 2012.
The enhanced industrial applications of new nanostructured materials, as well as the need to understand the behaviour, as well as the degradation processes, of older, more conventional, materials having nanometer size structures, necessitate an increased use of High Resolution Scanning Transmission Electron Microscopy (HR-STEM) for analytical analyses at the subnanometer level. One of the main advantages of a dedicated STEM, for in-situ mechanical experiments, is the possibility to shift instantly between three different observation modes: bright and dark field transmission modes and secondary electrons, greatly helping the understanding of complex structures. The main tools for the present work are a Hitachi HD-2700C cold FEG dedicated STEM with a CEOS aberration corrector that is equipped with a newly designed SDD EDS detector and a Hitachi NB5000 Focussed Ions Beam. These two tools, in combination with the use of traction and nano-indentation holders allow rapid observation of changes in material nanostructures and/or the propagation of crack lines upon the presence of different mechanical stresses. It is now possible to study in details the relation between the mechanical properties and the degradation mechanisms of materials such as steel alloys, used for example in the fabrication and repair of hydraulic turbines, with their particular complex nanostructures, looking principally at grain boundaries, phase transformation and crack propagation.As an example, Figure-1 shows nano-indentation experiments of a martensitic steel sample prepared by FIB, using a nano-indenter holder from Nanofactory. Rapid switching between bright field (BF) and dark field (ZC) were shown to help understand the structural transformations happening in this material. By moving the tip along the surface of the sample, Force vs Displacement curves, like the one in Figure-1c, can thus be obtained for very small structures. Moreover, since this nano-indenter holder can also go directly in the FIB, it offers the unique possibility to remove deformed regions at the surface of a sample and redo indentation measurements on subsequent areas as shown in Figure-2a and 2b. Finally, because of the high count rate in this dedicated STEM, the mechanical deformation observations can be further combined with fast chemical analyses to further comprehend the mechanical degradation mechanisms of these materials.But, in-situ mechanical traction experiments are also very important to identify the mechanical degradation mechanisms in such materials. Using FIB machining, two setups have been developed to performed mechanical traction on FIB thinned samples. In Figures 3a to 3d we show a setup using a normal copper TEM grid. The grid was cut at one place to allow the positioning of a FIB sample, which was subsequently thin down. The grid was then placed in a Gatan holder design for mechanical traction of bulkier samples. Also, using FIB machining we also prepared samples as presented in Figure-4a that can be placed in the Nanofactory nano-indenter for in-situ...
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