Microsc. Microanal. 23, 1091–1095, 2017 doi:10.1017/S1431927617012703
© MICROSCOPY SOCIETY OF AMERICA 2017
Selectively Electron-Transparent Microstamping Toward Concurrent Digital Image Correlation and High-Angular Resolution Electron Backscatter Diffraction (EBSD) Analysis
Timothy J. Ruggles,1,* Geoffrey F. Bomarito,2 Andrew H. Cannon,3,4 and Jacob D. Hochhalter2
1National Institute of Aerospace, Hampton, VA, USA 2Langley Research Center, National Aeronautics and Space Administration, Hampton, VA, USA 31900 Engineering, LLC, Clemson, SC, USA 4Department of Chemical and Biomolecular Engineering, Clemson University, Clemson, SC, USA
Abstract: Digital image correlation (DIC) in a scanning electron microscope and high-angular resolution electron backscatter diffraction (HREBSD) provide valuable and complementary data concerning local deformation at the microscale. However, standard surface preparation techniques are mutually exclusive, which makes combining these techniques in situ impossible. This paper introduces a new method of applying surface patterning for DIC, namely a urethane microstamp, that provides a pattern with enough contrast for DIC at low accelerating voltages, but is virtually transparent at the higher voltages necessary for HREBSD and conventional EBSD analysis. Furthermore, microstamping is inexpensive and repeatable, and is more suitable to the analysis of patterns from complex surface geometries and larger surface areas than other patterning techniques.
Key words: high-angular resolution EBSD, EBSD, digital image correlation, lithography, microstamping
INTRODUCTION High-angular resolution electron backscatter diffraction (HREBSD) can determine the local elastic strain and geo- metrically necessary dislocation (GND) content of crystal- line materials (Jiang et al., 2016; Ruggles et al., 2016b). Digital image correlation (DIC) performed in a scanning electron microscope (SEM) allows for microscale resolution of the total local deformation (Gupta et al., 2014; Yan et al., 2015). Data from these microscopy techniques can be used to vali- date and develop high fidelity plasticity models (Lim et al., 2014, 2015; Zhang et al., 2014; Dingreville et al., 2016). The information from DIC (total strain) and HREBSD (stress and GND density) is complementary, providing a decom- position of the elastic and plastic behavior of a crystalline material at themicroscale, which offers extended insight into model development. However, these characterization tech- niques have been mutually exclusive on the same surface at the same length scale, given the current state of sample preparation necessary for each technique. A sample well polished for EBSD generally does not contain sufficient fea- tures for DIC measurement, and patterns for DIC disrupt EBSD diffraction. This has limited the use of EBSD on pat- terned surfaces to initial microstructural characterization, sampling between speckles of the pattern at a higher length scale, and postmortem analysis. This paper introduces a new method of DIC pattern application, microstamping, which
*Corresponding author.
timothy.ruggles@
nasa.gov Received September 6, 2017; accepted October 16, 2017
leaves a pattern thin enough to be sufficiently transparent to EBSD at high accelerating voltages, but still capable of provid- ing enough contrast for DIC. This technique allows for the simultaneous collection of HREBSD and DIC data in situ. There are a number of established methods of pattern-
ing the sample for microscale DIC, that have variable com- patibility with EBSD techniques. Some patterning techniques degrade EBSD pattern quality but are nondamaging to the surface and removable, allowing for postmortem EBSD ana- lysis. Nanoparticle decoration and lithography are two such methods (Kammers & Daly, 2011; Di Gioacchino & Quinta da Fonseca, 2013; Gupta et al., 2014; Yan et al., 2015; Githens & Daly, 2017). It should be noted that EBSD patterns sometimes can be collected in spite of these patterning techniques, most notably in the case of colloidal silica nanoparticles, because they are small, amorphous and have a low effective atomic mass (Yan et al., 2015). This allows the recovery of the underlying grain structure. However, the EBSD pattern quality degrades. The extent to which this affects HREBSD methods, which require a greater precision than conventional indexing software, has not been reported. Etching (Ruggles et al., 2016a) and focused ion beam (FIB) patterning (Choi et al., 2014) are also available methods of applying a DIC pattern, but these methods are destructive and preclude the use of EBSD methods. This paper proposes residual layer urethane micro-
stamping as a method for concurrent HREBSD and DIC because of its selective transparency to electrons of different energy. A low atomic number, amorphous material is applied such that it is thin enough to be effectively
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