![]() There are three different methods developed so far to use the stamp to apply a pattern once it is fabricated. Selection of the specific elastomer largely depends on the desired method of application. The material is vacuum cast to the master, expelling all entrapped gas and allowing the material to flow into the sub-micron features of the mold and then polymerized. For the stamp, a castable elastomer is selected which must be soft and flexible to gently peel away from the master and be useful for pattern reproduction on curved substrates while having sufficient toughness to avoid tearing at the corners of the micro features of the stamp. Once the mold is fabricated, many stamps can be replicated from it inexpensively. Rather than a random pattern, a pattern was optimized to minimize correlation error consistent with Bomarito et al. The stamp mold used in this study is 1 × 1 mm with a speckle size of 1 micron fabricated with photolithography. It has been observed that a 500 nm feature depth is ideal for microstamping. For sub-micron speckle sizes, the molds should be fabricated using e-beam lithography (EBL) or FIB machining to achieve sufficient resolution in the speckle edges. Standard methods for photolithography work well for fabricating these molds quickly when the speckle sizes are greater than 1 micron. In this study, a selectively electron transparent microstamp with a 1 micron speckle size is applied to an aluminum oligocrystal, and its effect on HREBSD techniques and its compatibility with HRDIC are assessed.įlexible microstamps are fabricated by first creating a rigid master mold and then applying a polymer using a vacuum casting process. A low atomic number, amorphous material is transferred such that it is thin enough to be effectively transparent to the higher energy electrons used for EBSD (typically 20–30 keV), but thick enough to give sufficient contrast for HRDIC at low accelerating voltages (5 keV for this study). This paper proposes a residual layer urethane rubber microstamping as a method for concurrent HREBSD and HRDIC because of its selective transparency to electrons of different energy. Etching ( Ruggles et al., 2016a) and focused ion beam (FIB) patterning ( Choi et al., 2014) are also available methods of applying a HRDIC pattern, but these methods are destructive and completely preclude EBSD methods. However, the EBSD pattern quality degrades, making this method potentially problematic for HREBSD techniques, which require a greater precision than commercially available indexing software. It should be noted that EBSD patterns sometimes can be collected despite these patterning techniques, most notably colloidal silica nanoparticles because they are small, amorphous and have a low effective atomic mass ( Yan et al., 2015). Nanoparticle decoration and lithography are two such methods ( Kammers & Daly, 2011). Some patterning techniques degrade EBSD pattern quality but are non-damaging to the surface and removable, allowing for post mortem EBSD analysis. There are a number of established methods of patterning the sample for microscale HRDIC, which have variable compatibility with EBSD techniques. This technique allows for the simultaneous collection of HREBSD and HRDIC data in situ. This paper introduces a new method of HRDIC pattern application, microstamping, that leaves a pattern thin enough to be mostly invisible to EBSD at high accelerating voltages, but still capable of providing enough contrast for HRDIC. This has limited the use of EBSD on HRDIC patterned surfaces to initial microstructural characterization, sampling between speckles of the pattern at a higher length scale, and post mortem analysis. A sample well polished for EBSD does not contain sufficient features for HRDIC measurement, and applied patterns for HRDIC disrupt EBSD diffraction. However, these characterization techniques have been mutually exclusive on the same surface at the same length scale, given the current state of sample preparation necessary for each technique. The information from HRDIC (total strain) and HREBSD (stress and GND density) is complementary, together providing a decomposition of the elastic and plastic behavior of a crystalline material at the microscale, which offers extended insight into model development. Data from these microscopy techniques can be used to validate and develop high fidelity plasticity models ( Lim et al., 2014 Zhang et al., 2014 Lim et al., 2015 Dingreville et al., 2016). Recent advances in two scanning electron microscopy (SEM) characterization techniques high resolution electron backscatter diffraction (HREBSD) and high resolution digital image correlation (HRDIC), allow for microscale resolution of the total local deformation (HRDIC) ( Yan et al., 2015 Gupta et al., 2014), and the local elastic strain and geometrically necessary dislocation (GND) content (HREBSD) ( Jiang et al., 2016 Ruggles et al., 2016b) of crystalline materials. ![]()
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