Micron-Scale Deformation 1077
(Hütsch & Lilleodden, 2014). Lathe milling uses an ion beam (almost) perpendicular to the axis of the micropillar. The substrate is rotated during milling, resulting in a cylindrical micropillar. Conversely, in annular milling, the pillar axis is parallel to the ion beam and the ions etch the surface of the substrate. Both methods have advantages and disadvant- ages. Lathe milling provides perfect control over the final geometry of the sample; however, there are significant effects from ion implantation on the mechanical properties of the micropillars. The annular method always produces tapered micropillars and the height is only loosely controlled. How- ever, the annular method is considerably faster and the negative effects of ion damage are much weaker (Hütsch & Lilleodden, 2014). The new method outlined here combines the advantages of both techniques. With this procedure, non-tapered micropillars can be milled anywhere on the surface of a bulk material, with well-defined geometry and preparation times comparable with the annular method (Wurster et al., 2015). The long-term goal is to determine the fundamentals
of plastic deformation at the micrometer scale. The new micropillar fabrication procedure presented in this work enables us to produce a high number of samples in a shorter time. This, in turn, allows for good statistical analysis of the erratic deformation behavior of micropillars arising from the stochastic response of dislocation ensembles to the acting force (Miguel et al., 2001, 2002; Dimiduk et al., 2006; Zaiser, 2006; Ispánovity et al., 2010). Thus, even though a single measurement cannot provide important material parameters, this information can be derived from extensive systematic testing (Ispánovity et al., 2010, 2013). Introductory compression tests were performed on
Al-5%Mg alloy micropillars fabricated onto the surface of the bulk material. Importantly, this alloy exhibits the Portevin–Le Chatelier (PLC) effect (Tabata et al., 1980; Chinh et al., 2000; Gubicza et al., 2004; Yilmaz, 2011). In this type of bulk sample, the intermittent stress–strain response originates in the repeated pile-ups and break-outs (pinning and unpinning) of dislocations from the atmosphere of solute atoms where these atoms act as obstacles for mobile dislocations (Gubicza et al., 2004; Yilmaz, 2011). This mechanism generates strong acoustic signals with typically greater energies than those caused by non-PLC dislocation avalanches during plastic deformation. As the stress drops caused by the PLC effect and dislocation avalanches lead to a measurable AE, the tests on this material allowed us to verify the sensitivity of our AE detection system.
SAMPLE PREPARATION
Before fabrication, the geometry and size of the micropillar must be decided. As this study focuses on collective dis- location phenomena, samples with high initial dislocation density are required. At the same time, the sample size should not be too large, which would lead to bulk deforma- tion, thus hindering the occurrence of clearly distinguishable stress drops. Normally, in fcc metals such as Al, the
dislocation density varies between 1011 and 1014m−2. Therefore, it can be derived that the average spacing between dislocations is ~0.1–3 µm. As the dislocations tend to form cell-like structures with a characteristic size of approximately 10 times the dislocation spacing, the pillar size is selected to be on the order of the cell size. It should be noted that in deformed micropillars the cell size may also depend on the sample size; nevertheless, the cell size-to-dislocation spacing ratio remains ~10 (Yu et al., 2014; El-Awady, 2015). In situ micropillar deformation tests demand very careful
and precise sample
preparation.To obtain the required surface properties, orientation, and initial dislocation density, the following steps were performed. After a short etching, the Al-5%Mg bulk substrate was electropolished in perchloric electrolyte D2 solution with a 60mA/mm2 current density. The lattice orientation was measured using electron back- scatter diffraction (EBSD). The sample was cut with an electric dischargemachine to have the normal surface oriented parallel to the <123> direction. Next, further electropolishing was performed, followed by a heat treatment for 72h at 200°C. Finally, the surface was electropolished again with a 30mA/mm2 current density, and the orientation was checked again with EBSD. The sample was predeformed along the <123> direction with a load of 20MPa. An initial dislocation density of 2×1013m−2 was measured with transmission electron microscope and X-ray line profile analysis.With this value, a pillar geometry with a rectangular cross-section of 4×4µm2 and a height of 12 µm was selected, corresponding to an aspect ratio of 3:1:1, which was commonly applied in earlier studies. After the abovementioned sample preparation
processes, a “surrounding hole” was milled by a 30nA ion current around the micropillar. The FIB milling pattern is marked by the grid in Figure 1a. The sample was oriented so that the normal vector of the surface was parallel to the ion beam direction. A thin Pt layer was then deposited onto the top surface of the micropillar. The cap ensures the ion beam will fabricate a smooth side surface on the pillar. Moreover, due to its amorphous structure, the cap is very hard; hence it helps (eliminate the effects related to a possible misalign- ment between the surfaces of the compressing tip and the sample during the compression test. Therefore, the stress becomes more homogeneous below the Pt layer than at the contact between the tip and the sample surface. Next, the stage is tilted by 7°. In combination with the 52° ion–electron beam angle, this results in a 45° ion beam direction with respect to the surface normal. To ensure precise and easy positioning of the scanning electron microscope (SEM) stage, a small cross (2 µm) was imprinted onto the geometric center of the top surface of the micropillar. Afterwards, the individual milling steps were performed. These steps are explained in detail in Figure 2. To further decrease tapering (Li et al., 2006), a final “polishing” step was performed with 100pA ion beam over-tilted by 1° with respect to the pillar axis (normal to the top surface of the pillar). Apart from the first step explained above, the entire fabrication is performed with high milling angles.
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