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Tracking Microstructure Evolution in Complex Biaxial Strain Paths: A Bulge Test Methodology for the Scanning Electron Microscope

Abstract

In this work, a novel method is presented to track site-specific microstructure evolution in metallic materials deformed biaxially along proportional and complex strain paths. A miniaturized bulge test setup featuring a removable sample holder was designed to enable incremental measurements to be performed in a scanning electron microscope, by probing the same position on the sample at different deformation levels, with electron backscatter diffraction (EBSD), electron channeling contrast imaging (ECCI) and other imaging modes. Validation experiments were performed at room temperature on samples prepared from commercial sheet metal (dual-phase steel) and foils (stainless steel). Local strain measurements with the digital image correlation technique confirmed that proportional strain paths with a strain ratio up to 5 can be investigated using elliptical dies in the bulge test holder. It is also shown how complex strain paths can be obtained using a combination of overlapping elliptical dies. Incremental EBSD and ECCI were conducted in configurations relevant for the multi-scale investigation of localized plasticity and damage mechanisms in dual-phase steel. Quantitative information regarding microstructure evolution (phase fractions, orientation fields, dislocation structures, etc.) and regarding local strain distributions could be successfully obtained. This type of data sheds light on underlying deformation mechanisms and provides opportunities to calibrate crystal plasticity models.

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Notes

  1. Cruciform geometry enforces fracture to take place in the thinned center region of the sample, overriding local microstructural effects. The Marciniak test, on the other hand, creates a homogeneous multi-axially strained region where the fracture point is dictated by the microstructure.

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Acknowledgements

We gratefully acknowledge Michael J. Tarkanian, James Hunter, David Bono, Qilong Cheng and Jiyun Kang for their input in the design of the bulge test setup. Valuable help provided by Ashley Brown Raynal is gratefully acknowledged. The authors thank Pr. Tomasz Wierzbicki and Pr. Ian Hunter for their kindly support. Dr. Guilhem Martin is acknowledged for his assistance in the finalization of this manuscript. This work made use of the shared experimental facilities supported in part by the MRSEC program of the National Science Foundation under award number DMR – 1419807.

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Appendix 1: Development of a Thinning Process for DP Steel

Appendix 1: Development of a Thinning Process for DP Steel

DP600 and DP800 steel used in this study were non-commercial grades provided by Tata Steel (IJmuiden, the Netherlands) in the shape of 1.3 mm-thick sheets. The following thinning method was developed to prepare thin samples from the thick commercial sheets. The goal was to obtain EBSD-polished samples with a homogeneous thickness (typically 100 μm ±5 μm) to perform well-controlled bulge tests. The composition and overall properties of these two steels are provided in Table 2 for reference.

Table 2 Characteristics of the DP steel used in this work

The first preparation step (illustrated in Fig. 11(a)) consisted in cutting a 300 μm-thick sample out of the 1.3 mm-thick sheet using a wire EDM machine (Charmilles Robofil 1020SI). To perform a homogeneous cut, the sheet was clamped in a precision tool maker vise on both sides. Then, a disk of diameter 24 mm was cut out with the same process. As schematized in Fig. 11(b), the cutting introduces damaged layers on both surfaces which had to be removed by polishing. These layers bear compressive residual stress and make the polishing step a complex task as samples progressively bend when the damaged layer is removed from one side. A cross section observation of the damaged layer was performed to estimate its depth below 60 μm (cf. Figure 11(c)). As a safe target, 100 μm of mater was removed on both sides, by polishing, to obtain a sample with a microstructure fully representative of the original one.

Fig. 11
figure 11

Overview of the thinning process used to prepare polished samples from DP steel sheets. (a) Wire electrical discharge machining is used to extract 300 μm-thick disks from commercial sheet metal. (b) Schematic view of a disk cross-section at different stages of polishing. Damaged surface layers inherited from the cutting step are present initially as shown in inset (SE image). Thickness inhomogeneity and slight shape waviness results from the two-step polishing process. (c) Illustration of the experimental configuration used for automated polishing. (d) Profilometery measurements used to check the performance of the gluing process. The optical image shows a sample glued on a glass stub and polished on one side. Sacrificial rings were not used here to keep the glass surface as a reference. The profile measurements shown on the right were carried out along the two black lines

The following procedure gave the best results for polishing the sample while keeping a homogeneous thickness. First, sacrificial rings were cut out using a waterjet cutter from hardened stainless steel foils (yield strength of 1240 MPa). When the first side of the sample was polished, a 200 μm-thick foil was used whereas a 100 μm-thick one was used for the second side. One sacrificial ring was fixed with super glue on a borosilicate glass stub (McMaster-Carr) fitting in an automated polisher (cf. Figure 11(c)). The surface of the glass stub was extremely flat, within ±1μm (measurement realized with a stylus profilometer DekTakTX from Bruker). To glue the ring, the stub was positioned in a vertical precision vise, the glue was deposited, and then the ring, a foil of Teflon and finally a second glass stub before closing the vise. The Teflon foil keeps the glue from adhering on the second glass stub and can simply be removed by peeling it off. The sample was then glued in the center of the ring with the same procedure. The quality of the gluing was checked by looking at the transparent backside of the stub. Three glass stubs were inserted in an automated polishing machine (to maintain balance and avoid wobbling movements) and they were polished with a 9 μm diamond solution on a hard polishing cloth. The polishing step was stopped when the sacrificial ring (originally thinner than the sample) started to get polished on the whole surface. Conventional polishing steps were then used to reach a 1 μm polishing grade (first side) or a OPS polishing grade (second side). Three baths of acetone were used to unglue and clean the samples.

One difficulty we faced was to monitor the sample thicknesses as the thinning process introduces a slight waviness in their shape. This waviness is likely caused by released residual stress initially present in the rolled thick sheet. The challenge with profilometery techniques was to hold and flatten the sample properly (with glue or a magnet) and still get an absolute measurement of the thickness. In further experiments, one should consider using X-ray tomography to get a 3D visualization of the sample and hence decouple shape waviness from thickness variations. A mechanical profilometer (DekTakTX, Bruker) was used several times to make sure the top surface after gluing was parallel to the glass stub (cf. Figure 11(d)). However, because of the variability in the layer of glue deposited, an estimation of the absolute thickness by this method was hardly reliable. As a consequence, the thicknesses reported in Table 3 have been measured simply with a caliper at four points on the sample. For DP600 samples, polished without using sacrificial polishing rings but with a similar process, the scatter found was in the order of 10 μm. For DP800, the scatter found was in the order of 5 μm. This decreased in scatter is attributed to the stability and corrective mechanism provided by the sacrificial rings. Note that when only the central part of the sample is considered (the tested area), the thickness scatter reported in Table 3 can be considered an upper bound.

Table 3 Sample thicknesses evaluated after polishing. The * symbol indicates samples polished with a sacrificial ring following the procedure detailed above

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Plancher, E., Qu, K., Vonk, N. et al. Tracking Microstructure Evolution in Complex Biaxial Strain Paths: A Bulge Test Methodology for the Scanning Electron Microscope. Exp Mech 60, 35–50 (2020). https://doi.org/10.1007/s11340-019-00538-8

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Keywords

  • Biaxial loading
  • SEM
  • EBSD
  • ECCI
  • Micro-mechanics