On the use of SEM correlative tools for in situ mechanical tests

Abstract

In situ SEM mechanical tests are key to study crystal plasticity. In particular, imaging and diffraction (EBSD) allow microstructure and surface kinematics to be monitored all along the test. However, to get a full benefit from different modalities, it is necessary to register all images and crystallographic orientation maps from EBSD into the same frame. Different correlative approaches tracking either Pt surface markings, crystal orientations or grain boundaries, allow such registrations to be performed and displacement as well as rotation fields to be measured, a primary information for crystal plasticity identification. However, the different contrasts that are captured in different modalities and unavoidable stage motions also give rise to artifacts that are to be corrected to register the different information onto the same material points. The same image correlation tools reveal very powerful to correct such artifacts. Illustrated by an in situ uniaxial tensile test performed on a bainitic-ferritic steel sample, recent advances in image correlation techniques are reviewed and shown to provide a comprehensive picture of local strain and rotation maps.

Measuring topographies from conventional SEM acquisitions

“…The reconstructed sample surface is identical to the inverse pole figure of the sample before Pt layer deposition, which can be found in Ref. [31]. It should be noted that due to the discrete nature of the FIB milling process, the topography reconstructed by this procedure has terraces, each of height 100 nm (or 1 pixel).…”

“…This is interpreted as a slow-scan direction error (due to the repositioning inaccuracy at the beginning of each new scanned line). An adapted global DIC procedure has been developed to correct this specific error by acquiring two images with different scanning directions [30,31], yet the algorithm cannot be applied to two images on the inclined sample by the current SEM. Without correcting the slow-scan direction error, the root mean square (RMS) average of u x shown in Figure 2 It should be noted that an uncertainty level of 8.1 nm is very satisfactory, with respect to the pixel size of 53.7 nm.…”

“…The tensile test sample described in Ref. [31] is used to validate the tilt-imaging method. The same Pt speckles described in Section 2.2 have been deposited onto the sample surface.…”

“…The SEM drift distortion for the aforementioned conditions have been analyzed in Ref. [31], and amount to approximately 1 % in both x and y directions. The uniform linear SEM drift distortion will not lead to topography measurement errors, since its effect will be removed in the linear fitting step.…”

“…The EBSD acquisitions are first rectified to correct the coordinate distortions [31], then analyzed by the rotation-integration method. Figure 14(a) shows the crystal rotation magnitude and the components of the rotation axis director between the reference state where the surface is again flat (i.e., initially polished surface), so that in the absence of plastic strain, the integrated displacement u z would be equal to the elevation.…”