*2.4. Micro-Compression Test in SEM*

The in-situ compression tests were carried out in high vacuum at room temperature in a Carl Zeiss ΣIGMA series SEM, by an in-situ nanoindenter (UNAT-SEM II) which had a load noise floor level at about 0.002 mN. The micro-pillars were compressed by a diamond flat punch. Furthermore, in order to control the orientation of the micro-pillar well for a pure compression test, such as the loading direction parallel to GB and the top surface of sample perpendicular to flat punch, an additional rotation stage for the sample holder performing rotations in two dimensions was developed. Because the original sample holder could only rotate in one direction, it was necessary to combine it with a complementary rotation stage with two more degrees of freedom. All the compression tests were carried out in displacement controlled mode with several cycles of loading and unloading with small increment as shown in Figure 2a for Ni and in Figure 2b for *α*-Brass. The idea of loading and unloading cycles is to have enough time to make an SEM picture of high quality in order to well observe slip lines. Once slip lines were observed or yield stress was reached, the compression test was stopped during

the unloading step in order to prevent from the extra force on the pillar coming from the tip vibration when the test was manually interrupted. The loading force and corresponding displacement were recorded during the mechanical tests, so that the stress–strain curves can be calculated after the tests. Even though the cyclic loadings were performed with small increments, the tests were stopped when the first slip event was activated. The first slip event was identified when slip lines were observed in SEM or when yield stress of the bi-crystal was reached. Thus, only the last cycle (marked as red line in Figures 2a,b) contributed to plastic deformation, and all the previous cycles were in the elastic state.

**Figure 2.** Loading curves obtained with displacement control mode for the compression tests of (**a**) Ni and (**b**) *α*-Brass. Only red lines contribute to plastic deformation, and all the previous cycles are in the elastic state. Stress–strain curves were corrected by CPFEM simulations for (**c**) the Ni micro-pillar and (**d**) the *α*-Brass micro-pillar. The applied stress at the end of the compression test is about 289.4 MPa for the Ni sample and 127.4 MPa for the *α*-Brass sample.
