*2.2. Sample Preparation and Characterization*

Macrosamples were prepared in the form of cubes of 20 × <sup>20</sup> × 5 mm3. These macrosamples were ground with grit paper and then homogenized by heat treatment. For Ni, the produced samples were homogenized at 1100 ◦C for three days in vacuum. However, for *α*-Brass, in order to prevent dezincified phenomenon in vacuum [36], the heat treatment was realized in argon at 980 ◦C for 1 min with a special sample holder. Then, the samples were ground again and polished using a diamond suspension of particle sizes until 1 micron on zeta cloth. After that, the sample surfaces were finally polished by OP-U-NonDry solution with water on chem-cloth for 5 mins. At the end, the samples were electropolished at a voltage of 24 V for 20 s with electrolyte A2 (65∼86% Ethanol, 10∼15% 2-Butoxyethanol and 5∼15% water) for Ni and with electrolyte D2 (15∼35% Phosphoric acid, 15∼25% Ethanol, <10% Propane-1-OL, <1% urea and 50∼70% water) for *α*-Brass in order to eliminate hardening and damage (including scratches) occurred during the mechanical polishing.

Each grain orientation was measured by EBSD (Oxford, UK) in a Carl Zeiss SIGMA series SEM (Oberkochen, Germany). Orientation maps were acquired with a 20 kV acceleration voltage and a spatial step size of 20 μm. Data processing was performed by Flamenco Channel 5 software (HKL Technology, Oxford Instruments, Abingdon, UK) with the indexing rate always greater than 99%. In order to investigate dislocation pile-ups behavior and their interaction with GBs by AFM measurements and slip lines characterization on the upper surface, special GBs were chosen from the hereinafter described conditions following the surface notations presented in Figure 1b,d. The first condition is that GB is roughly perpendicular to the upper surface. The second one is that with a mechanical loading parallel to GB plane, for the target grain, the Burgers vector of the likely activated slip system (i.e., the one with the maximum Schmid factor) must not be perpendicular to GB. Meanwhile, the slip plane of this slip system must not be parallel to the upper surface. These conditions ensure that the Burgers vector has a component perpendicular to the upper surface where AFM measurements will be performed. Based on all the above conditions, one GB of each material was used to analyze the dislocation distributions and the slip transmission phenomenon.
