*3.1. Weak Hardening Associated with One Activated Slip System*

The self-interaction of dislocations refers to the interaction between dislocations belonging to the same slip system. To study the mechanical response of micropillars with one dominant slip system, we selected grains with specific orientation in a polycrystalline sample according to the SF analysis on each slip system with Euler angle measured from EBSD mapping. As shown in Figure 2a, we chosen a grain with the orientation where [5 1 5] direction is parallel to the loading direction. Under uniaxial loading, the SF of (12-1)[-111] slip system reaches the maximum value of 0.5, the SF of (0-11)[-111] slip system is the second highest and equals 0.438, and SFs of other slip systems are lower than 0.428. Therefore, the (1-1)[-111] slip system would be primarily activated. Similarly, the grain with the orientation where [20 9 2] is parallel to the loading direction and shown in Figure 2b favors (10-1)[111] slip system, because this slip system has the maximum SF of 0.5, the second highest SF is 0.4683 associated with the slip system (101)[11-1], and SFs of other slip systems are lower than 0.435. The orientation of the two grains is demonstrated by the inverse pole figure in Figure 2c. Figure 2d,e

show the pillars with initial diameters of 8.9 μm and 8.5 μm. Figure 2f,g show the compressed pillars where the apparent slip traces well match the slip traces on (12-1) plane and (10-1) plane, respectively.

**Figure 2.** Electron backscatter diffraction (EBSD) IPF mappings showing the grains with the orientation of (**a**) [515] favoring slip system {112}<111> and (**b**) [20 9 2] favoring slip system {110}<111>. (**c**,**d**) Micropillars FIBed in the two grains. (**e**,**f**) Compressed micropillars and schematic diagrams with traces. (**g**) Stress-strain curves and strain hardening rates.

We thus concluded that (12-1)[-111] slip system and (10-1)[111] slip system were obviously activated in each pillar, respectively. Figure 2h shows the typical stress-strain curves. It is noted there is no apparent strain hardening behavior once continuous plastic flow occurs. Several slight stress drops correspond to shear instability due to free surface. The weak hardening effect is ascribed to the lack of dislocation pileup because the size of the pillar is too small to form dislocation pileup. Videos 1 and 2 record the in situ compression testing of the two pillars. Corresponding to the 0.2% offset yield strengths of 460 MPa and 450 MPa, the glide resistance of dislocations are estimated to be 230 MPa and

220 MPa for the two slip systems {12-1}<-111> and {10-1}<111>, respectively. The similar resistance for the two slip systems is consistent with our previous study [52]. The strain hardening rate is around 1.0 GPa for the two slip systems {12-1}<-111> and {10-1}<111> as the strain exceeds 0.015.
