*3.2. Intermediate Hardening Associated with Two Activated Slip Systems*

Micropillars with two orientations were prepared to study the interactions between two {1-21}<111> slip systems and between two {110}<111> slip systems, respectively. When the grain is orientated along the crystallographic direction [1 48 8], the (1-21)[111] and (-1-21)[-111] slip systems have the largest SFs of 0.494 and 0.487, and other slip systems have the SF smaller than 0.462. Figure 3a,d show two micro-pillars with diameters of 5.8 μm and 8.7 μm. Figure 3g shows the stress-strain curves, revealing apparent strain hardening with a hardening rate of ~1.9 GPa for the strain exceeding 0.02. Figure 3h schematically show two slip traces associated with the slip planes (1-21) and (-1-21). Apparent slip traces are identified to be associated with dislocation motion on the slip plane (1-21), as observed in the compressed pillars in Figure 3b,e. The slip trace associated with the slip plane (-1-21) was not obvious. Thus, we magnified the red square regions in Figure 3b,e' which is the backside of the pillar. The high magnification images in Figure 3c,f reveal the slip traces associated with dislocation motion on the slip plane (-1-21). Thus, the slightly increased strain hardening rate is ascribed to the junction formation interaction between (1-21)[111] and (-1-21)[-111] slip systems. Video 3 records the in situ compression testing of the pillar with a diameter of 8.7 μm. Corresponding to the 0.2% offset yield strength of 430 MPa in Figure 3g, the critical resistance for dislocation glide on {121} slip planes is calculated to be 212 MPa, which is consistent with the result obtained from Section 3.1.

**Figure 3.** (**a**,**b**) Micropillars with a diameter of 5.8 μm before and after compression, showing apparent slip trace associated with slip plane (1-21); (**c**) the magnified image of the red square in (**b**), showing slip traces associated with slip plane (-1-21). (**d**,**e**) The micro-pillar with a diameter of 8.7 μm before and after compression; (**e'**) the back-side of the image (**e**); (**f**) The magnified image of the red square in (**e'**), showing slip traces associated with two slip planes (1-21) and (-1-21). (**g**) Stress-strain curves. (**h**) A schematic showing slip traces associated with slip planes (1-21) in blue and (-1-21) in green.

When the grain is orientated along the crystallographic direction [-6 9 -5], the (01-1)<1-1-1> and (-110)<-1-11> slip systems have the largest SFs of 0.399 and 0.344, and other slip systems have the SF smaller than 0.31. Figure 4a and d show two micro-pillars with diameters of 5.9 μm and 8.9 μm. Figure 4h schematically show two sets of slip traces associated with the slip planes (0 1 -1) and (-1 1 0). Apparent slip traces are associated with dislocation motion on the two slip planes (0 1 -1) and (-1 1 0), as observed in the compressed pillars in Figure 4b,e and the magnified images in Figure 4c,f. Especially in Figure 4c, the two slip systems seem equally activated. The stress-strain curves in Figure 4g show apparent strain hardening with a hardening rate around 4.0 GPa for the strain exceeding 0.02. Thus, the strain hardening rate is ascribed to the junction formation interaction between (01-1)[1-1-1] and (-110)[-1-11] slip systems, and is greater than the case associated with the interaction between two {110}<111> slip systems where one slip system is more activated. Video 4 records the in situ compression testing of the pillar with a diameter of 5.9 μm. Corresponding to the 0.2% offset yield strength of 580 MPa in Figure 4c, the critical resistance for dislocation glide is 231 MPa for (01-1)[111] slip family, which is consistent with the results in Section 3.1.

**Figure 4.** (**a**,**d**) Micro-pillars with a diameter of 5.9 μm and 8.9 μm. (**b**,**e**) The compressed micro-pillars. (**c**,**f**) The magnified images of the compressed micro-pillars, showing apparent slip traces. (**g**) Stress strain curves. (**h**) A schematic showing traces associated with slips on (-110) plane in green and (01-1) plane in blue.
