*3.3. Strong Hardening Behaviors Associated with Multiple Activated Slip Systems*

To reveal hardening behaviors of micropillars associated with multiple activated slip systems, we fabricated two types of micropillars with the crystallographic orientations [100] and [111]. When the micropillars are oriented along the [100], four slip systems have comparable SFs around 0.47, including (-1-1-2)[11-1], (-1-12)[111], (-11-2)[1-1-1], and (-112)[1-11], as shown in Figure 5f. Figure 5a,c show the two pillars with a diameter of 5.8 μm and 8.5 μm, respectively. Figure 5b,d show the pillars at a

compression strain of 0.1. Figure 5e shows the stress strain curves. Although the 0.2% offset yield strength is approximately same about 430 MPa for the two micropillars, the strain hardening behaviors are different. The micropillar with the bigger diameter of 8.5 μm shows a high strain hardening rate varying from 20 GPa to 2.0 GPa as the strain increases from 0.01 to 0.02. A relatively stable strain hardening rate is 2.0 GPa as the strain exceeds 0.025. A homogenous deformation was observed before the strain reaches 0.02, indicating multiple slip systems were activated. Video 5 records the in situ compression testing of the pillar with a diameter of 8.5 μm. The pillar with a diameter of 5.8 μm shows a low strain hardening rate varying from 5.0 GPa to 2.0 GPa for the strain from 0.01 to 0.02. Video 6 records the in situ compression testing of the pillar with a diameter of 5.8 μm, showing homogenous deformation which implies the activation of multiple slip systems. The lower strain hardening rate is thus ascribed to the smaller diameter.

**Figure 5.** (**a**,**b**) The micropillar with a diameter of 5.8 μm before and after compression at a strain of 0.1. (**c**,**d**) The micropillar with a diameter of 8.5 μm before and after compression at a strain of 0.1. (**e**) Stress strain curves. (**f**) A schematic showing four slip systems, (-1-1-2)[11-1] in blue, (-1-12)[111] in green, (-11-2)[1-1-1] in red, and (-112)[1-11] in black.

A much stronger hardening behavior was observed in Figure 6a when the micropillar is oriented along the [111]. The strain hardening rate decreases from 35 GPa to 5 GPa as the strain increase from 0.01 to 0.05. Four slip systems, (1-1-2)[1-11], (1-2-1)[11-1], (10-1)[1-11] and (1-10)[11-1] as shown in Figure 6b, have comparable SFs in the range from 0.32 to 0.35, while other slip systems have SFs less than 0.29. Corresponding to the 0.2% offset yield strength of 600 MPa, these slip systems have the glide resistance of about 213 MPa which is consistent with the results from other tests. Figure 6c shows a dog-bone shaped micro-pillar with the cross-section of 3.3 μm × 3.0 μm. Figure 6d,e shows two snapshots at a strain of 0.075 and 0.085, respectively, indicating homogeneous deformation before cracking takes place. Video 7 records the in situ tension testing of the pillar, showing homogenous deformation during tension testing and implying the activation of multiple slip systems before the pillar failures at a strain of 0.08.

**Figure 6.** (**a**) Stress strain curve and strain hardening rate. (**b**) A schematic showing four slip systems, (1-1-2)[1-11] in blue, (1-2-1)[11-1] in green, (10-1)[1-11] in red, and (1-10)[11-1] in black. (**c**) A dog-bone micropillar. (**d**,**e**) Two snapshots at strain of 0.075 and 0.085, respectively.
