*2.2. Orientation Selection of Micro-Pillars*

Three families of slip systems, {110}<111>, {112}<111>, and {123}<111>, are usually observed in BCC materials, as depicted in Figure 1a. For most BCC metals (e.g., Fe, V, and W), {110}<111> and {112}<111> slip systems are generally activated but {123}<111> slip system is rarely activated at room temperature. {123}<111> slip system is observed in Na and K at room temperature [49] and in Fe at high temperature [50]. For BCC FeCrAl alloy, the predominant slip systems at room temperature are {110}<111>and {112}<111> slip systems [51,52].

There will be 300 combinations amongst {110}<111> and {112}<111> slip systems, but only 17 configurations of interactions are unique as considering the symmetry of crystal [27]. As depicted in Figure 1b, these dislocations interactions are correspondingly classified into four types, self-, dipolar-, junction-, and collinear-interactions. Most interactions are belonging to junction interaction where both slip plane and slip direction of two slip systems are different. Dipolar interaction is the interaction between dislocations on the same slip plane with different slip vectors, and thus only happen in {110}<111> slip systems because two slip directions are available on a slip plane {110}. Collinear interaction is the interaction between dislocations on two slip planes with same slip direction. Self-interaction refers to the interaction between dislocations on same slip system.

**Figure 1.** (**a**) Schematic diagram of three slip systems in a body centered cubic (BCC) unit cell; (**b**) Schematic diagrams show four categories of interactions between slip systems.

For single-crystal micropillars, strain gradients are generally lacking because of free surface [44,53]. When a single slip system is predominately activated in a micropillar, self-interaction is to a great extend reduced, and an obvious yielding and continuous shearing with weak hardening is generally observed. Thus, maximizing the Schmid factor (SF) of one specific slip system can be used to measure the glide resistance of dislocations associated with the slip system. Meanwhile, an apparent strain hardening behavior was expected to occur when the least two slip systems could be activated in the micropillar. Correspondingly, we choose the orientation of micropillars with the following criteria: (a) maximizing the SF of one slip systems in order to assess glide resistance of dislocations in a specific slip system, and (b) maximizing the SFs of two slip systems in order to study the effect of junction interaction on strain hardening. As a comparison, micropillars with multiple activated slip systems were fabricated and tested.

For bi-crystal micropillars, GBs generally act as pinning barriers for impeding dislocation propagation. Dislocation-GB interactions are heavily dependent on the transferability or continuity of slip systems in the two grains across a GB. Geometrical compatibility factor (m') is successfully used to estimate the deformation compatibility across GBs [54–58], where m'=cos(ϕ)×cos(k), ϕ is the angle between the slip plane normal directions and *k* is the angle between the slip directions. We chose bi-crystals with m' varying from 0.44 to 0.84.

## **3. Results and Discussion**
