*2.1. Materials*

The FeCrAl alloy tested in the study has a nominal composition of Fe-13Cr-5Al-2Mo-0.2Si-0.05Y (wt.%). The ingot of alloy was prepared by arc-melting under an argon atmosphere using elemental ingredients with a purity above 99.9 at. %. The ingot was homogenized, hot-rolled at 1100 ◦C, warm-rolled at 800 ◦C in three steps from its initial thickness ~5 mm to a final thickness ~2 mm and annealed at 800 ◦C for 1 h. The grain size of the materials is around 50 μm. The sample was mechanically polished with silicon carbide grinding papers from grit 800# to 4000# and electropolished in a solution of 10 mL perchloric acid and 90 mL methanol at −5 ◦C for 15 s at 15 V. The following SEM observation and FIB fabrication were conducted on a FEI Helios NanoLab 660 dual beam system. The orientation of polished sample was characterized by using electron backscatter diffraction (EBSD). The orientation information was collected with an EBSD detector from EDAX in this dual-beam system. The OIM software was used to control the data collection process, during which a step size of 0.5 μm was adopted.

Based on the orientation mapping from EBSD, we choose the grain with specific orientation where SF of slip systems with designed interaction are maximized. On these grains, we prepared micropillars by using FIB. The acceleration voltage of Ga<sup>+</sup> beam was 30 kV. The currents for initial cut and final cut are 64 nA and 0.24 nA respectively. The height-to-diameter ratio of each pillar was 1.5–2.5. The taper angle is within 2 to 5 degrees. The in situ mechanical tests were performed by using a Hysitron PI85 PicoIndentor equipped with a 20-μm flat punch tip. The tests were displacement-controlled while the loading rate was set to reach a strain rate of 10−<sup>3</sup> s<sup>−</sup>1. During in situ testing, the indenter was controlled to minimize the misalignment between the tip and the top surface of the pillars, and a minimum of three tests were performed for each type of pillar to ensure the reproducibility of interested phenomena. The in situ compression testing stops when obvious slip bands can be observed on the surface of the samples.

Slip trace analysis based on self-developed Matlab program is used to identify the activated slip system in all deformed micropillars. The true stress is estimated using a full width half maximum (FWHM) approach [22,46], where the diameter in the middle of the pillar is estimated from known (1) top surface diameter, (2) taper angle, and (3) total length of the pillar. The true strain is obtained by correcting the engineering strain with the Young's modulus correction formula for a pillar [47,48].
