Abstract: Precipitation-strengthened high entropy alloys (HEAs) exhibit excellent strength-ductility combinations due to precipitation hindering dislocation gliding and work hardening ability of the matrix. However, the effect of compositions on the microstructure and related deformation mechanism of HEAs is still unclear. In this study, we developed two types of L1₂-strengthened Al₅Ti₈Fe_x(CoNi)_86.9-xB_0.1 (x = 17, 28) HEAs to study the effect of Fe content on the deformation mechanism. Our results reveal that an increased Fe concentration substantially increases the strength and ductility of Al₅Ti₈Fe_x(CoNi)_86.9-xB_0.1 HEAs at room temperature. For the Al₅Ti₈Fe₁₇(CoNi)_69.9B_0.1 HEA, the presence of a large amount of ordered L1₂ phase leads to strain strengthening governed by dynamically refined slip bands. For the Al₅Ti₈Fe₂₈(CoNi)_58.9B_0.1 HEA, the increasing Fe content raises the stacking fault energy of the matrix and reduces the stability of the FCC matrix, making it less stable than the BCC structure. Additionally, the reduced volume fraction of the ordered L1₂ precipitated phase and the increased stack fault energy of the FCC matrix lead to an increase in the cross-slip frequency during deformation, which in turn promotes avalanche glide of dislocations on highly stressed crystallographic slip planes and the generation of microbands. The microbands and phase transformation inside the microbands promote the strain strengthening, resulting in enhanced strength and ductility. These findings clarify the effect of the Fe content on the deformation behaviours and provide new insight into the formation mechanism of microbands in precipitation-strengthened HEAs, which will open new avenues for the design of ultra-strong yet ductile alloys in the future.

Key words: L1₂-strengthened, High-entropy alloys, Fe content, Microband-induced plasticity, FCC→BCC transformation