Enhancing fatigue resistance of Cr-Mn-Fe-Co-Ni multi-principal element alloys by varying stacking fault energy and sigma (σ)-phase assisted grain-size reduction

Abstract

This study investigates two key aspects of the low cycle fatigue (LCF) behavior of alloys from the Cr-Mn-Fe-Co-Ni system at room temperature: (1) the influence of stacking fault energy (SFE) in single-phase face-centered cubic (FCC) alloys and (2) a grain size reduction triggered by the precipitation of a small amount of σ-phase. The first effect is investigated using model alloys (Cr₂₆Mn₂₀Fe₂₀Co₂₀Ni₁₄ and Cr₁₄Mn₂₀Fe₂₀Co₂₀Ni₂₆ in at.%, grain size: ∼60 µm), which have distinct SFEs at room temperature. A reduction in SFE from 69 to 23 mJ/m² results in a 10 to 20 % increase in tensile/compressive peak stresses, i.e., cyclic strength, across all examined strain amplitudes (±0.3 %, ±0.5 %, and ±0.7 %) while maintaining comparable fatigue lives. Despite its higher cyclic strength, the low-SFE alloy exhibits delayed, and less evolved dislocation substructures than the other alloy. In both single-phase alloys, fatigue cracks originated from the surface reliefs, surface-exposed coherent annealing twin boundaries, and occasionally from high-angle grain boundaries. However, the crack propagation rate was slower in the low-SFE alloy, contributing to its superior fatigue resistance. By aging the low-SFE Cr₂₆Mn₂₀Fe₂₀Co₂₀Ni₁₄ alloy differently, we could induce the precipitation of ∼5 % σ-phase during recrystallization, which strongly reduced the FCC grain size to ∼5 µm. With this microstructure, the cyclic strength increased by 50–65 % and remained more stable during fatigue testing while maintaining a comparable life. The σ-precipitates were found to deflect and arrest fatigue cracks, while extensive deformation twinning around cracks complements slip activity and reduces crack propagation rate. Overall, the σ-phase-assisted grain size reduction is 3 to 5 times more effective in improving cyclic strength than SFE reduction.