Impact of Anisotropy and Strengthening Mechanisms on the Fatigue Behaviour of Laser Powder Bed Fusion Processed (FeCoNi)₈₆Al₇Ti₇ High-Entropy Alloy

Abstract

The structural integrity of additively manufactured components is frequently limited by anisotropic fatigue properties. However, the influence of precipitation strengthening on this anisotropic behaviour in high-entropy alloys remains insufficiently understood. This study investigates the fatigue crack growth resistance of a laser powder bed fusion processed (FeCoNi)₈₆Al₇Ti₇ alloy. A pronounced anisotropy in the fatigue threshold (ΔKₜₕ) is observed, samples where the crack growth direction is at a 45° angle relative to the build direction (BD) exhibit a threshold approximately 40% higher than those at 0°, parallel, and 90°, perpendicular to BD. Examination of the fatigue crack surfaces near the ΔKₜₕ region has revealed a dominant step-wise intercellular crack growth with an appearance of cyclic plastic tearing in 45° samples, while transgranular and transcellular fatigue crack growth is dominant in samples with either 0° or 90° to BD. L2₁ particles have not been found to contribute to crack propagation, but TEM analysis reveals that dense dislocations have piled up in these particles in 45° samples and this is not the case in 0° or 90° samples. It is thus proposed that the macroscopic alignment of the cell boundaries in the 45° sample obstructs the crack advance despite the favourable activation of {111} slip systems. This geometric barrier forces the crack to follow a high-energy, intermittent inter-cellular and overall a step-wise crack growth path. Ultimately, it is concluded that L2₁ particles act as effective obstacles that induce dislocation pile-ups and force crack deflection, a mechanism activated in the 45° orientation that drives its high-energy fatigue propagation mode and enhanced damage tolerance.