Elevated fatigue crack growth resistance of Mo alloyed CoCrFeNi high entropy alloys

Fatigue crack growth resistance of powder metallurgy (PM) CoCrFeNiMo0.2 and CoCrFeNi high entropy alloys (HEAs) is investigated in this paper. The effect of minor element alloying on fatigue properties is explored in order to promote long fatigue lifetime design for these promising engineering structural materials. The fatigue crack growth tests are conducted on single-edge notched tensile (SENT) thin plate specimens by in-situ scanning electron microscope (SEM) fatigue experiment system at room temperature. The cyclic cumulative irreversible strain evolution at crack tip is characterized using the digital image correlation (DIC) technique in SEM. The results show that CoCrFeNiMo0.2 exhibits a relatively superior fatigue crack resistance with respect to CoCrFeNi. A lower fatigue crack growth rate and less crack tip plastic damage accumulation at the same effective driving force are found in CoCrFeNiMo0.2 which are associated with an enhanced slip reversibility. The enhanced slip reversibility is attributed to the improved slip planarity induced by the addition of Mo element which leads to a combination of decreasing the stacking fault energy (SFE) and increasing the lattice friction stress and shear modulus. In addition, stacking fault mediated deformation mechanism in CoCrFeNiMo0.2 plays an effective role in inhibiting fatigue-induced plastic deformation. This study provides a fundamental understanding of fatigue crack growth mechanisms in the studied HEAs, and offers a certain guiding significance on alloying design to improve fatigue resistance.