The effect of grain size on the low-cycle fatigue behaviours of a CrMnFeCoNi high entropy alloy

Abstract

High-entropy alloys (HEAs) alloys are alloys formed by at 5 or more elements, which have high strength, excellent ductility, and wear, corrosion and creep resistance. While the above-mentioned properties are very important and widely studied, fatigue properties are a more critical issue in most industrial applications. Fatigue causes over 90% of failure. Yet, the fatigue properties of HEAs have been less investigated. So, it worth to study the fatigue properties. It has been well-known that grain size plays a critical role in determining the mechanical properties of HEAs. However, the grain size effects on fatigue properties of HEAs has not been clear. This project aims to study the effect of grain size on the low-cycle fatigue (LCF) properties of a CrMnFeCoNi HEA. Results show that the fatigue life increased with decreasing grain size and/or decreasing strain amplitude. Fatigue-induced structural evolution of the HEA was complicated. Deformation of samples with fine and coarse grain sizes occurred mainly via planar slip at low strain levels, while wavy slip dominated the deformation of samples with the intermediate grain size. Planar slip was suppressed at a high strain regime. Dislocation cell structures, which are classic wavy slip microstructures, were commonly seen in intermediate grains at the high strain regime. The fatigue cracking behaviour at twin boundaries (TBs) in the HEA was also explored. Irrespective of grain size, the change from slip band cracking to TB cracking occurred with increasing the difference in the Schmid factors between matrix and twin. However, the required critical difference of Schmid factors for the transition of the dominant cracking mode decreases with decreasing grain size due to the reduced slip band spacing that increases the impingement sites on the TBs and facilitates the coalescence of defects and voids to initiate TBs cracks