Uncovering the critical factor governing the fatigue crack propagation under the coexistence of multi-interfaces in AlCoCrFeNi2.1 eutectic high-entropy alloy

Abstract

The AlCoCrFeNi2.1 eutectic high-entropy alloys (EHEAs) with a typical dual-phase lamellar microstructure can reach a good balance of strength and ductility, which have gained great attention recently. However, their high-temperature microstructural evolution and corresponding fatigue-crack growth (FCG) behavior have not yet been reported. In the present work, the microstructure of the AlCoCrFeNi2.1 EHEAs aged at 900 °C for different durations was investigated using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD). After that the FCG tests were conducted for the EHEAs with different microstructures to reveal the mechanism of fatigue-crack propagation. The results showed that the B2Ⅱ precipitates had a priority in the precipitation sequence during thermal exposure. Particularly, the EHEAs before and after thermal exposure exhibited a waved da/dN-ΔK curve, which should be ascribed to the coexistence of multi-interfaces driven frequent and excessive crack deflections during the FCG tests. In addition, long-term thermal exposure (for example for 200 h) led to a significant increase in FCG resistance. The inherent reason was revealed based on the evolution of high-angle grain boundaries, average size and number density of precipitates in the aged EHEAs. The data obtained will contribute to understand the EHEAs' high-temperature microstructural stability and provide a valuable way to enhance the FCG resistance.