Four-point bending fatigue behavior of an equimolar BCC HfNbTaTiZr high-entropy alloy: Macroscopic and microscopic viewpoints

Abstract

High-entropy alloys (HEAs) have attracted a great deal of interest and high expectations not only from the scientific community, but also from the industrial point of view, mainly because of a remarkable combination of promising properties. However, the characterization of HEAs under fatigue loading is still very scarce, especially in the case of body centred cubic (BCC) HEAs. As an attempt to fill this gap, we report here the first study on the fatigue behavior of a refractory BCC HEA in four-point bending loading conditions. Equimolar HfNbTaTiZr HEA was subjected to load-control fatigue tests in air and at room temperature to assess its fatigue properties in the high-cycle regime. Experimental results show that the maximum stress level reached within one fatigue cycle exceeds the uniaxial yield stress of the material. This particularity is observed within the whole range of fatigue life investigated, i.e., from approximately 104 to 107 cycles, showing no macroscopic plastic deflection of the beam in the process. A fatigue endurance of approximately 430 MPa (at 107 cycles) is found, for a stress ratio of approximately 0.1. A fatigue crack initiation mechanism occurs at the specimen surface through an intergranular phenomenon and propagates in a transgranular crystallographic manner. The fatigued samples underlined the low local lattice rotation, implying a restrained cyclic strain hardening. Finally, the fatigue study of the HfNbTaTiZr HEA highlights a fatigue endurance over an ultimate tensile strength ratio of 0.43, which is significantly larger than the actual conventional structural metallic materials under similar conditions.