Low cycle fatigue properties of refractory high-entropy HfNbTiZr alloy

Considerable effort has been applied toward developing refractory high-entropy alloys (RHEAs) with both high strength and good ductility. However, the deformation and fatigue failure mechanisms of these alloys under cyclic loading remain poorly understood. The present work addresses this issue by investigating the low-cycle deformation behavior and microstructural evolution of an HfNbTiZr RHEA with a nominal composition of 25 mol% each of Hf, Nb, and Ti, and Zr forming the balance under symmetric tension-compression conditions at room temperature. Analysis of the cyclic stress responses of the alloy reveals a variety of internal stress trends at different strain amplitudes (Aca), which implies the operations of a variety of cyclic deformation mechanisms. Further analysis demonstrates that cyclic softening is mainly caused by a rapid decrease in frictional stress during the initial cycles representing roughly 10% of the total number of cycles to failure. In general, the microstructural characteristics of the alloy observed with an increasing number of cycles under various values of Aca demonstrate that softening occurs due to the formation of slip bands (SBs) caused by high-density dislocations arising in conjunction with increasing plastic strain accumulation. Specifically, dislocation structures forming at a low value of Aca = 0.8% mainly consist of planar SBs (PSBs), while low-density cross SBs (CSBs) form at Aca = 1.1%, and the density of the CSBs increases with further increasing Aca. In addition, a fatigue lifetime prediction model was presented that obtained good prediction accuracy for the HfNbTiZr RHEA investigated herein. Accordingly, the present study can be expected to provide a fundamental basis for understanding the deformation mechanisms of other RHEAs.