High-temperature low cycle fatigue behavior of an equiatomic CoCrFeMnNi high-entropy alloy

In the present work, low cycle fatigue (LCF) behavior of an equiatomic CoCrFeMnNi high entropy alloy (HEA) is correlated to the microstructural evolution at 550 degrees C. The fully reversed strain-controlled fatigue tests were conducted in air under strain amplitudes ranging from 0.2% to 0.8%. The measured cyclic stress response showed three distinct stages which include initial cyclic hardening followed by a quasi-stable cyclic response until failure. The rate and amount of cyclic hardening increased with the increase in strain amplitude. In comparison to common austenitic stainless steels, CoCrFeMnNi HEA shows comparable strength and improved LCF lifetime at similar testing conditions. Electron-microscopy investigations after failure reveal no noticeable change in grain size, texture and annealing twins density. Initial cyclic hardening is attributed to the dislocations multiplication and dislocation-dislocation as well as dislocation-solute atom interactions. The quasi-stable cyclic response is associated with the equilibrium between dislocation multiplication and annihilation, which also leads to the formation of complex dislocation structures such as ill-defined walls and cells, particularly at higher strain amplitudes. Besides, the material exhibits serrated plastic-flow due to interactions between mobile dislocations and diffusing solute atoms (such as Cr, Mn and Ni). Lastly, segregation in the form of Cr- and NiMn-enriched phases were observed near grain boundaries, which appears to have a detrimental effect on the fatigue life.