Abstract

The current work compares the deformation behavior of CoCrFeMnNi and CoCrNi in the temperature interval between 295 K and 8 K through a series of quasi-static tensile tests.Temperature-dependent yield stress variation was found to be similarly high in these two alloys.Previous investigations only extended down to 77 K and showed that a small amount of εmartensite was formed in CoCrNi while this phase was not observed in CoCrFeMnNi. The present study extends these investigations down to 8 K where similar low levels of ε-martensite were presently detected. Based on this result, a rough assessment has been made estimating the importance of deformation twinning to the strength. The relative work hardening rates of CoCrFeMnNi and CoCrNi were comparable in value despite the differences in ε-martensite formation during deformation. CoCrFeMnNi deforms by dislocation slip and deformation twinning while deformation in CoCrNi is also accommodated by the formation of ε-martensite at cryogenic temperatures. Additionally, CoNi, a solid solution from the Co-Cr-Fe-Mn-Ni system with low strength, was used for comparison, showing contrasting deformation behavior at cryogenic temperatures.

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 °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 lifetimes 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 dislocation multiplication and dislocation-dislocation as well as dislocationsolute atom interaction. The quasi-stable cyclic response is associated with the equilibrium between dislocation multiplication and annihilation, which also leads to the formation of heterogeneous dislocation structures such as ill-defined walls and cells, particularly at higher strain amplitudes. Besides this, the material exhibits serrated plastic-flow due to interactions between mobile dislocations and diffusing solute 2 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.