He-ion irradiation effects on the microstructure stability and size-dependent mechanical behavior of high entropy alloy/Cu nanotwinned nanolaminates

High entropy alloys (HEAs) have attracted extensive attention due to their excellent properties in harsh environments. In this work, we prepare HEA/Cu (HEA = FeCoCrNi) nano-twinned nano-laminates (NTNLs) with equal individual layer thickness (h) and study the He-ion irradiation effects on their deformation behavior and mechanical properties. With decreasing h the microstructure of HEA/Cu NTNLs transits from the nanolayered structure with nanotwins confined in the isolated layers at large h >= 25 nm to the columnar grained single crystal-like structure with nanotwins penetrated across the coherent layer interface(s) at small h < 25 nm. Compared with the as-deposited HEA/Cu NTNLs showing notable detwinning behavior, in particular at small h, the nanotwins in irradiated samples are difficult to detwin owing to the He defects pinning effect on twin boundaries. It appears that both as-deposited and irradiated HEA/Cu NTNLs manifest the trend of "smaller is stronger". Unlike the reported monotonically increased/reduced irradiation hardening with h in conventional bimetal nanolaminates, a minimum irradiation hardening occurs in the present HEA/Cu NTNLs. Unexpectedly, the as-deposited HEA/Cu NTNLs manifest positive reduced strain rate sensitivity (SRS) m with decreasing h, while their irradiated siblings exhibit the transition from positive to negative SRS m. The size-dependent mechanical properties of HEA/Cu NTNLs are rationalized by the partial-based mechanism in combination with the stability of nanotwins.