Neutron irradiation response of a Co-free high entropy alloy

Abstract Low dose neutron irradiation has been conducted on a NiFeMnCr (Co-free) high entropy alloy (HEA) near room temperature. Post-irradiation examination (PIE) at room temperature revealed that this HEA exhibited qualitatively similar change in mechanical properties (hardness, strength and ductility) as conventional austenitic stainless steels. Isochronal annealing was performed to determine the hardness, electrical resistivity and defect property evolution with respect to annealing temperature. Different annealing behavior was observed. Irradiation hardening nearly completely anneals by 650 °C, although the annealing trend is different following exposure to 0.1 and 1 displacements per atom (dpa). Positron Annihilation Spectroscopy (PAS) measurements indicate that the temperature to initialize vacancy cluster (stage V) recovery is ∼400 °C, and that the density of vacancy-type defects is reduced to that of the unirradiated control specimen at 500 °C. No significant change in the chemical ordering near defects was measured by PAS after neutron irradiation or after isochronal annealing up to 700 °C. Electrical resistivity measurements revealed a large increase in resistivity following irradiation, and this resistivity increase does not significantly recover even after the 700 °C anneal. Overall, these annealing results indicate the HEA vacancy cluster annealing behavior is similar to conventional face centered cubic (FCC) alloys whereas solute diffusion is limited up to 700 °C (significantly more sluggish than conventional FCC alloys). X-ray diffraction (XRD) and transmission electron microscopy (TEM) results indicated that this HEA exhibit good phase stability upon neutron irradiation up to 1 dpa and post-irradiation annealing up to 700 °C.