Radiation-Induced Grain Growth of Nanocrystalline Al_xCoCrFeNi High-Entropy Alloys

Grain growth of nanocrystalline Al_xCoCrFeNi high-entropy alloys with varying Al contents (x = 0, 1, 2) is studied. The alloys are fabricated by high-energy ball milling and subjected to a 1 MeV Kr²⁺ ion irradiation bombardment at room temperature up to a dose of 5.625 displacements per atom (dpa). X-ray diffraction (XRD) and transmission electron microscopy (TEM) characterizations show that the crystal structure is face-centered cubic (FCC) for CoCrFeNi (Al-0) alloy and BCC + FCC for Al₁CoCrFeNi (Al-1) and Al₂CoCrFeNi (Al-2) alloys. In situ TEM observations show that the grain size increases with irradiation dose from 13.8 ± 3, 7.4 ± 1, and 11 ± 1 nm before irradiation to 36 ± 8, 25 ± 5, and 26.6 ± 3 at 5.625 dpa for Al-0, Al-1, and Al-2 alloys, respectively, and a significant chemical composition dependence on grain growth was seen, where the highest grain growth rate is observed for the Al-2 alloy as a result of the lowest cohesive energy, which results in the lowest activation energy for atomic jump under ion irradiation. The grain growth kinetics is elucidated by the thermal spike model, and its mechanism is attributed to a disorder-driven mechanism for the initial fast growth, which is caused by the loss of the crystalline order as a result of ion-irradiation-induced large lateral damage volume and a defect-driven mechanism for the later slow growth stage, which is driven by the defect concentration difference near grain boundaries (GBs) under ion irradiation. Finally, this paper shows the effect of atomic collision cascades on grain growth, demonstrating the possibility to control grain sizes using the ion beam technique for nanostructured materials in nuclear applications.