Effect of Al Addition to The Multi-Principal Elemental CrFeMnNi Alloy System in Terms of The Resulting Microstructure and Radiation Resistance

In recent times, High-Entropy Alloys (HEAs) or Multi Principal Elemental Alloys (MPEAs) have been attracting much attention as new structural materials for nuclear reactor in-core applications due to their structural stability and excellent mechanical properties. However, their microstructural behaviour under irradiation still requires special attention. Particularly, the impact of specific chemical elements on the irradiation response of the alloys still needs to be evidenced and is at the core of this NSF funded project. The current work compares the radiation behaviour of CrFeMnNi, Al_0.3CrFeMnNi and Al_0.8CrFeMnNi alloys. The alloys, which were prepared through Plasma Spark Sintering (SPS), were irradiated in-situ in a Transmission Electron Microscope (TEM) using 1 MeV Kr⁺ ions up to 10 dpa at room temperature (RT) and at 300 °C. Since the starting microstructure depends not only on the composition but also on the way of processing, thorough pre-irradiation characterization of all the three alloys was carried out using X-Ray Diffraction (XRD) and TEM which showed the formation of (BCC+FCC) phases in major proportion. Addition of Al resulted in an increase in the proportion of BCC phase along with the formation of NiAl based ordered phase. At RT, with increased irradiation dose, the phases with more uniformly distributed elemental composition showed the dynamic formation and annihilation of irradiation induced defects. However, the Cr-segregated phase in the (Al_0.3CrFeMnNi) alloy showed amorphization behaviour with increasing irradiation dose. At RT, the alloys did not show the formation of voids or Radiation induced Segregation (RIS). However, at 300 °C, the alloys showed the formation of voids at doses around ~5 dpa. Also, Ni segregation at the void/matrix interfaces has been observed in the Al_0.3CrFeMnNi alloy at 300 °C. Overall, the void formation was observed to depend on the local chemistry and temperature of irradiation. Additionally, irradiation at 300 °C also resulted in Radiation Induced Precipitation (RIP) in these alloys, the existence of which were confirmed through extensive post-irradiation TEM diffraction analysis, which showed low intensity crystalline spots and streaks around the major diffraction spots. The Cr-rich phase in Al_0.3CrFeMnNi alloy showed the formation of nano-crystalline features. The Ni-rich phase showed the formation and Nano-structuring of irradiation-induced defects. The in-situ TEM experiments allowed to clearly evidence how variations in local chemistry and microstructural features in these MPEAs affect the local response to irradiation (at the nm/micron level).