Effects of sintering temperature and Y2O3/Ti addition on microstructure and hardness of ODS-AlCrFeNi HEAs

Oxide dispersion strengthened (ODS) alloys are leading candidate structural materials for advanced nuclear reactor application due to their excellent high strength and radiation tolerance. In this paper, ODS AlCrFeNi high entropy alloys (HEAs) were synthesized via mechanical alloying (MA) and spark plasma sintering (SPS) to study the effects of sintering temperature (from 1050 degrees C to 1250 degrees C) and different Y2O3/Ti addition amounts on the microstructure and hardness. The microstructure was characterized by XRD, SEM, EBSD and HRTEM, and the micro-hardness was tested. The results demonstrated that after MA and SPS, spinodal decomposition occurs in the matrix, in which Fe/Cr enriched disordered BCC phase and Ni/Al enriched ordered B2 phase are formed. At the higher sintering temperature (1250 degrees C), additional FCC phase appears in 0.3 YT, which is contributed to the formation of higher volume fraction of Al2O3 at powder/grain boundaries so that spinodal decomposition is partly suppressed. At the lower sintering temperature (1100 degrees C), FCC phase could be formed when Y2O3+Ti amount is lower (0.6 wt%, 1.0 wt%). The hardness of ODS-AlCrFeNi is notably improved when Y2O3 and Ti are added in the same weight percent which is up to 1 wt% or above. The average grain size of 3.0 YT alloy (3 wt% Y2O3+3 wt% Ti) is the smallest, which is 240 nm. The average size of precipitates in 3.0 YT alloy is 8.7 nm, while the number density is the highest (3.3 x 1022/m3). The addition of Y2O3/Ti can modify the types of nanoscale oxides in the ODS-AlCrFeNi. The precipitates in 0Y alloy (without Y2O3 and Ti addition) are Al-rich oxides, while the precipitates observed in 3.0 YT alloy include AlYO3, Al2Y4O9, Y2TiO5, YTiO3, Al2TiO5 and Al2O3. The grain boundary strengthening and precipitation strengthening are dominated in 3.0 YT alloy leading to the highest hardness increment compared with 0Y alloy and the hardness of 3.0 YT alloy reaches 751 HV.