A comparative study of microstructure and mechanical properties of ODS CrFeNi-based medium- and high-entropy alloys

Medium- and high-entropy alloys (MEAs and HEAs) intrigue extensive interest of worldwide scholars for their unique properties. In this paper, oxide dispersion strengthened (ODS) CrFeNi-based MEA and HEAs with 1 wt% Y2O3 and 1 wt% Zr addition were fabricated by mechanical alloying (MA) and spark plasma sintering (SPS). The differences between ODS-MEA and ODS-HEA in microstructure (especially nanoscale oxides) and mechanical properties were characterized and compared by XRD, XPS, BSE, HRTEM, STEM, Vicker's hardness and compression test. MAed ODS-CrFeNi MEA, ODS-CoCrFeNi and ODS-CoCrFeNiMn HEA powders are of single face-centered cubic (FCC) crystal structure. After sintering, all three alloys consist of FCC matrix, body-centered cubic (BCC) structured Cr-rich phases, few Cr/Mn-rich oxides and high-density nanoscale oxides. Nanoscale oxides formed in the three alloys are all identified as Y4Zr3O12. In ODS-CrFeNi, the average size of oxides is 10 nm, and the number density in the BCC phase (3.2 ×1022/m3) is higher than that in FCC matrix (3.9 ×1021/m3). In ODS-CoCrFeNi and ODS-CoCrFeNiMn, the average size and number density of oxides increase to 19 nm, 20 nm and decrease to 2.3 × 1021/m3, 1.9 × 1021/m3, respectively. The amount of BCC phase decreases significantly in ODS-CoCrFeNi and ODS-CoCrFeNiMn HEAs compared with ODS-CrFeNi MEA. With the increasing principal element number, high entropy effect promotes more Y2O3 and Zr dissolve in the matrix during MA, but higher Y/Zr solubility and less precipitation of nanoscale oxides Y4Zr3O12 in the alloys during SPS/annealing. ODS-CrFeNi MEA exhibits the highest hardness of 475 HV and the highest compressive yield strength of 1531 MPa.