Effects of Y2O3/Ti/Zr addition on microstructure and hardness of ODS-CoCrFeNi HEAs produced by mechanical alloying and spark plasma sintering

Abstract

Oxide dispersion strengthened (ODS) high entropy alloys (HEAs) are potential structural materials for advanced nuclear reactor application due to their excellent radiation tolerance and high-temperature mechanical properties. ODS-CoCrFeNi HEAs with different type and amount Y2O3/Ti/Zr addition were synthesized by mechanical alloying (MA) and spark plasma sintering (SPS) to reveal the effects of Y2O3/Ti/Zr addition on the types, size, and number density of nanoscale oxides and hardness. The microstructure was characterized by XRD, XPS, EBSD and HRTEM, and the micro-hardness was tested. The results showed that ODS-CoCrFeNi HEAs mainly consist of face-centered cubic structural CoCrFeNi matrix, high-density nanoscale oxides together with few coarse carbides/oxides. The alloys exhibit the bimodal grain size distribution, and the addition of Y2O3/Ti/Zr reduce the average grain size. The type of nanoscale oxides in the ODS-HEAs is strongly related to the type of oxide-forming elements added. The cubic or monoclinic Y2O3 oxides are identified in the ODS-HEAs with the addition of only Y2O3. The nanoscale oxides, orthorhombic Y2TiO5 and orthorhombic YTiO3 or cubic Y2Ti2O7, are observed in the HEAs with the addition of both Y2O3 and Ti. Hexagonal Y4Zr3O12 are found in the HEAs with both Y2O3 and Zr. The number density and average size of nanoscale oxides depend on both the number and type of Y2O3/Ti/Zr addition. In ODS-HEAs with 3 wt% Y2O3 and 3 wt% Zr, the number density of nanoscale oxides is the highest, which is ∼3.14 × 1022/m3, and the hardness is up to 676 HV. This demonstrates that the joint addition of Y2O3 and Zr has the most obvious influence on microstructure and hardness compared to the addition of Y2O3 or Y2O3+Ti.