Simultaneously enhanced strength and ductility in heterogeneous grain structured Ni₂₆Co₂₆Fe₂₅Cu₁₇Ti₆ high entropy alloy by introducing an appropriate amount of Y₂O₃ nanoparticles

The strength and ductility trade-off is a long-standing issue of single face-center-cubic (FCC) high entropy alloys (HEAs). In the present work, the in-situ formation of heterogeneous grain structure coupled with the introduction of Y₂O₃ nanoparticles in FCC structure Ni₂₆Co₂₆Fe₂₅Cu₁₇Ti₆ HEA is proposed as a strategy to enhance the strength and ductility synergy. The oxide dispersion strengthened (ODS) Ni₂₆Co₂₆Fe₂₅Cu₁₇Ti₆ HEAs were fabricated by mechanical alloying (MA) and spark plasma sintering (SPS) methods and the influences of different Y₂O₃ additions on the evolutions of oxide precipitates, heterogeneous microstructure and mechanical properties of ODS-HEAs were systematically investigated. The results show that the introduction of Y₂O₃ facilitates the new precipitation of finer and semi-coherent Y₂Ti₂O₇ oxide nanoparticles, in addition to the protogenous TiO in the Ti-containing HEA matrix. All the studied ODS-HEAs display the bimodal grain microstructure, in which numerous oxide dispersoids are distributed exclusively in the fine-grained (FG) region. With the increase of Y₂O₃ addition, the average grain size of FG region first decreases and then increases in ODS-HEA, while the proportion of FG region shows the opposite trend. When the content of Y₂O₃ is 1.05 wt%, the yield strength, ultimate tensile strength and elongation to fracture of ODS-HEA at room temperature are 21%, 27% and 157%, respectively, higher than those of Ni₂₆Co₂₆Fe₂₅Cu₁₇Ti₆ base alloy. The simultaneous enhancement of strength and ductility in ODS-HEA by introducing a proper amount of Y₂O₃ is mainly attributed to the additional precipitation strengthening contribution of generated Y₂Ti₂O₇ oxide nanoparticles having semi-coherent interfaces with the FCC matrix.