Effects of nano scale Y2O3 additions on microstructural stability and mechanical properties of equiatomic CoCrCuFeNi based high entropy alloys

Abstract

The use of a blend of pre-alloyed powder aggregate with a measured amount of nano-dispersoids is a powerful method to fabricate near-net-shaped components via the Powder metallurgy route. In the current investigation, CoCrCuFeNi -based multicomponent alloys with controlled addition of Y2O3 (Y2O3 wt. % = 0, 1, 2 and 3) were synthesized by mechanical alloying (MA) and subsequently consolidated by vacuum hot pressing (VHP). The effect of the Y2O3 addition of the as-sintered Y2O3 dispersed CoCrCuFeNi aggregate on the microstructural and mechanical characteristics was investigated to better understand their behaviour. The XRD analysis confirms that the as-prepared oxide dispersion strengthened high entropy alloy (ODS-HEAs) including the base HEA, and established a metastable nanostructure after the MA process. The microstructure eventually turned up into a thermodynamically stable aggregate once it is hot pressed. In addition, following 30 h of MA in CoCrCuFeNi HEA with 2 wt% and 3 wt% Y2O3, a stable intermetallic phase of Co14.2Cr2.7Y2 and an FCC phase were observed, but with 0 wt% and 1 wt% Yttria addition, only an FCC phase was observed. During the milling of CoCrCuFeNi HEA, the average particle size decreases from 13.864 μm to 9.541 μm (start to the end of the milling process). Furthermore, for 3 wt % Y2O3 dispersed CoCrCuFeNi ODS-HEA, the change in particle size is from 15.582 μm to 5.712 μm at the start and end of milling, respectively, indicating that the particle size has decreased to much lower values compared to base HEA. After increasing the amount of Y2O3 added from 0 to 3 wt %, there was a significant increase in the microhardness of CoCrCuFeNi HEA from 370 ± 6 HV to 548 ± 5 HV. Higher Yttria wt.% (2 wt% and 3 wt%) was found to result in the formation of complex intermetallic Co14.2Cr2.7Y2 particles, which were able to pin the migration of the grain boundaries during sintering due to their high thermal stability, and were thus responsible for the increase in hardness for these alloys.