Y2O3 evolution and dispersion refinement in Co-base ODS alloys

Co-base ODS alloys are potential materials for high-temperature applications. The evolution of Y2O3 during mechanical alloying (MA) and subsequent annealing were studied with the main focus on the refinement of particle size by the addition of minor alloying elements. The results reveal that the MA powder is characterized by ultrafine grains and high dislocation density, which facilitate the decomposition of Y2O3. The decomposed Y2O3 thermally precipitates at about 690 °C. Ti, Zr and Ca cause the coarsening of oxide, while Hf refines the oxide particle size and improves the homogeneity. The optimum Hf content is 1.2 wt.% and the resulting particle size is reduced to 9.6 nm with the precipitation of partially coherent Y2Hf2O7 oxides. Higher Al content tends to result in the formation of Al2O3 and Y3Al5O12 oxides, which are of larger size. The threshold stress is predicted to be enhanced by the reduction of inter-particle spacing. The refinement effect is also verified by hardness measurements.