Characteristics of complex oxides in Co based ODS alloys

Co based oxide dispersion strengthening (ODS) alloys strengthened by oxide nanoparticles and γ' precipitates are promising high temperature structural materials. Phase evolution, thermal stability and interfacial structure of the oxides were characterised, and the formation mechanism of the complex oxides was clarified. Co based ODS alloys exhibit inhomogeneous dispersion of the oxides due to the presence of relatively large Y-Al-O and fine Y-Hf-O complex oxides. Oxide particle size was controlled by the relative amount of Y-Al-O and Y-Hf-O oxides. The addition of Hf inhibits the formation of Y-Al-O oxides, resulting in the refinement of the oxides. The coherency of the matrix/oxide interface is size dependent. Large Y-Al-O complex oxides are incoherent with the matrix, while small Y-Hf-O nanoparticles tend to be partially coherent with the matrix. The extremely fine Y₂Hf ₂O₇ complex oxides demonstrate excellent thermal stability during heat treatment. Based on the observations of partially crystallised nanoparticles and the oxides with core/shell structure, a three-stage mechanism was used to elucidate the formation mechanism of the complex oxides.