Effect of Ti and Cr on dispersion, structure and composition of oxide nano-particles in model ODS alloys

Three model ODS alloys (Fe–0.3Y₂O₃, Fe–0.2Ti–0.3Y₂O₃ and Fe–14Cr–0.2Ti–0.3Y₂O₃) were prepared by ball milling and then hot extrusion to study the effect of Ti and Cr on the size, distribution, crystal structure and composition of the nano-oxide particles. All alloys were characterized by high resolution transmission electron microscopy (HRTEM), atom probe tomography (APT) and synchrotron-X-ray diffraction (S-XRD) to determine the distribution, structure and composition of the oxide nanoparticles samples. The median particle sizes were 9.6 nm, 7.7 nm and 3.7 nm for the Fe–Y₂O₃, Fe–Ti–Y₂O₃ and Fe–Cr–Ti–Y₂O₃ alloys, respectively, so the presence of Ti resulted in a significant reduction in oxide particle diameter and the addition of Cr gave a further reduction in size. In the Fe–0.3Y₂O₃ alloy, the particles are found to be bcc Y₂O₃, whereas in the other two alloys (Fe–Ti–0.3Y₂O₃ and Fe–Cr–Ti–Y₂O₃), the oxide particles were found to be structurally consistent with both orthorhombic Y₂TiO₅ and fcc Y₂Ti₂O₇. Detailed APT studies showed Cr shells around oxide particles of all sizes in the Fe–Cr–Ti–Y₂O₃ alloy, that a range of cluster compositions are present and that the particle chemistry varies with cluster size. We show that the addition of Cr has a strong effect on both the size and stoichiometry of the particles