Microstructure characterization of a novel austenitic ODS steel by transmission electron microscopy

Transmission electron microscopy (TEM) was applied to study the microstructure and composition of a novel austenitic oxide dispersion strengthened (ODS) steel (16 wt.% Cr, 14 wt.% Ni, 1 wt.% W). Special attention was paid to characterize the dispersed nano-oxide particles. It was found, that these embedded particles have a size distribution ranging from 1 to 70 nm. The particles with sizes up to 20 nm, have a density of ∼10²³ m⁻³, are mainly Y₂O₃ or Y₂Ti₂O₇. The Y₂Ti₂O₇ particles with size up to 3 nm were found to be fully coherent with the γ-Fe matrix (without defects) with relationships 〈001〉 Y₂Ti₂O₇ II 〈001〉 Fe, 〈111〉 Y₂Ti₂O₇ II 〈111〉 Fe and (110) Y₂Ti₂O₇ II (110) Fe. The larger Y₂Ti₂O₇ were found semi-coherent. Due to the misfit of ∼6% between the Y₂Ti₂O₇ crystal lattice and the matrix, which have similar FCC structures, misfit dislocations were observed. The Y₂O₃ particles, having a smaller misfit of ∼2%, but with BCC structure, reveal coherence with the matrix with relationship 〈001〉 Y₂O₃ II 〈001〉 Fe, 〈111〉 Y₂O₃ II 〈111〉 Fe, and {110} Y₂O₃ II {110} Fe. It was observed that different nanoparticles have a tendency to agglomerate which leads to the loss of their coherency. Independent of the sizes and composition, the nano-oxide particles (which are either fully- or semi-coherent) were able to pin dislocations under repulsive interaction which is known to improve mechanical properties.