Decomposition of Y₂Ti₂O₇ Particles in 8 Pct Cr Oxide-Dispersion-Strengthened Martensitic Steel during Tempering

摘要

Chemical composition analysis using inductively coupled plasma spectroscopy and phase identification using X-ray were performed on the extracted residues of 8 pct Cr martensitic steel strengthened by nanoscaled complex oxides, which consist of yttrium, titanium, and oxygen. Some Y₂Ti₂O₇ particles, which were stable during normalizing, decomposed into Y₂O₃ and Ti₂O₃ during tempering. This reaction reversibly occurred between normalizing and tempering. Y₂Ti₂O₇ particles formed in the steel had other constituents in solid solution as compared to the completely stoichiometric Y₂Ti₂O₇ particles synthesized artificially in air. As for the mechanism of the decomposition of Y₂Ti₂O₇ particles in the steel, segregation of oxygen to dislocations induced by normalizing caused the decomposition of Y₂Ti₂O₇ during tempering. In addition to that, the interfacial strain between Y₂O₃ particles or Ti₂O₃ particles within Cr₂₃C₆ carbides, which are formed by tempering, was lower than the strain between Y₂Ti₂O₇, which precipitated in Cr₂₃C₆ carbides or the matrix. This difference in interfacial strain could also promote the decomposition of the Y₂Ti₂O₇ particles in the steel.