Microstructure of nano-structured ODS CLAM steel by mechanical alloying and hot isostatic pressing

Oxide dispersion strengthened (ODS) China Low Activation Martensitic (CLAM) steel was prepared by mechanical alloying (MA) and hot isostatic pressing. The morphology and microstructure of MA powders were studied by X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy and laser diffraction scattering to optimize the milling time. The microstructure of nano-structured ODS CLAM steel was characterized by electron backscatter diffraction, atom probe tomography, and transmission electron microscopy. The results showed that, during mechanical alloying, the morphology of MA powders changes from an initial fine spherical shape to a coarse laminar shape (at 0.5 h) back to a fine near-spherical shape (after 20 h), and the alloying elements and yttria (Y) gradually dissolve into the iron (Fe) matrix. With increased milling time, the mean grain size decreases rapidly (from 0 to 3 h), then gradually (from 3 to 8 h), then remains constant thereafter. The average grain size of as-received ODS CLAM steel that has undergone hot isotopic pressing is 368 nm. High-density Y-Ti-O-rich nano-clusters, a few Y2Ti2O7 precipitates, and some Cr-Mn-rich precipitates are formed. The size of Y2Ti2O7 and Cr-Mn-rich precipitates increases with increasing temperature. The nano-clusters show excellent temperature stability.