Effects of Zr and/or Ti addition on the morphology, crystal and metal/oxide interface structures of nanoparticles in FeCrAl-ODS steels

FeCrAl oxide dispersion strengthened (ODS) steel is one of the most promising candidate cladding materials for Generation IV nuclear reactors due to its superior high temperature strength and excellent resistance to creep, corrosion and irradiation. The morphologies of matrix grains and oxides, the crystal and metal/oxide interface structures of nanoparticles in two FeCrAl ODS steels, i.e., 3.3Al–0.5Zr (Fe–15Cr–2W–3.3Al–0.5Zr–0.35Y ₂O ₃) and 3.7Al–0.1Ti–0.6Zr (Fe–15Cr–2W–3.7Al–0.1Ti–0.6Zr–0.35Y ₂O ₃), were studied by transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM) and high resolution transmission electron microscopy (HRTEM). The average matrix grain sizes of 3.3Al–0.5Zr ODS steel and 3.7Al–0.1Ti–0.6Zr ODS steel are 1.01 and 0.97 μm, respectively. The dispersion morphology of oxides in 3.7Al–0.1Ti–0.6Zr ODS steel is much better, relative to that of 3.3Al–0.5Zr ODS steel. Crystal & interface structures of 88 and 171 oxides in 3.3Al–0.5Zr ODS steel and 3.7Al–0.1Ti–0.6Zr ODS steel were characterized, respectively. The proportions of Y–Zr–O, Y–Al–O, Y–Cr–O, ZrO ₂ and Y ₂O ₃ in 3.3Al–0.5Zr ODS steel are 77.2%, 9.1%, 2.3%, 2.3% and 2.3%, respectively and, however, the proportions of Y–Zr–O, Y–Ti–O, Y–Al–O and ZrO ₂ in 3.7Al–0.1Ti–0.6Zr ODS steel are 86.5%, 8.3%, 3.5% and 1.7%, respectively. The much better dispersion morphology of oxides in 3.7Al–0.1Ti–0.6Zr ODS steel, relative to that of 3.3Al–0.5Zr ODS steel, is due to the much higher proportion of Y–Zr–O, the considerably lower proportion of Y–Al–O and the fairly high number fraction of Y–Ti–O. The mechanisms of the formation and polymorphic transition of various kinds of oxides and, moreover, the strengthening mechanisms in both ODS steels were discussed.