Polymorphic and coherency transition of Y–Al complex oxide particles with extrusion temperature in an Al-alloyed high-Cr oxide dispersion strengthened ferritic steel

The phase and metal/oxide interface structure of the nanometer-scale particles in an Al-alloyed high-Cr oxide dispersion strengthened ferritic steel extruded at 1150 °C and 1050 °C were characterized by high-resolution transmission electron microscopy and diffraction contrast techniques, including weak beam electron microscopy. After extrusion at 1150 °C, yttrium–aluminum–hexagonal (YAH, YAlO3) and yttrium–aluminum–perovskite (YAP, YAlO3) oxides (diameter ⩽10 nm) constitute ∼55% and 38% of the particles, respectively; ∼78% of the particles (4.5–10 nm in diameter), which include 40% YAH oxide and 38% YAP phase with misfit (translational) moiré fringe spacing of 2.15 nm and 1.65 nm, respectively, are semi-coherent with the matrix. After extrusion at 1050 °C, almost all the particles are YAH phase, and ∼86.5% (diameter <4.5 nm) are coherent with the matrix. The coherency of the oxides is size dependent. The crystallographic orientation correlations of the oxides and matrix were found.