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 degrees C and 1050 degrees C were characterized by high-resolution transmission electron microscopy and diffraction contrast techniques, including weak beam electron microscopy. After extrusion at 1150 degrees C, yttrium aluminum hexagonal (YAH, YAlO(3)) and yttrium-aluminum-perovskite (YAP, YAlO(3)) oxides (diameter <= 10 nm) constitute similar to 55% and 38% of the particles, respectively; similar to 78% of the particles (4.5-10 nm in diameter), which include 40% YAH oxide and 38% YAP phase with misfit (translational) moire fringe spacing of 2.15 nm and 1.65 nm, respectively, are semi-coherent with the matrix. After extrusion at 1050 degrees C, almost all the particles are YAH phase, and similar to 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. (C) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.