Nanometer-sized δ*-Al2O3-reinforced aluminum composites using the friction stir process: A TEM study

Abstract In-situ Al2O3 nanocomposites of metallic aluminum were fabricated by friction stir processing (FSP) of Al and CeO2 mixed powders having been sintered in an argon atmosphere. Transmission electron microscopic observations of these cermets showed multiple variants of the δ⁎-Al2O3 nanoplates in the fcc-Al matrix following the definite crystallographic orientation relationship 223 δ ∗ / / 111 Al ; 1 ¯ 10 δ ∗ / / 1 ¯ 10 Al as constrained by the semicoherent habit plane (223)δ⁎/{111}Al, giving rise to the characteristic diffraction streaks and stacking faults induced by Shockley partial dislocations of the matrix. The transformation path and formation mechanisms of the platy δ⁎-Al2O3 arrays in terms of oxidation precipitation and redox reactions under the applied stress and temperature are discussed.