Abstract
Slow or fast oxidation of ultrafine FeCr alloy particles leads to a core/shell like microstructure (1). A slowly oxidized sample revealed FeO (wüstite) and spinel oxide FeCr 2O 4 to be located at the shell, which is built up of very small crystals (3–5 nm). The FeO phase is observed to be very stable, being still present 360 days after the oxidation. The core consists usually of a bcc FeCr alloy, however, some particles contain the σ-FeCr phase, which is the expected form during slow cooling of FeCr particles. A fast oxidized sample contains only two phases, namely °-Fe 2O 3 and CrO 3, which coexist at the shell, leaving the core empty or filled with a lightly scattering substance, e.g. a gas. The outer surface of the shell is CrO 3 free but consists of °-Fe 2O 3. However, CrO 3 is located at the core/shell interface and forms a homogeneous layer around the core and is therefore not mixed with °-Fe 2O 3, which is built up of 10–20 nm crystals.
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