Nitride Dispersion Strengthened Steel Development after Sintering of Nitrided Fe-4.6 at% Al Alloy Powder

The development of a composite microstructure with hard aluminum nitrides (AlN) dispersed in a soft ferritic iron-based solid is achieved through the powder metallurgy route. Fe-4.6 at% Al alloy powders subjected to two different thermochemical treatments viz. nitriding and nitriding followed by hydrogen reduction are consolidated by spark plasma sintering (SPS) at 900 °C. The final sintered microstructure is characterized with the help of optical, scanning electron, and transmission electron microscopy along with X-ray diffraction for detailed phase analysis. The microstructures of the nitrided and sintered compacts are observed to be significantly affected by two factors: 1) presence of oxide phases on the surface of the powder particles before SPS and 2) evolution of nitrogen gas due to dissociation of iron nitrides during sintering. These result in a very complicated microstructure. However, for nitrided powders, performing a hydrogen reduction step before SPS lead to a microstructure composed of precipitation-hardened particle cores due to dispersion of nanosized AlN precipitates, with a soft, ferritic interparticle region. This composite microstructure achieved through the powder metallurgy route, which can be finely tuned by controlling the nitriding, oxidation, and sintering parameters, is envisioned to be greatly beneficial for several engineering applications such as automotive components.