Microstructure characterization of bulk nanocrystalline Fe–0.8C alloy produced by mechanical milling and spark plasma sintering

The microstructure of bulk nanocrystalline Fe–C alloy produced by mechanical milling (MM) and spark plasma sintering (SPS) has been characterized using transmission electron microscopy (TEM) and a three-dimensional atom probe (3DAP) to understand the origin of the unusually high yield strength and plastic strain observed from the sample. A bimodal grain structure consisting of fine (∼250 nm) and coarse (∼900 nm) grains was observed as a result of partial recrystallization. The fine grained region was found to be a duplex phase structure comprised of ferrite and cementite grains. From TEM and 3DAP analyses, the presence of fine oxide particles containing chromium was confirmed. The presence of the recrystallized coarse grains is attributed to the large plastic strain in compression.