The morphology and crystal structure of the carbon cluster in a low-temperature aged low-carbon ferritic steel were clarified by performing conventional and in-situ TEM, and atomically-resolved ADF-STEM observation. The carbon cluster grow parallel to <001> direction of the matrix, whose morphology was presumed to be either disk or plate. Lattice constant of the carbon cluster was measured by peak fitting of atomic columns observed by ADF-STEM. The result clarified that the structure of the carbon cluster was martensite-like body-centered tetragonal (bct) structure. This martensite-like bct structure suggested that the carbon cluster was supersaturated with carbon atoms occupying specific octahedral sites.
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