Novel ultrafine Fe(C) precipitates strengthen transformation-induced-plasticity steel

A transmission electron microscopy study was conducted on nanoprecipitates formed in Ti microalloyed transformation-induced-plasticity-assisted steels, revealing the presence of Ti(N), Ti2CS and a novel type of ultra-fine Fe(C) precipitate. The matrix/precipitate orientation relationships, sizes and shapes were investigated in detail. The ultrafine, disc-shaped Fe(C) precipitates have sizes of 2–5 nm and possess a hexagonal close packed crystal structure with lattice parameters a = 5.73 ± 0.05 Å, c = 12.06 ± 0.05 Å. They are in a well-defined Pitsch–Schrader orientation relationship with the basal plane of the precipitate parallel to the [1 1 0] habit plane of the surrounding body-centred-cubic ferritic matrix. Detailed analysis of precipitate distribution, orientation relationship, lattice mismatch and inter-particle spacing suggests that these ultrafine precipitates contribute considerably to the strengthening of these steels.