The phase transformations and twinning in a nano-sized face-centred cubic (FCC) crystal of iron with different carbon contents are studied by molecular dynamic (MD) simulations. The simulation cells are stretched uniaxially along the direction. It is found that the FCC matrix transforms into the body-centred cubic (BCC) phase following the Nishiyama–Wassermann orientation relationship (OR). Then, the ‘BCC
hexagonal close-packed (HCP)’ and ‘BCC
FCC’ transformations occur following different variants of the Burgers OR and the Pitsch OR, respectively. The twins form between two neighbouring BCC grains. Additionally, the carbon atoms can destabilise the BCC phase, inhibit the phase transformation and pin phase interfaces, leading to tiny BCC twins.
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direction. It is found that the FCC matrix transforms into the body-centred cubic (BCC) phase following the Nishiyama–Wassermann orientation relationship (OR). Then, the ‘BCC
hexagonal close-packed (HCP)’ and ‘BCC
FCC’ transformations occur following different variants of the Burgers OR and the Pitsch OR, respectively. The 
twins form between two neighbouring BCC grains. Additionally, the carbon atoms can destabilise the BCC phase, inhibit the phase transformation and pin phase interfaces, leading to tiny BCC twins.