High-resolution electron microscopy observation and dislocation reaction mechanism of fivefold twinning in a Cu-rich precipitate in a cold rolled ferritic steel containing copper

•Fivefold twin is observed in a Cu-rich precipitate in cold rolled ferritic steels.•A dislocation reaction mechanism for the fivefold twin formation is proposed.•Two particular mismatching α-Fe/Cu-rich precipitate interfaces play a critical role.

Ferritic steels containing copper have been studied as model systems for clusters/precipitate formation in reactor pressure vessel steels. The samples were aged at 400 °C for 4000 h and subsequently cold rolled to 30% reduction at room temperature. The microstructural characteristics of the samples were analyzed using high-resolution transmission electron microscopy. Direct evidence was found that the fivefold twinning occurs via simultaneous emission of two Shockley partial dislocations from two particular α-Fe/Cu interfaces, and then the pileup tips of the twofold twin.