Evolution of twins and formation mechanism of special fringes inside nanoscale Cu precipitates in ferritic steel

The evolution of twins and the formation mechanism of special fringes inside the Cu precipitates in ferritic steel were studied on the atomic scale in this paper. The results show that the small Cu precipitate (~ 10 nm) already has the fcc structure at the early stage of aging, without conventional 9R or 3R structure, which may attribute to high Si content. The special fringes in Cu precipitates are formed by the co-diffraction between the precipitate and the Fe-bcc matrix, which have the included angle of 17° with the (101) planes of the matrix. When the precipitate grows to 14 nm, twins begin to appear with a spacing of 6 atomic layers and the included angle of 34° between adjacent twinning layers are formed. The thickness of twinning layers increases significantly with the growth of Cu precipitate, while the quantity of twins increases slightly. In addition, the Cu-fcc precipitate with no twins is formed again after sufficient aging (5 h). The appearance and proportion of fringes depend on the relative size and position between the precipitates and the foil of TEM samples. The results of this study are of great significance for understanding the twinning and structural evolution of Cu precipitate in the ferritic matrix. • The complete fcc structure was found in nanoscale Cu precipitates (10 nm) in ferritic steel. • The growth of Cu precipitates is accompanied by the evolution of twins in the precipitates. • The special fringes in Cu precipitates is formed by the co-diffraction of precipitate and matrix.