Direct observations on the crystal structure evolution of nano Cu-precipitates in an extremely low carbon steel

The structural evolution of nano Cu-precipitates was studied with high-resolution transmission electron microscopy. 9R Cu precipitates were mainly multiple-twinned. However, the unusual 9R Cu precipitates with no twins were also observed. The 9R→face-centered tetragonal (fct) transition can occur without complete detwinning with 9R Cu structure, which has been confirmed by fast Fourier transform pattern and calculation of Moire' fringes. Instead of commonly-accepted evolution process of body-centered cubic (bcc)→twinned 9R→3R→face-centered cubic (fcc), a new transformation mechanism of bcc→9R→twinned fcc or fct→fcc has been proposed. Finally, a crystallographic model was established to explain the bcc→9R→fcc transformation processes.