HIGH-RESOLUTION ELECTRON-MICROSCOPY STUDIES OF THE STRUCTURE OF CU PRECIPITATES IN ALPHA-FE

Transmission electron microscopy and high-resolution electron microscopy investigations of the structure of Cu precipitates in the size range 4-30 nm were carried out as part of a wider investigation into Cu precipitation from thermally aged Fe-Cu and Fe-Cu-Ni ferritic model alloys. A twinned 9R structure was found to be present in precipitates slightly larger than 4 nm in diameter, following transformation from b.c.c. Two twin-related 9R variants were observed in all the smallest 9R particles studied. The 9R precipitates were observed to grow subsequently as spherical, multiply twinned particles up to approximately 17 nm, indicating that further twinning must occur during growth in the 9R phase. At sizes larger than 17 nm, a second transformation to the more stable 3R structure takes place. Observations on these 3R particles indicate that, following transformation from 9R, the precipitates are untwinned and have a distorted fc.c. structure. The particle-matrix orientation is close to, but not exactly, the Kurdjumov-Sachs relationship. Larger 3R particles are observed to have the expected f.c.c. structure, aligned according to the Kurdjumov-Sachs relationship, suggesting that lattice relaxation occurs during diffusional growth of the 3R precipitates.