High-resolution transmission electron microscopy was used to investigate the transformation of twinned 9R-type Cu precipitates in an aged alloy. In the case of 9R-type precipitates which were larger than about 13nm, motion of the twin boundaries and elimination of regular stacking faults on every third (009)9R close-packed plane occurred almost simultaneously. Direct evidence was found that 3R was a non-cubic structure which was obtained when the regular stacking faults on the (009)9R basal planes were removed. In precipitates with sizes of up to about 26nm, lattice-plane rotations and plane spacing changes in 3R-type variants took place towards stable face-centered cubic and face-centered tetragonal structures. This suggested the occurrence of a lattice relaxation which involved the diffusion of atoms. The face-centered tetragonal structure had larger lattice constants (a = b = 0.369, c = 0.366nm) than those of bulk face-centered cubic Cu (a = 0.361nm). Precipitates with sizes of 26 to 40nm consisted of nearly twin-related variants which had both face-centered cubic and face-centered tetragonal segments, and were aligned with the Fe matrix in accord with the Kurdjumov-Sachs orientational relationship. Larger precipitates were observed to be face-centered cubic and face-centered tetragonal single crystals.

Structural Changes of 9R Copper Precipitates in an Aged Fe-Cu Alloy. R.Monzen, M.Iguchi, M.L.Jenkins: Philosophical Magazine Letters, 2000, 80[3], 137-48