Structural changes of 9R copper precipitates in an aged Fe-Cu alloy

High-resolution transmission electron microscopy and conventional transmission electron microscopy have been used to investigate in detail the transformation processes of twinned 9R copper precipitates to final stable structures via a 3R structure in a thermally aged Fe-Co model alloy. In 9R precipitates of size greater than about 13nm, the motion of the twin boundaries and the elimination of the regular stacking faults on every third (009)9R close packed plane were observed to occur nearly simultaneously. Direct evidence was found that 3R is a non-cubic structure obtained when the regular stacking faults on the (009)9R basal planes are removed. In larger precipitates (of size up to about 26 nm), lattice plane rotations and plane spacing changes in 3R variants took place towards stable fcc and fct structures, suggesting the occurrence of lattice relaxation involving the diffusion of atoms. The fct structure had larger lattice constants a=b=0.369 nm and c=0.366 nm than bulk fcc copper (a=0.361 nm). Precipitates of size 26-40 nm consisted of nearly twin-related variants with both fcc and fct segments, and aligned with the iron matrix according to the Kurdjumov-Sachs orientation relationship. Larger precipitates were observed to be fcc and fct single crystals.