A HIGH-RESOLUTION ELECTRON-MICROSCOPY STUDY OF COPPER PRECIPITATION IN FE-1-CENTER-DOT-5 WT-PERCENT CU UNDER ELECTRON-IRRADIATION

High-resolution electron microscopy has been used to study the structure of copper-rich precipitates in an Fe-1.5 wt% Cu alloy irradiated at 295 degrees C with 2.5 MeV electrons to a dose of 3.1 x 10(23) m(-2) (1.4 x 10(-3) (displacements per atom (dpa)) at a dose rate of 4.1 x 10(17) m(-2) s(-1) (2 x 10(-9) dpas(-1)). Most of if not all the precipitates smaller than about 8 nm in diameter were found to have a twinned 9R structure similar to that seen in thermally aged model alloys. Some precipitates larger than about 8 nm in diameter appeared to have transformed wholly or partly to 3R or f.c.c. The results confirm that the usual assumption that the main effect of irradiation is to enhance the diffusion of copper, and hence to accelerate the diffusion kinetics, is substantially correct. The smaller size at which the 9R-3R-f.c.c. transformation seems to occur under irradiation may be due to incorporation of vacancies in the precipitates or to nucleation of small dislocation loops at the precipitate-matrix interface, either of which may modify the critical size at which the 9R-3R transformation is triggered.