The precipitation of copper in abnormal ferrite and pearlite in hyper-eutectoid steels

The precipitation of copper within abnormal ferrite and pearlitic phases in hyper-eutectoid Fe-C-Cu and Fe-C-Mn-Cu experimental steels has been examin ed, principally by transmission electron microscopy, in terms of morphology , mechanism and crystallography. Interphase precipitation of copper during austenite decomposition was found to be a primary precipitation mechanism. However, precipitation due to ageing was also observed in both abnormal and pearlitic ferrite. The latter case appeared either as uniformly distribute d precipitates within the ferrite or in association with dislocations. Copp er precipitates adopted either the Kurdjumov-Sachs or the Nishiyama-Wasserm an orientation relationship with respect to the ferrite. Disc-shaped precip itates thought to form by ageing within abnormal and pearlitic ferrite deve lop on {001}(ferrite) habit planes. Twinning observed within copper precipi tates formed by interphase precipitation was attributed to a requirement to accommodate the growth of precipitates at the interphase boundary. Multi-t winned precipitates were found in pearlitic ferrite and may be evidence for an intermediate transitional state between b.c.c. and f.c.c. structure. (C ) 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.