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

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 examined, 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 distributed precipitates within the ferrite or in association with dislocations. Copper precipitates adopted either the Kurdjumov–Sachs or the Nishiyama–Wasserman orientation relationship with respect to the ferrite. Disc-shaped precipitates thought to form by ageing within abnormal and pearlitic ferrite develop on {001}ferrite habit planes. Twinning observed within copper precipitates formed by interphase precipitation was attributed to a requirement to accommodate the growth of precipitates at the interphase boundary. Multi-twinned precipitates were found in pearlitic ferrite and may be evidence for an intermediate transitional state between b.c.c. and f.c.c. structure.