Phase transformation of Cu precipitates from bcc to fcc in Fe-3Si-2Cu alloy

Phase transition of Cu precipitates during aging of an Fe-3Si-2Cu alloy was studied by transmission electron microscopy. The precipitation of 3-5-nm-sized body-centered cubic (bcc) Cu in ferrite matrix was confirmed by high-angle annular dark-field scanning transmission electron microscopy imaging. The bcc Cu precipitates transformed to 9R Cu as they grew. Many 9R Cu precipitates were twinned, but untwinned 9R Cu particles were also observed. The 9R Cu transformed to twinned face-centered cubic (fcc) Cu by the glide of +/- a/3 [100](9R) Shockley-type partial dislocations. Formation of the 3R structure previously reported could not be confirmed in this study. Finally, twins in fcc Cu precipitates disappeared to form stable fcc Cu particles. The importance of electron beam-orientation-dependent moire fringes in the correct identification of Cu structure is discussed in detail. (C) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.