The heterogeneous nucleation and growth of precipitates at dislocations in Al–Cu–Mg alloy were investigated by examining sample aged at 195C for various times from 600s to 9h. High-resolution transmission electron microscopic observations and electron energy loss spectrometry analysis showed that the precipitation sequence of the S (Al2CuMg) phase along dislocations should be SSS → GPB zones → S (Type I) → S (Type I) + S (Type II). Type II S precipitate could nucleate and grow separately at some dislocation parts which were unfavorable for Type I S precipitate formation, or could transit from Type I S precipitate during lateral growth along dislocations by means of continuous or stepped changing in lattice orientations. Both of these modes were accomplished under the action of dislocation strain field. The deviation extent of Type II S precipitate was considered not changing with time but closely related with the interactions between the transformation strain during nucleation and the dislocation strain field.

Precipitation Process Along Dislocations in Al-Cu-Mg Alloy During Artificial Aging. Z.Feng, Y.Yang, B.Huang, M.Han, X.Luo, J.Ru: Materials Science and Engineering A, 2010, 528[2], 706-14