Precipitation process along dislocations in Al–Cu–Mg alloy during artificial aging

▶ HRTEM observations was carried out on the microstructure evolution along dislocations during the aging process. ▶ The precipitation sequence of the S phase at dislocation is suggested to be SSS → GPB zones → S (Type I) → S (Type I) + S (Type II). ▶ Two probable formation modes for Type II S precipitate, namely independently nucleation and growth and lateral growth, were proposed. The heterogeneous nucleation and growth of precipitates at dislocations in Al–Cu–Mg alloy were investigated by examining sample aged at 195 °C for various times from 10 min to 9 h. High resolution transmission electron microscopy (HRTEM) observations and electron energy loss spectrometers (EELS) analysis show that the precipitation sequence of the S (Al 2CuMg) phase along dislocations should be SSS → GPB zones → S (Type I) → S (Type I) + S (Type II). Type II S precipitate can nucleate and grow separately at some dislocation parts which are unfavorable for Type I S precipitate formation, or can 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 are accomplished under the action of dislocation strain field. The deviation extent of Type II S precipitate is considered not changing with time but closely related with the interactions between the transformation strain during nucleation and the dislocation strain field.