MECHANISM OF AL2CULI (T1) NUCLEATION AND GROWTH

Conventional strain contrast transmission electron microscopy (CTEM) and high-resolution transmission electron microscopy (HRTEM) were performed to establish the nucleation and growth mechanism of Al2CuLi (T1) precipitates in an Al-Li-Cu alloy. It is shown that the growth mechanism of T1 precipitate plates occurs by the diffusional glide of growth ledges composed of b = 1/6 [112] partial dislocations on {111} matrix planes and that the growth ledges migrate by the ledge-kink mechanism, as previously suggested by Cassada et al.[1] for this system. T1 plate nucleation is modeled as the dissocaiation of a perfect b = 1/2 [110] matrix dislocation in the vicinity of a dislocation jog. The coordinated dissociation of the dislocatioN line segments on each side of the sessile jog provides the displacement necessary for the formation of a new hexagonal plate or plate ledge. Strain contrast analysis of the Burgers vector of plate edges and the edges of growth ledges indicates the stacking of partial dislocations is of mixed displacement.