Precipitation on low-angle grain boundaries in Al-Zn-Mg

Abstract

Weak beam microscopy was used to examine the precipitation of the 17 phase (MgZn2) on low angle grain boundaries which had formed in an Al-2.84 wt pct Zn-1.95 wt pct Mg alloy. The low angle boundaries observed in the partially recovered structure can be separated into two categories, planar and nonplanar. The planar boundaries form either lozenge or hexagonal configurations. The nonplanar boundary dislocation intersections form a “chair”-shaped figure. The geometry of the dislocation strain fields in the boundaries controls the precipitate growth and the size, shape, and position of the precipitates on each boundary type. The lozenge boundary can have both equiaxed and lath shaped precipitates growing upon it. The equiaxed precipitates form only at four dislocation junctions because at this position the strain field distribution is symmetric. The lath precipitates are restricted to grow along dislocations in the boundary whose line direction is coincident with the long axis of the precipitate since in these pinned dislocation segments there are high line tension forces present to resist dislocation bowing. The “chair” boundary contains only lath precipitates, which begin to grow from alternate three dislocation intersections and continue growth along the dislocation line. The contrast observed bordering the lath on each side is produced by lattice dislocations. A proposed explanation for the above observations involves assuming that the dislocations in the chair boundary dissociate into Lomer-Cottrell locks which have constricted-extended node pairs. A lath can begin growth from the constricted node forcing the stair rod dislocation to dissociate. The reaction products of the stair rod can combine with the partials in the Lomer-Cottrell lock to form the observed lattice dislocations.