Transformation defects on theta ' precipitates in Al-Cu alloys

摘要

Growth defects in the form of transformation disconnections (TDs) are considered for the precipitation of theta' in an Al - Cu alpha solid solution. The minimum sized TDs are predicted to be 1 - 2 nm in height, in agreement with experimental observations. One type of TD is glissile and the other ( more prevalent) type is sessile, requiring climb for motion. The presence of a fault plane on one side of the sessile TD does not impede its formation. The results are in accord with symmetry theory and with the topological model for precipitation.