THE EFFECT OF RETROGRESSION AND RE-AGING ON THE DUCTILE FRACTURE-TOUGHNESS OF AL-ZN-MG ALLOYS CONTAINING DIFFERENT DISPERSOID PHASES

The effects of an incoherent and of a semi-coherent dispersoid phase u pon the fracture toughness of a 7000-series AlZnMg alloys has been inv estigated. Standard crack opening displacement tests were successfully applied, the crack propagation being shown to be predominantly interg ranular. The presence of manganese bearing incoherent dispersoids lowe rs the fracture toughness due to their effect of promoting microvoid f ormation. Retrogression and re-ageing led to a coarsening of the grain -boundary phases, but because the surface-to-surface separation of the se particles remained approximately constant, there was no effect of t his treatment, at constant yield stress, upon the fracture toughness. The toughness was only enhanced in the system (CZH) where retrogressio n and re-ageing led to a reduced yield stress.