Effect of retrogression and reaging treatments on the microstructure of Ai-7075-T651

Abstract

The effect of the retrogression and reaging treatments (RRA) on the microstructure of Al-7075 in the T651 temper, both in the matrix and on grain boundaries, was studied using transmission electron microscopy. The processes occurring in the matrix during the retrogression treatment are principally the dissolution of small particles of the η’ transition phase, transformation to η of the larger particles of η’, coarsening of the three commonly observed variants of the η phase precipitates (η₁, η₂, and η₄), and precipitation of new η phase particles, particularly the η₁ variant. The main process occurring during the reaging treatment is either growth of partially dissolved η’ particles or precipitation of the η’ phase. These lead to a microstructure containing many fine η’ precipitates and some larger η₁ and η₂ particles with a smaller amount of coarse η₄ particles, resulting in a broad particle size distribution. The high strength of the 7075 alloy in the RRA temper is believed to arise from the relatively high overall concentration of particles in this dispersion. The retrogression treatment produces rapid initial coarsening of the grain boundary particles, which are primarily η phase precipitates, resulting in an increase in their volume per unit grain boundary area,V _A . The beneficial effect of the RRA treatment on the susceptibility of 7075-T651 to SCC is believed to be due, at least partially, to the increased value ofV_A produced by the RRA treatment.