Microstructures of the commercial 7075 Al alloy in the T651 and T7 tempers

Abstract

The microstructures of the commercial 7075 Al alloy in the peak aged (T651) and overaged (T7) tempers were studied using transmission electron microscopy. The microstructure of 7075-T651 is characterized predominantly by the presence of a fine dispersion of the ή transition phase; the primarily plate-shaped ή particles have diameters ranging from 3 to 10 nm. The microstructure of 7075-T7 mainly consists of a bimodal size distribution of ή phase variants,i.e., plate-shaped ή₁ and ή₂ particles ranging from 12.5 to 30 nm in diameter and finer ή₁ particles 5 to 10 nm in diameter. The ή transition phase is identified as having a hexagonal crystal structure with lattice parameters, α = 0.496 nm andc = 1.403 nm. The maximum hardness of the 7075 alloy is believed to arise mainly from the fine dispersion of small ή particles.