High resolution electron microscope observations on precipitation in Al-3.0% Cu alloy

Abstract

A high resolution election microscope study (1) has been extended to longer aging times at 130°C to follow the precipitation sequence. Monolayer G.P. [1] zones and groups of parallel zones which were present initially, parallel to {100} matrix planes, tended to coarsen. Isolated zones of up to 300 Å dia. by nine atom layers in thickness were observed after 10 days aging, when continuous electron diffraction streaks still existed through {200} type matrix spots parallel to 〈100〉 directions. Breaking-up of these continuous streaks, normally attributed to G.P. [2] ( θ n ) formation, was not accompanied by any distinct microstructural change, rather, a continuous structure progression was observed from G.P. [1] to [2] to θ. Once formed, θ′ particles coarsened at the expense of G.P. zones. Resolution of (001) lattice fringes of 7.9 Å spacing at G.P. [2] zones permitted distinction from θ', which showed (001) lattice fringes of 5.8 Å spacing. Single and crossed (101) and/or (Oil (lattice fringes of 3.3 Å spacing were also resolved at θ' particles, providing positive identification. θ' particles tended to become lenticular on coarsening. Electron diffraction streaking attributable to θ' platelets was observed parallel to 〈001〉 matrix directions through spots at {110} matrix positions and tended to disappear at longer aging times. Brightfield and dark-field observations suggested that individual copper-rich planes viewed edge-on were resolved.