Effect of Trace Additions of Ag on Precipitation in Al–Mg Alloys

Abstract

Effect of Ag additions on the precipitate microstructures of the Al–10Mg alloys has been investigated using transmission electron microscopy (TEM) and electron diffraction.In the Al–10Mg alloy aged at 240°C, age-hardening response is attributable initially to an array of coarse-scale, sparsely distributed rod-like and/or plate-like β′ precipitate (particles). At the maximum hardness, the microstructure contained an increased volume fraction of coarse-scale β′ and β precipitate particles. In a well over-aged condition, the microstructure exhibited very coarse-scale β precipitate particles. However, in the Al–10Mg–0.5Ag alloy aged at 240°C, fine-scale and uniformly distributed icosahedral quasicrystalline precipitates were observed in the early stages of ageing, and it was to be replaced by the metastable crystalline T phase after the alloy is aged for 2 h at 240°C. The T phase formed as faceted rods parallel to 〈110〉α directions appeared to be the primary strengthening constitute exhibiting maximum hardness. The globular β precipitate particles were observed to be replaced by the metastable rod-like T precipitate particles after the alloy was aged 72 h at 240°C, and the β phase is confirmed to be the equilibrium constitute phase in the over-aged ternary Al–10Mg–0.5Ag alloy.