Effect of trace additions of Ag on precipitation in Al-Mg alloys

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 degrees C, age-hardening response is attributable initially to an array of coarse-scale, sparsely distributed rodlike and/or plate-like beta' precipitate (particles). At the maximum hardness. the microstructure contained an increased volume fraction of coarse-scale beta' and beta precipitate particles. In a well over-aged condition, the microstructure exhibited very coarse-scale P precipitate particles. However, in the Al-10Mg-0.5Ag alloy aged at 240 degrees 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 degrees C. The T phase formed as faceted rods parallel to < 110 >(a) directions appeared to be the primary strengthening constitute exhibiting maximum hardness. The globular beta precipitate particles were observed to be replaced by the metastable rod-like T precipitate particles after the alloy was aged 72 h at 240 degrees C, and the beta phase is confirmed to be the equilibrium Constitute phase in the over-aged ternary Al-20Mg-0.5Ag alloy.