X-RAY INVESTIGATION OF AGEING IN AN ALUMINIUM-5 PERCENT MAGNESIUM ALLOY WITH SMALL SILVER ADDITIONS

The structural changes associated with ageing of aluminium-5 wt.% magnesium alloys containing 0.5 and 1.0 wt.% silver have been investigated by X-ray diffraction methods. At lower temperatures and shorter times, the hardening is shown to be due to the formation of approximately spherical Mg-Ag zones having an AuCu I type structure. Initially the zones are coherent with the matrix but with further ageing their tetragonality increases, mainly due to a decrease in the c axis, and to accommodate the increasing stress, orientation changes occur such that (101)zone ∥(110)Al and [010]zone ∥[001]Al. The lattice parameters of the zones at this stage are a = 4.42 A ̊, c = 3.48 A ̊, c/a = 0.79. Line breadth measurements indicate that they have an average size of at least 60 Å. At higher temperatures the hardening is due to the precipitation of a T phase, Mg32(Al, Ag)49 with (100)T ∥(112)Al and [001]T ∥ [11̄0]Al. © 1968.