X-ray investigation of ageing in an aluminium-5% magnesium alloy with small silver additions

Abstract

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, Mg 32(Al, Ag) 49 with (100) T ∥(112) Al and [001] T ∥ [11̄0] Al.