Abstract
The present work was carried out so as to determine the phase diagram on the aluminium rich side in the Al–Mg system with 0.5 wt% silver by means of the micrographic method, electrical resistivity measurements and differential thermal analysis.
The solubility of magnesium in aluminium was decreased by the addition of 0.5 wt% silver since the solid solubility curve of the α phase in this alloy shifted to an appreciably higher temperature range than that in the binary Al–Mg alloy. The results obtained by electron probe microanalysis and electron diffraction analysis showed that the equilibrium precipitates in the alloys with 0.5 wt% silver (≤9wt% Mg) consisted mainly of a ternary compound, the T phase (Mg32(Al, Ag)49), at higher temperature (≥300°C). On the other hand, in an Al-9 wt% Mg-0.5 wt% Ag alloy aged at 300°C, the β phase was detected and the precipitation of the β phase became more dominant as the aging temperature lowered. The ageing behaviour of an Al-6 wt% Mg-0.5 wt% Ag alloy was also studied by hardness measurements and electron microscopic observations. The precipitates in the alloy containing silver were finer and more uniformly distributed than those in a binary Al-6 wt% Mg alloy, which accounts for the remarkable increase in its age-hardening. The improvement of the age-hardening by a small addition of silver might be partly associated with the direct effect of the increased degree of supersaturation for precipitation of the T phase. However, as the most important cause for the improved age-hardening, it is considered that the increased supersaturation and the chemical affinity between silver and magnesium would bring about the formation of the stable G–P zones, which act as the sites of nucleation of T phase precipitation.
