The formation of the Guinier-Preston zones in aluminium alloys is closely linked with the excess vacancies. Traces of tin added to an Al-Ag alloy exert an influence on the Guinier-Preston zones precipitation. Due to their high binding energy with vacancies, tin atoms trap some of these available to promote the diffusion of silver atoms for the formation of the Guinier-Preston zones. At 125°C, tin microalloying slows down the reaction of the Guinier-Preston zones precipitation. The diffusion coefficient of the solute atoms in the Sn free alloy and in the Sn added alloy are determined during the coarsening regime which obeys to the Lifshitz, Slyosov and Wagner theory.Keywords: Guinier-Preston zones; precipitation; coarsening; diffusion; vacancies; hardening 1 Introduction Al-Ag supersaturated solid solution evolves towards the equilibrium state following the sequence [1-4]: Supersaturated solid solution → Guinier-Preston (GP) zones → metastable γ' phase → equilibrium γ phase. The Guinier-Preston zones (GP), consisting of silver atom clusters, are coherent with the matrix. The metastable phase γ'(Ag2Al) is semi-coherent with the matrix and the equilibrium phase γ(Ag2Al), is incoherent with the matrix. Precipitation starts from the formation of GP zones, which are isomorphous with the matrix and, therefore, have a lower interfacial energy than intermediate or equilibrium precipitate phases that possess a distinct crystal structure. As a result, the nucleation barrier for GP zones is significantly smaller. The effect of microalloying elements on the behaviour of age-hardenable alloys such as Al-Ag is an interesting physical problem addressing the mechanisms of transport and aggregation of the solute. Trace elements have been found to exert a disproportionate influence on the structure and properties of Al alloys compared with the amounts added which may be less than 0,1 molar fraction . Most trace element effects arise because they modify the nucleation and the growth characteristics of the phases which form during precipitation [5][6][7][8][9]. It is well known that the formation of the GP zones in aluminium alloys is closely linked with the excess vacancies. A number of models of GP zones precipitation assisted by vacancies has been developed by several authors [10][11][12]. GP zones formation is governed by a transport mechanism of solute atoms by solute atom-vacancy complexes. The high binding energy between tin atoms and vacancies leads to the formation of vacancy-tin atom pairs and silver atom-vacancy-tin atom complexes.

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Using microhardness and electrical resistivity measurements, the contributions of the matrix and that of the Guinier-Preston zones to the hardening and to the electrical resistivity of the Al10at.%Ag alloy are determined separately during the Guinier-Preston zones precipitation. A linear correlation between the hardness and the electrical resistivity of the as quenched alloys exists. There is also a linear relationship between the contribution of the matrix to the hardening and that to the electrical resistivity of the isothermal aged alloy. However, no linear relation exists between the hardness and the electrical resistivity of the isothermal aged alloy.

The development of the transition metastable phases and the accompanying hardening of these transformations are studied for Al-20wt%Ag and Al-3wt%Cu-1wt%Mg supersaturated solid solutions. During the precipitation the GP and GPB zones form by a process of negative diffusion. The metastable phases are characterized by their hardness isotherms and their temperature range determined on the hardness isochronal.