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The influence of Mg concentrations on the dissolution of Guinier-Preston (GP) zones formed at room temperature (RT) and on the formation of more stable phases has been investigated during continuous heating of AI-4.5 at% Zn-xMg alloys. The Mg content was varied from 0.05 to 3 at %. After different aging periods at RT, calorimetric investigations were carried out at heating rates of 40 and 80 ~ C min -I . In the case of alloys with a lower Mg content (x ~< 0.5 at %) only the dissolution of GP zones could be observed during the heating, whereas in the case of alloys with a higher Mg content the formation of the ~?'-phase started before the total dissolution of GP zones and at higher temperatures the formation of the r/-phase also took place. These phases were identified by transmission electron microscopy. The heat-of-solution of GP zones shows saturation as a function of RT aging time. The time needed for the saturation increased monotonously with increasing Mg content. The reversion of zones was followed by in situ X-ray small angle scattering measurements. The change of the total scattered intensity was measured during continuous heating at a rate of 40 ~ C min -1 . These investigations have confirmed the results of the calorimetric measurements which indicate that the total dissolution of zones takes place only in the case of the alloys with a Mg content lower than 0.5 at%. In the case of alloys with a Zn concentration of 4.5 at % studied here, 1 at % Mg is sufficient to initiate the formation of more stable phases during the reversion of zones.

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AlZnMgCu alloys are important structural niaterials because of their high strengths and good stress corrosion resistance (SPEIDEL; ARCHAMBAULT, CHEVIER, BECK). The relationship between the properties and structure, as well as the formation of different phases in the ternary alloys are well known (UNGAR et al.; LOFFLER e t al.). However, the decornposition and trarisforniation processes in the quaternary AIZnMgCu alloys are not well known at present and there are several contradictions in the literature. Only a few systematical investigations have been made on the role of Cu i n the deconiFosition piocesses in the quaternary alloys (BRYANT; SOENO, CHIGASAKI).The aim of this work is to deterniine the effect of Cu addition on the precipitation processes i n the AlZn5Mg2 alloys. ExperimentalThe alloys were prepared of high purity alurniniuiu ancl alloying eleinents ancl cast in t t i~ induction furnace into iron mould. The composition of the alloys investigated can be seen in Table 1 . The samples were cut from sheets after cold rolling.Electrical resistance, tensile properties, inicrohardness measurements completed by clifferential scanning calorirnetry and X-ray sillall-angle scattering investigations have been conducted. The detttils of the experimental technics are published previously (UNQAR The processes investigated were the following : G1' zone formation at about room ternperature, Ostwald ripening above 60 "C and the fornmtion of transition phases between 130 .~i i d I80 "C. One part of kinetical iiieasureinents was carried out "in-situ" in a thermostat Lhe others a t roo111 teiiipertttirre or in liquid nitrogen after interrupting the heat treatrnent