Joint differential scanning calorimetry, electrical resistivity, and microhardness study of aging in two AlCuMg alloys
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S. Abis Techma s.r.l., Zona Industriale di Ottana, 08100 Bolotana (NU), Italy, and INFM, Torino Universita`, Italy (Received 11 January 2003; accepted 24 March 2003)
Two age-hardenable AlCuMg alloys (Cu 4.5 wt.% and Cu/Mg ratios of 2.5 and 8) were studied by electrical resistivity, differential scanning calorimetry, and microhardness measurements during natural and artificial aging at 110 °C. The results were interpreted in terms of Guinier–Preston/Guinier–Preston–Bagariatskij zone formation followed by a modification in ⬙ and S⬙ intermediate phases.
I. INTRODUCTION
Due to their interesting combination of technical properties in terms of strength-to-density ratio, good corrosion and stress corrosion properties, and welding ability, age-hardenable AlCuMg alloys have been widely investigated to study microstructure evolution during both natural and artificial aging. It is now accepted that agehardening after solution treatment and quenching proceeds via two main precipitation sequences based on early formation of metastable phases from the ␣ supersaturated solid solution, which evolves during time and/ or temperature; namely, ␣sss → (Cu)GP → ⬙ → ⬘ → (CuAl2)
,
␣sss → (CuMg)GPB → (S⬙) → S⬘ → S(CuMgAl2)
(1) , (2)
where GP and GPB stand for the Guinier-Preston and the Guinier-Preston-Bagariatskij zones, respectively. Depending on the Cu content and the Cu/Mg ratio, sequence (1) or (2) predominates. In the early stages of the supersaturated solid solution decomposition, a small amount of solute atoms starts to aggregate. In this condition two types of aggregation are proposed to take place, depending on size and order degree of the forming structures: clusters or GP/GPB zones. The distinction may be irrelevant in determining the final properties of the material, since it is generally accepted that the strength increase during aging is associated with the formation of a pattern of tetragonal ⬘-phase and orthorhombic S⬘-phase with their precursors, but it has recently assumed a certain importance in the interpretation of the first hardness increase following lowtemperature annealing. The vacancy-assisted migration 1522
J. Mater. Res., Vol. 18, No. 7, Jul 2003
of solute atoms to form aggregates having a different degree of order and different size may in fact be responsible for a different pinning of dislocations, which in turn determines the hardness of a material. Recently, the secondary aging process1 and its effect on mechanical properties have renewed the interest on the first decomposition stages of a wide composition range of aluminium alloys. In previous papers on this topic,2,3 we have already discussed some results on the transformation sequence in two AlCuMg alloys having different Cu/Mg ratios. Here we aim to complete the considerations made by presenting electrical resistivity data on the same alloys during the first stages of natural and artificial aging. The discussion is made by comparing the results with calorimetric and microhardness tests. Some transmission electron microscopy (TEM) observati
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