Precipitation of guinier- preston zones in aluminum- magnesium; a calorimetric analysis of liquid-Quenched and solid-Que
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INTRODUCTION
A G I N G of an aluminum-rich matrix (face-centered cubic) supersaturated with dissolved magnesium, can ultimately lead to the precipitation of/3 (AI3Mg2: face-centered cubic) particles (see phase diagramtt]). However, a number of metastable phases, which can occur before the emergence of the equilibrium phase (/3), have been reported. There is general agreement concerning the occurrence of an intermediate/3' (AI3Mg2: hexagonal) phase immediately preceding the/3 phase. But no consensus exists with respect to possible preprecipitates. Guinier-Preston (GP) zone formation can occur during aging at relatively low (e.g., room) temperature of A1Mg alloys homogenized at elevated temperature and subsequently quenched (denoted as solidquenched (SQ) alloys). Some authors report that at least about 10 at. pct Mg should be dissolved for GP-zone formation to occur at room temperature, [2'3] whereas others claim that this critical amount equals about 5 at. pct Mg. [4'5] Enhanced aging may lead to a more stable intermediate phase (as derived from a shift to a higher temperature of the endothermal DSC (Differential Scanning Calorimetry) peak ascribed to the GP-zone dissolution on nonisothermally heating (constant heating rate) an aged alloy). This phase has been interpreted as an ordered "GP-zone" (denoted as
M. VAN ROOYEN, Graduate Student, and E.J. MITTEMEIJER, Professor, are with the Laboratory of Metallurgy, Delft University of Technology, Rotterdamseweg 137, 2628 AL Delft, The Netherlands. J.A. SINTE MAARTENSDIJK, formerly Student at the Laboratory of Metallurgy of the Delft University of Technology, is with Kemira B. V., Moezelweg 151, 3198 LS Rozenburg, The Netherlands. Manuscript submitted June 17, 1987. METALLURGICALTRANSACTIONS A
fl,[4~ or 6'I52), possibly with an L12-type superstructure (A13Mg). [6'7]
Only a few data exist about the aging of aluminum alloys prepared by rapid solidification from the liquid state (denoted as liquid-quenched (LQ) alloys). As compared to SQ alloys, in general one may expect that the initially high cooling/solidification rate for LQ alloys has large effects on the aging kinetics. This has been demonstrated by recent experiments with A1Si-alloys and was ascribed to a relatively high amount of excess vacancies and a very fine grain structure. Is] However, in some cases relatively low values have been reported for the excess-vacancy concentration after liquid-quenching Eg~(see also results presented in this paper). The purpose of this paper is to investigate by calorimetric analysis heat effects and kinetics of the formation and dissolution of preprecipitates in both LQ and SQ A1Mg-alloys. Important stimuli for this work are the following: (i) the totally different precipitation behavior expected for LQ AIMg-alloys as compared to the previously investigated LQ A1Si-alloys[~~ (no preprecipitates or intermediate phases have been reported for A1Si); (ii) the role of the type of quenching performed; in the only previous work known to us on precipitation in LQ A1Mg alloys it was cla
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