A calorimetric study of precipitation in aluminum alloy 2219
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IT is often of interest to have an accurate assessement of the microstructure of age hardenable aluminum alloys. Particular items of concern are the identity of precipitate phases present, the relative amounts of the various phases and the particle sizes and distributions. Transmission electron microscopy, TEM, is well suited for this task and is generally the preferred technique. The principle disadvantages of TEM for this work are the time and skill required to obtain the data, the small sample volumes that can be examined and the difficulties of imaging small, densely precipitated particles. In previous work, 1-3differential scanning calorimetry, DSC, was used in conjunction with TEM to characterize the microstructures of three precipitation hardening, 7000 series, A1-Zn-Mg, alloys. After a detailed comparison of the TEM and DSC results, it was found that DSC could be used as a rapid method for identification and characterization of the precipitate phases and the DSC could provide a complimentary and, sometimes, more quantitative description of the overall microstructure. The present work was undertaken in order to apply this technique to an aluminumcopper commercial alloy, 2219. Aluminum alloy 2219 is nominally aluminum with 6.3 wt pct copper and small amounts of several other elements. Its age hardening behavior is comparable to that of the pure binary A1-Cu system,4 for which a wealth of experimental information is available. Reviews of the structures observed in A1-Cu have been given by Hardy and Heal, 5 Kelly and Nicholson, 6 and Phillips. 7 In a recent review,8 Lorimer gives the aging sequence as; supersaturated a ~ GP zones ~ 0" ~ 0' --* 0 Lorimer gives the structures of the various phases and also argues for the use of 0" rather than GP(II). In JOHN M. P A P A Z I A N is Staff Scientist, Research Department, Grumman Aerospace Corp., Bethpage, NY 11714. Manuscript submitted May 28, 1980.
addition, evidence has recently been found for room temperature spinodal decomposition of the solid solution before the appearance of GP zones. 9 Calorimetric techniques have been used to study precipitation in A1-Cu since the 1930's. The early work has been reviewed by Hardy2 More recent studies have been performed by Hirano and Iwasaki, ~~Thompson n and Zahra e t a l . 12-~4 All of these authors observed several peaks in the heat capacity v s temperature curves and assigned specific reactions to the peaks based upon the previous X-ray results of Silcock e t a l . ~5 Their results were generally in agreement with each other, but there was some disagreement on specific points, particularly the distinction between the GP zone dissolution peak and the 0" dissolution peak. 1~ These authors also used calorimetric measurements to study the aging sequence, reversion phenomena and the limits of stability of the various precipitate phases. Their results will allow us to make a detailed comparison between the behavior of the pure binary A1-Cu and the commercial alloy 2219. Specific points of comparison will be discussed in the Re
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