Processing and microstructure of
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I.
INTRODUCTION
THE growth of the aerospace industry has provided impetus for the development of a new generation of aluminum alloys capable of improved elevated-temperature performance. Initial attempts to upgrade the elevatedtemperature performance of aluminum alloys involved the optimization of microstructure through the control of composition and thermomechanical processing of ingot metallurgy alloys. Prime examples are the precipitation hardening alloys, such as 2219 and 2618.t~l These alloys are limited in service to temperatures below about 180 °C, since precipitate coarsening and attendant loss of strength occur at higher temperatures. For strength and retention of strength at elevated temperatures~ a high volume fraction of a fine-scale second phase (i.e., dispersoid) that is thermodynamically stable and uniformly dispersed in the aluminum matrix is mandated. At a given volume fraction, coarsening of the second phase at elevated temperature is accompanied by a decrease in yield strength, since the spacing between the second phase increases. The coarsening process is termed Ostwald ripening, t21 and mathematical expressions have been derived which predict the coarsening kinetics of the dispersoid; the controlling factor is the operative diffusion mechanism. L31The thermal stability of the dispersoids is determined by the diffusivity of the alloying elements in the aluminum matrix, the solubility of the alloying elements in the aluminum matrix, and the interfacial energy between the dispersoid and the matrix. The original theory of coarsening kinetics by Lifshitz, M.K. PREMKUMAR, Staff Engineer, is with the Fabricating Technology Division, Aluminum Company of America, Alcoa Center, PA 15069. A. LAWLEY and M.J. KOCZAK, Professors, are with the Department of Materials Engineering, LeBow Engineering Center, Drexel University, Philadelphia, PA 19104. Manuscript submitted October 17, 1991. METALLURGICAL TRANSACTIONS A
Slyozov, and Wagner (LSW) did not take into account the effects of a finite volume fraction of the dispersoid on diffusion rates, t4,51 As the volume fraction of the dispersoid increases, the diffusion fields surrounding individual particles overlap and accelerate coarsening; in addition, coalescence may be a contributing factor to growth at high volume fractions of dispersoid. Ardel1161 was the first to consider the effect of overlapping diffusion fields. Modification of the LSW model involved a multiplying factor, but the magnitude of the predicted effect was not substantiated by experimental observations, t71 Subsequently, Brailsford and Wynblatt181 predicted that the coarsening rate should be far less sensitive to the volume fraction of dispersoid than is calculated from the model by Ardell. I61 More recently, Voorhees and Glicksman tgl modeled the effect of second-phase volume fraction on coarsening; their results also predict volume fraction effects of the same magnitude as Brailsford and Wynblatt. ISI The high level of solute atoms required to produce a large volume fraction of dispersoid c
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