Zirconium vs manganese-chromium for grain structure control in an Al-Cu-Li alloy
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The authors wish to thank E.H. Hearn, R.T. Laing, R. G. Trimberger, and L. A. Wojcik for their technical support in metal processing.
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Zirconium vs ManganeseChromium for Grain Structure Control in an AI-Cu-Li Alloy R.C. DORWARD Most wrought, high-strength aluminum alloys have an elongated grain structure. This type of microstructure is intentional; compared to an equiaxed grain shape, it provides superior mechanical properties and stress corrosion resistance in the more highly stressed longitudinal and longtransverse directions. Grain structures are controlled by additions of alloying elements that form dispersoid particles (MnA16, Crml7, 'E' phase, ZrA13, etc.), which retard grain boundary migration. In A1-Li alloys, these dispersoids are also thought capable of affecting mechanical properties by dispersing slip. Although manganese was utilized for grain structure control in the first commercial lithium-containing alloys AA 2020 (A1-4.5 pct Cu-l.2 pct Li) and 01420 (A1-5 pct Mg-2 pct Li), it fell into disfavor when the more effective benefits of zirconium were realized. 2 Investigations into the specific influence of manganese on the mechanical properties of A1-Li alloys have not shown any significant benefits, 3-6 and in some cases indicated negative effects. Likewise, chromium has not proven effective in A1-Li alloys, 7 and has not been considered for commercial alloys. To determine if a combined addition of manganese and chromium has merit,* base alloys of nominal composition *Conventional aluminum alloys AA7039, 7079, and a number of 5XXX alloys contain both manganese and chrommm.
A1-2.7 pct Cu-2.1 pct Li and containing 0.12 pct Zr or 0.60 pct Mn plus 0.16 pct Cr were compared in sheet form. A manganese-to-chromium ratio of about 4 was chosen to maximize the probability of forming the voluminous MnA112 phase, 8 although the actual presence of this compound has not yet been confirmed. The alloys were cast as 16 cm square ingots, which were homogenized at 540 ~ for 10 hours. They were then reheated to 480 ~ and hot rolled to 2.5 mm thick sheet, maintaining a minimum temperature of 260 ~ The actual compositions of the sheets, as determined spectrographically by inductively coupled plasma analysis, are given in Table I. Sections of each alloy were solution heat-treated at 530 ~ for 20 minutes, quenched in cold water, stretched 4 pct, and aged for 48 hours at 138, 149, 160, and 171 ~ These aging practice
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