ABSTRACT7xxx (Al-Zn-Mg-Cu) alloys develop their strength, through the controlled decomposition (ageing) of a supersaturated solid solution to produce a precipitation hardened microstructure. The supersaturated solid solution is normally produced by water quenching from the solution heat treatment temperature. The kinetics of phase transformations occurring during cooling from the solution heat treatment temperature are limited at small undercoolings by the small chemical free energy, and as the undercooling increases, by reduced rates of substitutional solid state diffusion. This gives rise to the characteristic C shape of time-temperature-property (TTP) curves. The TTP curve for 7010 has been evaluated by a interrupted quench method into a salt bath at temperatures ranging from 450 to 210°C. A TTP curve for 7050T76 sheet data has then been used to predict tensile property inhomogeneity in large 7010 forgings with known cooling rates.
The CEC funded thematic network CREEPAL (BRRT ± CT98 ± 5101) was established in 1998 to collect and disseminate information on the long term creep and thermal mechanical cycling behaviour of aluminium alloys. This paper presents some of the information gathered by consortium members and some additional industrial information related mainly to forging alloys. A short history of the use of aluminium alloys at elevated temperature is presented. The principal considerations required when evaluating aluminium alloys for use at temperatures in the 100 ± 350°C range are detailed. The important microstructural mechanisms of creep in aluminium alloys are reviewed and the physical metallurgy of Hiduminium ± RR.58 (2618A) is examined. The alloys that have found commercial application at elevated temperature are listed. Recent developments are described and opportunities for further development considered.MST/5593
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