Abstract
A study on the development of ultra-fine grained (UFG) structures in AA 2099-T8 and recently-developed AA 2624-T351 aluminum alloys was conducted using thermomechanical processing (TMP). Specimens were cryogenically-rolled to 70% thickness-reduction at −196 °C and artificially-aged at 200 °C, 250 °C, and 300 °C for times between 30 mins and 8 h. Analyses show that T1 (Al2CuLi) and δ′ (Al3Li) precipitates are dominant in AA 2099-T8 while AA 2624-T351 contains no precipitate. Upon cryo-rolling, hardness of AA 2099 decreased by ∼24%, while that of AA 2624 increased by ∼11%. The optimum ageing conditions for obtaining UFG structures in cryo-rolled AA 2099 and AA 2624 are 250 °C for 4 h and 250 °C for 1 h, respectively. It was found that dispersoids from the starting coarse-grained microstructure were inherited at every stage of TMP. New phase (S′) evolved in the interior of ultra-fine grains in AA 2624, while only T1 re-evolved in the UFG structure of AA 2099.
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