63Cu NMR investigation of effect of small additions of Sn to Al-1.7 at.%Cu in promoting accelerated phase transformations on aging

Very recently 63Cu NMR has been shown to be extremely sensitive in detecting and differentiating between the precipitate phases that form in Al-Cu alloys during heat treatment. This technique is now used to quantify the effectiveness of small additions of Sn to the alloy Al-1.7 at.% Cu in promoting the rapid nucleation and growth of the θ'-phase precipitate. Two parallel series of 63Cu NMR spectra were recorded for Al-1.7 at.% Cu and Al-1.7 at.% Cu-0.01 at.% Sn: (i) aged at 130° C to observe the comparative rate of phase evolution and (ii) aged at 200° C to observe the rate of growth of θ'-phase and to compare with the Vickers hardness of the alloys aged at 200° C for similar periods. Evidence is presented that a metastable precursor phase to θ' (labelled TPHM2757math001) is formed in Al-Cu-Sn which transforms to θ' on further aging.