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

Abstract

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

) is formed in Al–Cu–Sn which transforms to θ′ on further aging.

Acknowledgements

We are grateful to Ian Polmear for many useful discussions and supplying references to previous work, and to him, Anita Hill and Kate Nairn for critical readings of the text. The initial stage of this work was supported by a Monash University Engineering Research Grant.