⁶³Cu NMR analysis of microstructure evolution in Al–Cu–Mg alloys

Abstract

⁶³Cu NMR spectroscopy has been used to detect metastable Guinier–Preston–Bagaryatsky (GPB) zones and nanoscale precipitates of equilibrium S-phase (Al₂CuMg) in dilute alloys of aluminium containing copper and magnesium with compositions which lie in the α + S phase field. The GPB zones are observed to form rapidly at room temperature with a time development closely related to the Vickers hardness. The final development of S-phase in the alloy has been confirmed by the observation of a line shape in the alloy identical to that observed in a specimen prepared from stoichiometric Al₂CuMg. Analysis of the hyperfine structure of the ⁶³Cu line shape observed for S-phase shows clearly that two Cu sites are present with approximately equal population. This result suggests that possibly two crystallographically distinct Al₂CuMg phases are present. The addition of small amounts of silver to Al–Cu–Mg alloys in the α + θ phase field is known to induce the formation of Ω-phase: a slight distortion of tetragonal θ-phase Al₂Cu. A hyperfine-structured ⁶³Cu line shape assigned to Ω-phase, indicating one distinct Cu site, has been observed in two separate Al–1.7 at.% Cu–0.33 at.% Mg alloys containing 0.1 and 0.18 at.% Ag, but not in the same Al–Cu–Mg alloy without Ag.

Acknowledgements

I am grateful to Professor Ian Polmear for generously supplying the high purity Al–Cu–Mg and Al–Cu–Mg–Ag alloys, to Professor Polmear and Dr Roger Lumley for numerous useful discussions, to Dr Mark Gibson for the Al₂CuMg ingot and to Dr A.J. Hill for the hardness measurements shown in figure 1b.