Structure evolution in dilute Al(Cu) alloys observed by 63Cu NMR Original Research Article

63Cu NMR has been used to observe the evolution of the precipitate structures in the solution treated and quenched dilute alloy Al-1.7 at%Cu following thermal aging. The distinguishable phases in this alloy form in the following sequence (i) fcc α-phase with Cu in random substitutional sites, (ii) Guinier-Preston zones, (iii) θ″-phase, (iv) θ′-phase and finally (v) θ-phase, where the last two phases are structural variants of CuAl2. The Cu site in all five of these phases has been detected as a resolved line in the 63Cu FT-NMR spectrum at room temperature, together with a line assigned to surface Cu sites on θ′-phase platelets. Two additional lines have been observed in spectra from specimens containing the θ-phase due to Cu substituting for Al in the CuAl2 structure, and Cu in the CuAl η2-phase. Density functional based ab initio electronic structure calculations have been used to reproduce accurately the observed electric field gradient (Vzz) at the Cu site in θ-phase CuAl2 and to explain the relatively narrow 63Cu lines observed for GP-zones, θ″-phase and θ′-phase.