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

Cu-63 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 alpha-phase with Cu in random substitutional sites, (ii) Guinier-Preston zones, (iii) theta"-phase, (iv) theta'-phase and finally (v) theta-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 Cu-63 Fr-NMR spectrum at room temperature, together with a line assigned to surface Cu sites on theta'-phase platelets. Two additional lines have been observed in spectra from specimens containing the theta-phase due to Cu substituting for Al in the CuAl2 structure, and Cu in the CuAl eta(2)-phase. Density functional based ab initio electronic structure calculations have been used to reproduce accurately the observed electric field gradient (V-zz) at the Cu site in theta-phase CuAl, and to explain the relatively narrow Cu-63 lines observed for GP-zones, theta"-phase and theta'-phase. (C) 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.