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

摘要

~(63)Cu NMR has been used to observe the evolution of the precipitate structures in the solution treated and quenched dilute alloy AI-l.7 at percent 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 CuAl_2. The Cu site in all five of these phases has been detected as a resolved line in the ~(63)Cu PT-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 CuAl_2 structure, and Cu in the CuAl eta_2-phase. Density functional based 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_2 and to explain the relatively narrow ~(63)Cu lines observed for GP-zones, theta"-phase and theta'-phase.