Positron studies of precipitation phenomena in Al-Li and in Al-LI-X (X=Cu, Mg or Sc) alloys

Positron lifetime measurements were used for studying decomposition during post-quench in situ annealing of binary Al-7.35 at.% Li and ternary Al-7.3 at.% Li-X (X=1.1 at.% Cu, 5.2 at.% Mg or 0.18 at.% Sc) alloys. A typical lifetime of 185+or-5 ps observed in Al-Li is attributed to coherent delta '(Al 3 Li) particles. In ternary alloys positrons show a strong response to the formation of semicoherent and incoherent phases such as T 1 (Al 2 LiCu) and Al 2 LiMg. Positron trapping in Al-Li-Sc is attributed to composite delta ' (Al 3 Li)/Al 3 Sc particles associated with vacancies. Quenched-in vacancies were detected in Al-Li-Sc and Al-Li-Mg but not in Al-Li-Cu and Al-Li.