Precipitation in Al-Li-Cu ternary alloys has been investigated mainly
by a resistivity study. The precipitation sequence in this alloy is as
follows: supersaturated solid solution->delta'->T-1. The resistivity
decreases at first and then decreases again passing through a peak. Th
is anomalous behaviour has been interpreted using transmission electro
n microscopy and it is suggested that the resistivity decreases accomp
anying the precipitation of delta' (Al3Li), while it increases with pr
ecipitation of the T-1 (Al2CuLi) phase and then decreases through a ma
ximum. The time taken to arrive at the maximum in resistivity is compa
rable with the time to peak hardness, which suggests that the increase
in resistivity may be attributed to the strain field around T-1 preci
pitates. Contrast experiments by transmission electron microscopy show
the strain field due to the T-1, which disappears on prolonged aging.