Abstract
The scanning electrical resistivity (SER) technique has been used as a method of investigating the phase transformations in an Al-Cu alloy during the formation and subsequent dissolution of fully coherent and/or semicoherent phases, resulting from an annealing at 190°C after solution treatment and quench.
The dissolution of a structure of coherent precipitates produces an SER trace where the contribution to the resistivity of the Al matrix of the solute Cu atoms alone is not sufficient to explain the resistivity trend. The theory of Hillel-Edwards-Wilkes (HEW) is invoked to take into account the contribution of precipitated particles to the resistivity in terms of direct scattering of the conduction electron. Good agreement with experimental data is obtained.
The SER results are compared with DSC traces, and it is shown that the two techniques are able to supply complementary information during the dissolution phenomena when coherent particles are involved.
The results obtained suggest that the precipitation sequence at 190°C is characterized by the appearance of a fully coherent phase (GPII) followed by the growth of the semicoherent θ′ phase (CuAl2).
The GPII particles grow during about ten hours of ageing, and then θ′ precipitates appear while the GPII phase tends to disappear.