  The influence of different types of precipitates (either Guinier-Preston
zones, $\theta''$ or $\theta'$) on the critical resolved shear stress and
strain hardening was determined by means of micropillar compression tests in an
Al - 4 wt. \% Cu alloy. The size, shape and volume fraction of the precipitates
were measured in each case. It was found that size effects were negligible for
micropillars with diameter $\ge$ 5 $\mu$m. Micropillars with Guinier-Preston
zones showed strain localization due to precipitate shearing. The best
mechanical properties were obtained with either a fine dispersion of the
$\theta''$ precipitates or a coarser dispersion of $\theta'$. Both precipitate
shearing and Orowan loops were observed around the $\theta''$ precipitates and
the micropillar strength was compatible with the predictions of the Orowan
model. In the case of the alloy with $\theta'$ precipitates, the strengthening
contribution associated with the transformation strain around the precipitates
has to be included in the model to explain the experimental results. Finally,
the micropillar compression tests in crystals with different orientations were
used to calibrate a phenomenological crystal plasticity. This information was
used to predict the mechanical properties of polycrystals by means of
computational homogeneization.
