  In this paper, we study the effects of $\theta ^\prime$-Al$_2$Cu plate-like
precipitates on the plasticity of Al-Cu micro-pillars, with a sample size
allowing the precipitates to cross the entire micro-pillar. {100}-slip traces
are identified for the first time in Al and Al alloys at room temperature. We
investigate the underlying mechanisms of this unusual {100}-slip, and show that
it operates along the coherent $\theta ^\prime$-Al$_2$Cu
precipitate/$\alpha$-Al matrix interface. A combination of molecular dynamics
simulations and stress analysis indicates that screw dislocations can
cross-slip from the {111} plane onto the {100} $\theta
^\prime$-Al$_2$Cu/$\alpha$-Al interface, then move on it through a kink-pair
mechanism, providing a reasonable explanation to the observed {100}-slips. The
roles of the $\theta ^\prime$-Al$_2$Cu/$\alpha$-Al matrix interface on the
properties of interfacial dislocations are studied within the Peierls-Nabarro
framework, showing that the interface can stabilize the {100} screw
dislocations from the spreading of the core, and increases the Peierls stress.
These results improve our understanding of the mechanical behavior of Al-Cu
micro-pillars at room temperature, and imply an enhanced role of interfacial
slip in Al-Cu based alloys at elevated temperature in consideration of the
underlying kink-pair mechanism.
