  The mechanical properties of Mg-Al alloys are greatly influenced by the
complex intermetallic phase Mg$_{17}$Al$_{12}$, which is the most dominant
precipitate found in this alloy system. The interaction of basal edge and
30$^\text{o}$ dislocations with Mg$_{17}$Al$_{12}$ precipitates is studied by
molecular dynamics and statics simulations, varying the inter-precipitate
spacing ($L$), and size ($D$), shape and orientation of the precipitates. The
critical resolved shear stress $\tau_c$ to pass an array of precipitates
follows the usual $\ln((1/D + 1/L)^{-1})$ proportionality. In all cases but the
smallest precipitate, the dislocations pass the obstacles by depositing
dislocation segments in the disordered interphase boundary rather than shearing
the precipitate or leaving Orowan loops in the matrix around the precipitate.
An absorbed dislocation increases the stress necessary for a second dislocation
to pass the precipitate also by absorbing dislocation segments into the
boundary. Replacing the precipitate with a void of identical size and shape
decreases the critical passing stress and work hardening contribution while an
artificially impenetrable Mg$_{17}$Al$_{12}$ precipitate increases both. These
insights will help improve mesoscale models of hardening by incoherent
particles.
