  Fracture in aluminum alloys with precipitates involves at least two
mechanisms, namely, ductile fracture of the aluminum-rich matrix and brittle
fracture of the precipitates. In this work, a coupled crystal plasticity-phase
field model for mixed ductile-brittle failure modes is formulated and used to
investigate the effect of precipitate morphology and size distribution on
damage evolution in aluminum alloys. A thermodynamically consistent framework
for elastic and plastic work dissipation in the fracture process zone is used
to formulate the coupled constitutive behavior for brittle and ductile damage,
respectively. Representative Volume Elements (RVE) with varying particle
morphology and orientation were generated and their uni-axial loading was
simulated to assess the damage resistance of the different model
microstructures. For critical energy release rate $G_c$ values of the aluminum
matrix ranging from 4 to 8~Jm$^{-2}$ for single crystals and from 10 to
16~Jm$^{-2}$ for polycrystals, the model predicts a change in failure modes
from particle debonding to cracking followed by ductile matrix failure. The
change of particle failure mechanism as a result of increased $G_c$ is observed
for both, single and polycrystalline model microstructures. For the case of
particle debonding, microstructures with circular or ellipsoidal particles
(with the major axis perpendicular to the loading direction) show a higher
ductility and fracture work compared to the other studied cases. In the case of
particle cracking, microstructures with ellipsoidal particles aligned parallel
to the loading axis show a higher ductility and fracture work among the
investigated cases. The simulations qualitatively reproduce the experimentally
observed particle failure mechanisms (cracking and debonding) for two different
matrix alloy classes (commercially pure and 2xxx alloys) reinforced with
ceramic particles.
