  We consider the stability of precipitates formed at grain boundaries (GBs) by
radiation-induced segregation in dilute alloys subjected to irradiation. The
effects of grain size and misorientation of symmetric-tilt GBs are quantified
using phase field modeling. A novel regime is identified where, at long times,
GBs are decorated by precipitate patterns that resist coarsening. Maps of the
diffusional P\'eclet number indicate that arrested coarsening takes place when
solute advection dominates over thermal diffusion right up to the
precipitate/matrix interface, overwhelming capillary effects. This contrasts
with liquid-solid mixtures where convection only accelerates coarsening.
