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Ductile crack growth in a grain boundary layer is modeled under mode I plane-strain, small-scale yielding conditions. The calculations are carried out for planar polycrystals using an elastic–viscoplastic constitutive relation for a progressively cavitating solid with two populations of void nucleating second-phase particles. The material properties in the grain boundary layers are taken to be representative of those for the α phase boundaries in metastable Ti β alloys, while the grain properties are homogenized representations of the grain properties in such alloys. Void nucleation is taken to occur rather early in the deformation history in the grain boundary α phase layers by a stress-controlled nucleation criterion. Subsequent strain-controlled void nucleation occurs within the grains as well as in the grain boundary α layers. The effects of grain flow strength, average grain size, grain morphology and grain boundary α …