When dislocation meet the grain boundary. The grain boundary present obstacles to dislocation motion. A usual point is that the dislocation will pile up against the grain boundary(But from the expriments , it seldomly see this phenomenon.). Macroscopic yielding occours when the adjacent grain can deform plastically Which maybe effected by the emission of dislocation from the grain boundary, or in another way , the pileup dislocation can produce a stress concentration which can active the dislocation source in the adjacent grain. As we knowe ,in polycrystalline, the material will express a Hall-Petch relationship between the yielding stress and the grain dimension. There are many models to explain the H-P relationship,such as pile-up dislocations, average fress distance....