How do grain boundaries affect the mechanical property of metallic materials?

Grain boundaries play a crucial role in the mechanical properties of metallic materials by acting as barriers to dislocation movement, which is the primary mechanism of plastic deformation. A higher density of grain boundaries typically increases the strength of the material through mechanisms like Hall-Petch strengthening, where smaller grains lead to greater resistance to deformation. However, excessive grain boundaries can also lead to embrittlement, particularly at high temperatures or in certain environments. Therefore, the size, distribution, and character of grain boundaries significantly influence the balance between strength and ductility in metals.