Which property generally increases along with the size of crystalline grains in a metal?

As the size of crystalline grains in a metal increases, the property that typically improves is ductility. Ductility refers to a material's ability to deform under tensile stress, which allows it to be stretched into wires or molded without breaking. Larger grains generally allow for more slip systems to operate during deformation, thereby enabling the material to absorb energy and undergo plastic deformation more readily.

In metals with larger grains, the number of grain boundaries—where two crystal grains meet—decreases, which reduces the obstacles to dislocation motion. Dislocations are defects in the crystal structure that play a significant role in the deformation of materials. When grain size is increased, dislocations can move more freely, resulting in enhanced ductility.

In contrast, smaller grains tend to increase strength due to the Hall-Petch relationship, where smaller grains hinder dislocation movement, thus making the material harder and stronger. However, this increased strength often comes at the expense of ductility, leading to a more brittle behavior. Therefore, while larger grain sizes enhance ductility, they may not favor hardness or strength as effectively as smaller grains.