What characteristic is associated with small grains in metals?

Small grains in metals are often associated with brittleness due to the way they affect the metal's microstructure and mechanical properties. When the grain size is small, the metal has an increased number of grain boundaries, which can impede the movement of dislocations—these are defects in the crystal structure that allow deformation under stress. As a result, metals with fine grains tend to resist deformation more than those with larger grains, making them more susceptible to fracture rather than bending or stretching, which characterizes brittle behavior.

In contrast, ductility is more commonly found in materials with larger grain sizes that allow for more slip systems to activate, enabling the material to deform plastically before breaking. Toughness involves a metal's ability to absorb energy and plastically deform without fracturing, which is also typically enhanced in coarser-grained materials. Flexibility is not a primary consideration in this context, as it more directly relates to polymers and composite materials than to the intrinsic properties of metals.