Smaller grain size tends to increase which properties?

Smaller grain size tends to increase which properties?

• A. Tensile strength and ductility
• B. Electrical conductivity
• C. Thermal expansion
• D. Creep resistance

Answer: (A)

Smaller grain size increases the number of grain boundaries in a metal. These boundaries impede the movement of dislocations, which are the carriers of plastic deformation. That barrier effect makes it harder for the material to deform, so the strength—specifically yield and tensile strength—rises as grains become finer. This strengthening is described by the Hall-Petch relation. Ductility, however, typically does not rise with finer grains; it often decreases because the same boundaries that harden the material also limit how much uniform plastic deformation the material can undergo before fracture. The other properties shown—electrical conductivity and thermal expansion—are not strongly improved by grain refinement, and creep resistance at high temperature usually does not benefit from smaller grains (it can even worsen due to grain-boundary diffusion). So, the main, consistent effect of smaller grain size is increased tensile strength, which is why the option that pairs tensile strength with ductility is considered the best match among the given choices.

Smaller grain size increases the number of grain boundaries in a metal. These boundaries impede the movement of dislocations, which are the carriers of plastic deformation. That barrier effect makes it harder for the material to deform, so the strength—specifically yield and tensile strength—rises as grains become finer. This strengthening is described by the Hall-Petch relation. Ductility, however, typically does not rise with finer grains; it often decreases because the same boundaries that harden the material also limit how much uniform plastic deformation the material can undergo before fracture. The other properties shown—electrical conductivity and thermal expansion—are not strongly improved by grain refinement, and creep resistance at high temperature usually does not benefit from smaller grains (it can even worsen due to grain-boundary diffusion). So, the main, consistent effect of smaller grain size is increased tensile strength, which is why the option that pairs tensile strength with ductility is considered the best match among the given choices.