Grain size and cooling rate: faster cooling -> smaller grain sizes; harder; increasing hardness reduces toughness and ductility.

Grain size and cooling rate: faster cooling -> smaller grain sizes; harder; increasing hardness reduces toughness and ductility.

• A. Faster cooling increases grain size
• B. Slower cooling increases hardness
• C. Faster cooling decreases grain size and increases hardness
• D. Cooling rate does not affect grain size

Answer: (C)

Faster cooling leads to a finer grain structure and typically increases hardness. When a metal cools quickly, many small grains form and stop growing early, producing a fine-grained microstructure. The greater number of grain boundaries from these small grains hinders dislocation motion, which makes the material harder (a form of grain boundary strengthening). However, this higher hardness often comes at the expense of toughness and ductility, since the material becomes less able to deform plastically before fracturing. So, the statement that faster cooling decreases grain size and increases hardness is the best match. The other options conflict with the established effects of cooling rate: faster cooling does not increase grain size, slower cooling does not increase hardness, and cooling rate does affect grain size.

Faster cooling leads to a finer grain structure and typically increases hardness. When a metal cools quickly, many small grains form and stop growing early, producing a fine-grained microstructure. The greater number of grain boundaries from these small grains hinders dislocation motion, which makes the material harder (a form of grain boundary strengthening). However, this higher hardness often comes at the expense of toughness and ductility, since the material becomes less able to deform plastically before fracturing.

So, the statement that faster cooling decreases grain size and increases hardness is the best match. The other options conflict with the established effects of cooling rate: faster cooling does not increase grain size, slower cooling does not increase hardness, and cooling rate does affect grain size.