Answer:
The correct answer is: A. Solidify at a different temperature that increases the nucleation rate.
Step-by-step explanation:
The grain size of a metal is determined by the number of nuclei that form during solidification. The more nuclei that form, the smaller the grain size will be. The nucleation rate is increased by decreasing the temperature of the molten metal. This is because the lower temperature reduces the energy barrier for nucleation, making it more likely for nuclei to form.
The growth rate of the solid nuclei is also affected by the temperature. However, the effect of temperature on the growth rate is much smaller than the effect on the nucleation rate. Therefore, the best way to decrease the grain size is to solidify the metal at a lower temperature.
Adding more heterogeneous nucleating agents to the molten melt will also increase the nucleation rate and decrease the grain size. However, this is not as effective as decreasing the temperature. This is because the nucleating agents can only form nuclei at the surface of the molten metal. The lower temperature will cause nuclei to form throughout the molten metal, resulting in a smaller grain size.
Adding less heterogeneous nucleating agents to the molten melt will decrease the nucleation rate and increase the grain size. This is because the nucleating agents provide sites for nucleation to occur. Without the nucleating agents, it is more difficult for nuclei to form, resulting in a larger grain size.