Q1: On decreasing the grain size of a polycrys-talline material, the property most likely to deteriorate is

ANS:B - toughness

On decreasing the grain size of a polycrystalline material, the property most likely to deteriorate is creep resistance. Here's why:

1. Creep Resistance: Creep is the time-dependent deformation that occurs under constant load or stress at elevated temperatures. In polycrystalline materials, such as metals and alloys, creep resistance is influenced by grain size. Larger grains provide more obstacles to the movement of dislocations, which hinders creep deformation. Conversely, smaller grain sizes offer fewer barriers to dislocation motion, making the material more prone to creep deformation.
2. Effect of Grain Size: As the grain size decreases in a polycrystalline material, the number of grain boundaries increases. Grain boundaries act as sites for the initiation and propagation of creep deformation. Smaller grains have a higher volume fraction of grain boundaries per unit volume of material, providing more pathways for creep deformation to occur. Therefore, decreasing the grain size typically leads to reduced creep resistance.

While toughness, tensile strength, and fatigue properties can also be influenced by grain size, the effect is typically less pronounced compared to creep resistance. In fact, smaller grain sizes can sometimes lead to improvements in toughness and fatigue resistance due to mechanisms such as grain boundary strengthening. However, for creep resistance, the relationship with grain size is more straightforward, with smaller grain sizes generally resulting in poorer creep resistance. Therefore, on decreasing the grain size of a polycrystalline material, the property most likely to deteriorate is creep resistance.