ANS:B - metals of high stacking fault energy e.g. α-iron.

Dynamic recovery is a phenomenon that occurs during hot working processes, such as hot rolling or forging, where the material undergoes plastic deformation at elevated temperatures. During dynamic recovery, some of the dislocations in the material rearrange and form new dislocation configurations, leading to a partial recovery of the material's mechanical properties. Here's how dynamic recovery relates to the given options:

1. Metals of Low Stacking Fault Energy: These materials typically exhibit significant dynamic recovery during hot working. Low stacking fault energy materials, such as aluminum and copper, have a high propensity for dynamic recovery due to the ease with which dislocations can move and rearrange at elevated temperatures.
2. Metals of High Stacking Fault Energy (e.g., α-iron): These materials tend to undergo dynamic recrystallization rather than dynamic recovery during hot working. Dynamic recrystallization involves the formation of new grains within the material, which helps relieve the accumulated strain and leads to a more refined microstructure.
3. Single Crystal of Ni-based Superalloys: Single crystals of nickel-based superalloys are typically used in high-temperature applications such as gas turbine engines. These materials are engineered to have high creep resistance and maintain their mechanical properties at elevated temperatures. During hot working of single crystals, the deformation behavior is more complex, and dynamic recovery may occur depending on the specific alloy composition and processing conditions.