Final answer:
The strength of a metal is not always proportional to an increase in dislocation density. While initial increases in dislocation density typically strengthen the material, excessive dislocation can lead to work softening or material weakening, especially at higher temperatures. Option B is correct.
Step-by-step explanation:
The strength of a metal is not always proportional to an increase in dislocation density. While it's true that initially, as dislocation density increases, the metal's strength usually increases due to dislocations hampering the movement of other dislocations and making deformation more difficult, there is a limit to this strengthening mechanism.
At higher dislocation densities, work hardening can occur, where further deformation becomes very difficult, making the material stronger. However, too high a dislocation density can lead to a condition known as work softening, where the material may actually start to weaken.
Dislocations may combine or cancel each other out, or lead to crack initiation and propagation, which ultimately reduces the strength of the material.
Variables such as temperature also affect the relationship between dislocation density and material strength. At elevated temperatures, the increased atomic vibrations can allow dislocations to move more easily, thereby reducing the strength of the material despite high dislocation density. Hence, the correct answer to the student's question is B) No, strength may or may not increase with dislocation density.