Final answer:
Two screw dislocations of opposite sign can annihilate each other because one's helical turn can undo the twists of the other, similar to a clockwise twist being undone by a counterclockwise one. The atomic distances between their slip planes can affect the ease of interaction, but do not prevent annihilation.
Step-by-step explanation:
To answer the student's question, yes, it is possible for two screw dislocations of opposite sign to annihilate each other. Dislocations are linear defects in a crystal structure, and screw dislocations occur when there is a helical turn of the crystal lattice along the line of the dislocation. When two screw dislocations of opposite sign but on parallel slip planes approach each other, they can effectively cancel out as the helical twists of one undo the twists of the other. This process reduces the overall energy of the crystal lattice and subsequently removes the dislocations.
It is important to note that for annihilation to occur, the two dislocations must have opposite signs, meaning one is right-handed and the other is left-handed, analogous to how a clockwise twist can be undone by a counterclockwise twist. The concept of their slip planes being separated by several atomic distances doesn't prevent their annihilation. However, the distance can affect the ease with which they can move and interact; closer slip planes generally facilitate faster interactions.