Q1: The elastic strain in copper is due to the

ANS:A - motion of dislocations.

Dislocations are linear defects or irregularities within the crystal lattice structure of a material. The motion of dislocations refers to the movement of these defects through the lattice when an external force is applied to the material. This motion of dislocations plays a significant role in the plastic deformation of crystalline materials. Here's how the motion of dislocations works:

1. Initiation: Dislocations can start moving under an applied force when the stress reaches a critical value known as the yield strength. At this point, the force overcomes the resistance to dislocation motion within the material.
2. Dislocation Glide: The most common mechanism of dislocation motion is called glide. In this process, dislocations move through the crystal lattice by the sequential movement of atoms in the lattice planes. This movement is typically along specific crystallographic directions and planes that offer the least resistance.
3. Obstacles and Interactions: Dislocations encounter various obstacles as they move through the lattice, such as other dislocations, grain boundaries, and impurities. These obstacles can impede or hinder the motion of dislocations, leading to strain hardening and increased material strength.
4. Dislocation Types: Dislocations can take different forms, such as edge dislocations, screw dislocations, and mixed dislocations. The type of dislocation influences the mechanisms by which they move through the lattice.
5. Plastic Deformation: The motion of dislocations is responsible for the permanent deformation or plasticity observed in materials. As dislocations move and interact, they cause changes in the material's shape and structure, leading to plastic deformation.
6. Temperature and Rate Dependency: The motion of dislocations can be influenced by temperature and the rate of deformation. Higher temperatures and slower deformation rates generally facilitate dislocation motion by providing more energy for overcoming obstacles.

Understanding and controlling the motion of dislocations is crucial in materials science and engineering for tailoring the mechanical properties of materials, such as strength, ductility, and toughness. Techniques like alloying, heat treatment, and mechanical processing can be used to manipulate dislocation behavior and improve the performance of materials in various applications.