This lecture covers the phenomenon of hardening in materials due to plastic deformation, focusing on the increase in yield strength and decrease in ductility. It explains the role of dislocations, work hardening mechanisms, and obstacles hindering dislocation movement. The instructor discusses strategies to reduce hardening in crystalline materials, such as restoration, recrystallization, and joint movement. The lecture also delves into the principles of hardening, the evolution of load surfaces, and the mechanics of plastic flow in single crystals. Various stages of strain hardening, dislocation interactions, and the impact of friction energy on transformation hysteresis are explored.