Metals yield at a stress much lower than those calculated on the basis of their bond strength alone. This is explained by the presence of defects. Identify the principal defects responsible for this observation.
The principal defects responsible for the observation that metals yield at a stress lower than expected based on their bond strength alone are dislocations. Dislocations are linear defects in the crystal structure of metals, and they play a crucial role in determining the mechanical properties of metals.
To understand this concept, we need to first discuss the crystal structure of metals. Most metals have a highly ordered arrangement of atoms in a regular pattern called a crystal lattice. However, perfect crystals without any defects are extremely rare in practice. Instead, crystals often contain imperfections or defects that affect their mechanical behavior.
Dislocations are one type of defect found in metals. They occur when there is a deviation from the regular crystal lattice arrangement along a line, known as the dislocation line. The presence of dislocations helps explain why metals yield at stresses lower than predicted based on their bond strength alone.
When an external stress is applied to a metal, the dislocations can move and interact with each other. This movement and interaction of dislocations result in the plastic deformation of the metal, allowing it to yield and permanently deform. This process is called slip.
The presence of dislocations enables the metal to undergo plastic deformation at much lower stresses compared to the theoretical bond strength because dislocations provide an easier path for atoms to move within the crystal lattice. This movement of dislocations allows metal crystals to slip past each other, accommodating the stress and resulting in plastic deformation.
In summary, dislocations are the principal defects responsible for the observation that metals yield at stresses lower than expected based on their bond strength alone. Their presence allows for easier deformation and slip of metal crystal lattice, enabling plastic deformation to occur at lower stresses.