Cottrell atmosphere

A Cottrell atmosphere refers to the localized gathering of impurity atoms, such as carbon or nitrogen, around dislocation lines in a metal's crystal structure. These impurity atoms are attracted to the stress fields created by dislocations, effectively "pinning" them in place. This pinning increases the metal's strength and hardness because it makes it more difficult for dislocations to move, which is necessary for plastic deformation. The concept helps explain how certain impurities can strengthen metals at the microscopic level by forming these atmospheres that hinder dislocation movement.