What is the process known as age hardening?

Age hardening, also known as precipitation hardening, is a process that enhances the strength and hardness of certain alloys, particularly aluminum and titanium alloys. This process involves a series of heat treatments that lead to the formation of tiny, dispersed precipitates within the metal matrix. Over time, these precipitates form as the alloy is held at a specific elevated temperature and then cooled, which results in an increase in hardness and strength.

The term "age" refers to the time that the alloy is allowed to undergo this transformation at certain temperatures, which is crucial to achieving optimal mechanical properties. As the precipitates grow and coalesce, they impede the movement of dislocations, which is a primary mechanism of deformation in metals, ultimately leading to a harder and more brittle structure.

The other choices do not describe the age hardening process accurately. Rapid cooling from high temperatures refers to quenching, which is a different process entirely. The addition of alloying elements may improve hardness, but it doesn't encompass the aging process itself. Annealing is aimed at improving ductility and relieving internal stresses, which is contrary to the goal of age hardening, which is to increase hardness and strength.