Which of the following correctly describes the characteristics of copper/aluminium alloys subjected to age hardening?

In the context of age hardening, copper and aluminum alloys undergo a specific treatment that involves solution heat treatment followed by aging. This process allows the formation of fine precipitates within the metal matrix, which significantly enhances strength and hardness.

When subjected to the age-hardening process, these alloys indeed become hard and can exhibit increased brittleness as they age, particularly when they are in certain conditions of microstructure. The hardening occurs due to the precipitates obstructing dislocation movement within the crystal structure, which contributes to the material's overall strength. However, if these alloys lose too much ductility in the process, they may exhibit brittle behavior under stress.

The other choices do not accurately capture the behavior of copper and aluminum alloys during age hardening. These materials do not remain soft and ductile after aging; rather, they gain strength at the expense of some ductility. Significant temperature changes affect these alloys, making them subject to changes in mechanical properties, and they do not gain flexibility over time due to the hardening process. Thus, the statement that they become hard and brittle as they cool aligns with the fundamental principles of age hardening in metallurgy.