Describe the phenomenon of overaging and why it causes a dro

Describe the phenomenon of overaging and why it causes a drop in mechanical properties Why is it necessary to quench the aluminum after solutionizing to achieve the desired superior mechanical properties instead of lening il gradually cool in air?

Solution

Any Aluminium alloy that exhibits similar phase diagram as that of Al & Cu can be age hardened. Al-Cu alloys with less than 5.5% copper in it is heated to a temperature range so that a uniform phase \'alpha\' appears in the microstructure from that temperature, mterial is quenched to room temperature. As a result of that copper will be locked in the microstructure of Aluminium. As the time passes, copper starts coming out of the microstructure and produces precipitates of Al₂Cu. These precipitates creates an obstacle in the movement of dislocation. This increases the strength of material and this phenomenon is called age or precipitation hardening. As the time passes further more and more precipitates. So, dislocation will find more number of obstacles in its path so the strength increases further. Since the copper in the sample will be in the limited quantity. So after some time nucleation stops. This corresponds to the peak strength of the sample. Later on smaller precipitates are combining together and forming a bigger precipitate. This increases the average distance between the precipitates and hence dislocation find lesser number of obstacles in its path. This phenomenon is called Overaging.