The strength and hardness of some metal alloys may be enhanced by the formation of extremely small uniformly dispersed second-phase particles within the original phase matrix in a process known as "Precipitation (or Age) Hardening". The precipitate particles act as obstacles to dislocation movement and thereby strengthen the heat-treated alloys. Many aluminum based alloys, copper-tin, certain steels, nickelbased super-alloys and titanium alloys can be strengthened by age hardening processes. In order for an alloy system to be able to be precipitation-strengthened, there must be a terminal solid solution that has a decreasing solid solubility as the temperature decreases. The Al-Cu phase diagram shown in Figure 1 shows this type of decrease along the solvus between the α and α + θ regions. Consider a 96wt%Al – 4wt%Cu alloy which is chosen since there is a large degrease in the solid solubility of solid solution α in decreasing the temperature from 550°C to 75°C. The precipitation-hardening process involves three basic steps: Solution Treatment, Quenching and Aging.