ME 3701, Materials of Engineering Laboratory, LSU 1Experiment: Precipitation Hardening of Aluminum Alloys Objective The objective of this investigation is to learn how and why certain alloys (primarily Aluminum alloys) can be strengthened by Precipitation Hardening heat treatment processes. Abstract The strength and hardness of some metal alloys may be enhanced by the formation of extremely small uniformly dispersed particles of a second phase within the original phase matrix; this must be accomplished by appropriate heat treatment. The process is called Precipitation Hardening or Age Hardening which involves three distinct steps: Solution Treatment to minimize segregation in the alloy, Quenching to create a supersaturated solid solution and Aging to facilitate the formation of coherent precipitates which strengthen the alloy by interfering with dislocation movement. Both the Natural Aging and Artificial Aging processes will be investigated by measuring the hardness of 2024 aluminum specimens at various time intervals into the aging process following the quench. Phenomena related to precipitation hardening, such as Guinier-Preston zones, overaging, coherent participates, etc., will be discussed. Background 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, nickel-based 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 αand α+ θregions. Consider a 96wt%Al –4wt%Cu alloy which is chosen since there is a large degrease in the solid solubility of solid solution αin decreasing the temperature from 550°C to 75°C. The precipitation-hardening process involves three basic steps: Solution Treatment, Quenching and Aging.Solution Treatment, or Solutionizing, is the first step in the precipitation-hardening process where the alloy is heated above the solvus temperature and soaked there until a homogeneous solid solution (α) is Figure 1 - The aluminum rich end of the Al-Cu phase diagram showing the three steps in the age-hardening heat treatment and the microstructures that are produced. [Askeland, 1994]
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