Precipitation-Lab - Precipitation Hardening of Aluminum...

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Precipitation Hardening of Aluminum Alloy Objective : The purpose of this laboratory is for you to explore the kinetics and strengthening of aluminum alloy using the precipitation hardening method. Readings : Read Callister 8 th Edition section 11.9 on “ Precipitation hardening Introduction : Pure FCC metals such as Al have low yield strengths because the stress required to move a dislocation is small. One method of strengthening an alloy is through the precipitation reactions . Precipitation hardening is the process of hardening or strengthening of an alloy by precipitating finely dispersed precipitates of the solute in a supersaturated matrix. This process involves the following three basic steps: 1. The first step is the solution heat treatment or homogenization. In the Figure 1 shown below, we illustrate this phenomenon. During this step, an alloy of composition X1 is heated to a temperature T1, between the solvus and solidus temperatures and soaked there until all of the solute dissolves into the α phase and a uniform solid solution structure is produced. 2. The second step is quenching. Quenching is simply cooling the sample rapidly to a lower temperature, T3, usually room temperature, and the cooling medium is usually water at room temperature. During quenching, the solute is not immediately able to diffuse out of a phase and the alloy is said to be supersaturated . The rationale behind the quenching process is to preserve the uniform solid solution structure of the alloy below the homogenization temperature. 3. The final step in the precipitation hardening process is aging. It is the process of precipitating incoherent precipitates from a supersaturated solid solution. When aging occurs at room temperature, it is called natural aging. Aging above room temperature is called artificial aging.
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Figure 1. Hypothetical binary A-B phase diagram for the Precipitation Hardening scheme.
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