330 Section 4 Nonferrous Metallurgy At high temperatures, cold work cannot strengthen the metal, and solid solution hardening is not effective either. The only way to strengthen an alloy at high temperature and reduce creep is to add an element that forms precipitate particles that are stable at those temperatures. Refractory alloys that contain stable precipitates resist creep better than the same alloys without the precipitates. 15.3.5 Strengthening through Heat Treatment—Preci pitation Hardening Many nonferrous alloys can be precipitation hardened, including selected alloys of copper (Cu), aluminum (Al), nickel (Ni), and titanium (Ti). The copper alloy UNS C17200 (CDA 172), which is copper alloyed with 1.7% beryllium (Be), demonstrates the process well. Figure 15-10 shows the different stages of the heat-treatment cycle on the copper-beryllium phase diagram. Precipitation Hardening With precipitation hardening of nonferrous alloys, the general heat-treat procedure is very similar for each alloy: 1. The workpiece is heated to a high temperature until all of the second element is in solution in a single phase. 2. The workpiece is quenched to a low temperature rapidly. The second element remains in solution. 3. The workpiece is aged at a moderate temperature. Small, fine precipitates involving the second element form uniformly throughout the workpiece. PROCEDURE PROCEDURE When the C17200 alloy strip arrives at the product fabrication plant, it is in position A on the phase diagram, a two-phase region. The metal contains large CuBe particles, and is completely recrystallized through the final anneal in the copper production plant. The CuBe particles impede some dislocation motion, but they are so few that little strengthening occurs. Ductility is reasonable. Solutionizing When parts have been formed from the s heet, they are heated into the single-phase region near 1400°F (760°C), position B on Figure 15-10. The CuBe particles dissolve into the copper matrix. The same thing happens to the CuBe particles in the alloy as happens to the sugar dissolving in water in the Self-Discovery activity “Mixtures, Solutions, and Phases—Sugar in Water,” found at the end of Chapter 4. Position B is sometimes called the solutionizing temperature, because the alloys become a single solution at that point. For the same reason, the metal at 1800 Solid + liquid Liquid Cu + Be Cu + Cu2Be particles fcc Cu— single-phase solid solution fcc Cu + CuBe particles— two-phase mixture Cu2Be 1600 1400 1200 Percent Be A 0.0 1.0 2.0 3.0 4.0 5.0 Temperature, ° F 1000 800 600 400 Pure Cu D B C Goodheart-Willcox Publisher Figure 15-10. Pictured is the Cu-Be phase diagram, showing the composition-temperature locations of a UNS C17200 (CDA172) alloy during heat treatment. Positions A, B, C, and D indicate the key points in the precipitation hardening process. Copyright Goodheart-Willcox Co., Inc.