Chapter 15 Introduction to Nonferrous Metals 331 that point may be called solutionized, because all of the alloy elements have become a single solution in the metal. The metal in the single-phase region has a microstructure with “clean” grains and no second-phase particles. Formation of Second-Phase Precipitates After solutionizing, the parts are removed from the furnace and cooled by water quenching to position C in Figure 15-10. Most nonferrous heat-treatable alloys can be quenched in water. Some alloys are gas cooled with inert gas in a vacuum furnace. Some must be quenched in liquid nitrogen to achieve temperatures low enough to keep the second element in solution. Precipitates are expected to form in this two-phase region, but the temperature drops so fast that precipitate particles cannot form. Instead, a very large number of extremely small “pre-precipitate” regions develop. Typical nonferrous metals in this as-quenched condition have good ductility and somewhat greater strength than fully annealed material. Growing Precipitates When the Cu-1.7% Be alloy is quenched, at f irst no particles can be seen even at 20,000X magnification in a transmission electron microscope. In a short time at room temperature, very tiny pre-precipitate regions of CuBe form throughout the metal. The atoms in the crystal lattice of the developing CuBe precipitate almost line up with copper atoms in the copper crystal. Stressed regions surround each small precipitate, as the precipitates align atoms at the particle-copper matrix interface. These particles are called coherent precipitates, because of the near match. Dislocations must push past these stressed regions before the metal can deform. We see dramatically increased strength. Strengthening by aging at room temperature, natural aging, is used when the as-quenched alloy meets the design requirements. Artificial Aging. When the copper-beryllium (CuBe) alloy is heat -treated through positions A, B, and C on Figure 15-10, and then heated to position D, beryllium (Be) atoms diffuse from the copper (Cu) matrix to the new CuBe particles. CuBe precipitates grow in minutes, with a sharp increase in strength. This artificial aging step produces high strength, with reduced ductility. Any temperature inside the two-phase region on the phase diagram will develop the second-phase precipitates. Higher temperatures allow shorter aging time, and hence faster production. However, processing this way requires very close control of both time and temperature. Operators and technicians must be on full alert to achieve consistent results from batch to batch. Figure 15-11 shows the precipitation hardening cycle on a time-temperature diagram. Overaging. The furnace time for precipitation hardening can be a few minutes to an hour, depending on the soak temperature, size of the part, Goodheart-Willcox Publisher Figure 15-11. The general time-temperature profile used for precipitation hardening alloys requires a solutionizing (step B), a quench (step C), and artificial aging (step D). For naturally aged alloys, step D is room temperature. The steps, labels, and temperatures match those of Figure 15-10. 1800 1600 1400 1200 1000 Time, minutes Temperature, ° F 800 600 400 A B C D Copyright Goodheart-Willcox Co., Inc.