Chapter 15 Introduction to Nonferrous Metals 329 15.3.1 Measurement of Yield Strength in No nferrous Metals There is one significant difference in the stress-strain behavior of most nonferrous metals, compared to that of steel. In plain carbon steel, there is a distinct break in the stress-strain curve at the yield point, as discussed in Chapter 6. For most nonferrous metals, the stress-strain curve smoothly changes from the elastic region to the plastic region, instead of changing with a sharp drop. A new definition of yield stress and yield strain is shown in Figure 15-9. The yield strength is defined as the stress where the stress-strain curve crosses a 0.2% offset line. 15.3.2 Strengthening Alloys through Cold Work Cold work such as cold-rolling or cold forging increases the strength and hardness, and reduces the ductility, of nonferrous alloys by developing dislocation tangles. Flat sheet strengthened by cold-rolling is more dent and wrinkle resistant than annealed sheet, so sheet is usually offered for sale with a cold-roll strengthening option. Shipping tickets should include this information on each coil received, to assure that each sheet behaves the same way in all finishing operations. 15.3.3 Strengthening by Solid Solution Alloyi ng Some alloys are strengthened by adding an alloy element that does not change the crystal structure, called a solid solution alloy. Because of the different sizes of the added atoms, stresses develop around them, making dislocation motion more difficult. Thus, strength is increased. Up to 7.5% copper (Cu) added to silver (Ag) makes sterling silver, which has higher hardness and wear resistance than pure silver. Silver and precious metal alloys are discussed in Chapter 22. 15.3.4 Strengthening by Large Precipitates Hig h hot-work temperatures allow grains to slide past one another in fractions of a second. The metal deforms at relatively low stress. At warm-work temperatures, the metal deforms in longer times, even when the stress on the metal is less than the yield strength. Thus we have creep, as discussed in Chapter 6. Creep occurs when the load is applied for hours or days or months with the stress below the measured yield strength and the sample deforms anyway. You can demonstrate creep for yourself with the Self-Discovery activity “Creep in Solder,” found at the end of this chapter. Creep limits the upper operating temperature in high-temperature applications. If jet engine or steam turbine blades touch the outer wall of the turbine, the whole turbine will be destroyed, so the blades cannot change length during use. Yield strength 0.2% offset Stress (psi, Pa) Elongation (% of original length) Elastic Plastic region Goodheart-Willcox Publisher Figure 15-9. This graph represents the stress-strain curve for most nonferrous metals. The yield point is defined as where the stress-strain curve crosses a 0.2% offset line. The offset line is drawn parallel to the elastic strain portion of the stress-strain curve, and is offset, or displaced, by 0.2% of the initial gage length. This yield strength is reported as “Yield, 0.2% offset.” Copyright Goodheart-Willcox Co., Inc.