72822044-ASM-Metals-Handbook-Volume-09-Metallography-and-Micro-Structures-2004.pdf

Revised by R.N. Caron, R.G. Barth, and D.E. Tyler, Metallography and Microstructures of Copper and Its Alloys, Metallography and Microstructures, Vol 9, ASM Handbook, ASM International, 2004, p. 775–788

Metallography and Microstructures of Copper and Its Alloys Revised by R.N. Caron, R.G. Barth, and D.E. Tyler, Olin Brass, Division of Olin Corporation

Introduction COPPER AND COPPER ALLOYS have played an integral role in human technological progress since antiquity. Native copper and then bronze alloys were fashioned into the first metal tools and decorations. The combination of electrical and thermal conductivity, workability, corrosion resistance, strength, and its abundance makes this family of metals important to all industry.

Revised by R.N. Caron, R.G. Barth, and D.E. Tyler, Metallography and Microstructures of Copper and Its Alloys, Metallography and Microstructures, Vol 9, ASM Handbook, ASM International, 2004, p. 775–788 Metallography and Microstructures of Copper and Its Alloys >Revised by R.N. Caron, R.G. Barth, and D.E. Tyler, Olin Brass, Division of Olin Corporation

Alloying Systems Copper alloys have traditionally been classified by composition and as wrought or cast in the groups show in Table 1. The alloys are listed by UNS designation that is administered by the Copper Development Association. Selected alloys are listed for each group. Similar compositional grouping is present in international designation systems. Table 1 Nominal compositions of copper and copper alloys UNS Name No. Wrought copper C10100 Oxygen-free electronic copper (OFE) C10200 Oxygen-free copper (OF) C11000 Electrolytic tough pitch copper (ETP) copper, high-residual C12200 Phosphorus-deoxidized phosphorus (DHP) C12500 Fire-refined tough pitch copper (FRTP) C14520 Phosphorus-deoxidized copper, tellurium bearing C14700 Sulfur-bearing copper C15000 Zirconium copper C15720 Dispersion-strengthened copper Wrought high-copper alloys

Nominal composition, %

99.99 (min) Cu 99.95 (min) Cu 99.90 (min) Cu 99.90 (min) Cu, 0.028 P 99.88 (min) Cu 99.90 (min) Cu, 0.010 P, 0.55 Te 99.90 (min) Cu, 0.35 S 99.80 (min) Cu, 0.15 Zr 99.52 (min) Cu, 0.2 Al, 0.2 O2 (O2 present as CuO2)

Bal Cu, 1.90 Be, 0.40 Co C17200 Beryllium-copper Bal Cu, 0.9 Cr C18200 Chromium-copper Bal Cu, 1 Pb, 0.05 P C18700 Leaded copper 97 (min) Cu, 2.35 Fe, 0.125 Zn, 0.05 P C19400 Iron-bearing copper Wrought brasses 70 Cu, 30 Zn C26000 Cartridge brass, 70% 66 Cu, 34 Zn C26800 Yellow brass, 66% 60 Cu, 40 Zn C28000 Muntz metal, 60% 89 Cu, 2 Pb, 1 Ni, 8 Zn C31600 Leaded commercial bronze, nickel bearing 63.5 Cu, 0.5 Pb, 36 Zn C33500 Low-leaded brass 62 Cu, 3 Pb, 35 Zn C36000 Free-cutting brass 71.5 Cu, 27.5 Zn, 1 Sn, (0.04 As) C44300 Admiralty, arsenical 71.5 Cu, 27.5 Zn, 1 Sn, (0.06 Sb) C44400 Admiralty, antimonial 71.5 Cu, 27.5 Zn, 1 Sn, (0.06 P) C44500 Admiralty, phosphorized 61 Cu, 38 Zn, 1 Sn C46400 Uninhibited naval brass 60.5 Cu, 1.75 Pb, 0.75 Sn, 37 Zn C48500 High-leaded naval brass Wrought bronzes Bal Cu, 5.0 Sn, 0.2 P C51000 Phosphor bronze, 5% A Bal Cu, 8.0 Sn, 0.2 P C52100 Phosphor bronze, 8% C 82.2 Cu, 10 Al, 3 Fe, 4.8 Ni C63000 Aluminum bronze, 10% Bal Cu, 1.9 Ni, 0.6 Si C64700 Silicon-nickel bronze 58.5 Cu, 1 Sn, 38.7 Zn, 1.5 Fe, 0.3 Mn C67500 Manganese bronze A 77.5 Cu, 20.3 Zn, 2.2 Al, (0.04 As) C68700 Arsenical aluminum brass Wrought copper-nickel alloys and nickel silvers 88.6 Cu, 10 Ni, 1.4 Fe C70600 Copper-nickel, 10% 75 Cu, 25 Ni C71300 Copper-nickel, 25% 68.5 Cu, 31 Ni, 0.50 Fe C71500 Copper-nickel, 30% Bal Cu, 30.5 Ni, 2.6 Cr C71900 Copper-nickel 65 Cu, 10 Ni, 25 Zn C74500 Nickel silver, 65-10 65 Cu, 18 Ni, 17 Zn C75200 Nickel silver, 65-18 Cast high-copper alloy 98 (min) Cu, 1.0 Cr C81500 Chromium-copper Cast brasses, bronzes, and nickel silver 85 Cu, 5 Sn, 5 Zn, 5 Pb C83600 Leaded red brass 64 Cu, 26 Zn, 4 Al, 3 Fe, 3 Mn C86200 Manganese bronze 63 Cu, 25 Zn, 3 Fe, 6 Al, 3 Mn C86300 Manganese bronze 88 Cu, 8 Sn, 4 Zn C90300 Tin bronze 87 Cu, 10 Sn, 2 Zn, 1 Pb C92600 Leaded tin bronze 74 Cu, 20 Pb, 6 Sn C94100 High-leaded tin bronze 89 Cu, 10 Al, 1 Fe C95300 Aluminum bronze 85 Cu, 11 Al, 4 Fe C95400 Aluminum bronze 81 Cu, 11 Al, 4 Fe, 4 Ni C95500 Nickel-aluminum bronze 91 Cu, 7 Al, 2 Si C95600 Silicon-aluminum bronze 66 Cu, 25 Ni, 5 Sn, 2 Zn, 2 Pb C97800 Nickel silver Brazing alloys 80 Cu, 15 Ag, 5 P C55284 BCuP-5 brazing alloy Coppers. The alloys designated as coppers contain 99.3% or more copper. These have the highest electrical and thermal conductivity. Impurities such as phosphorus, tin, selenium, tellurium, and arsenic are detrimental to properties such as electrical conductivity and recrystallization temperature (Ref 1). If deliberately added, however, these alloying elements can enhance other desirable properties. Silver is the only impurity that does not significantly lower the conductivity of pure copper, so it is included in the percent weight of copper when calculating the minimum percent weight of an alloy.

High-copper alloys contain between 96 and 99.3% Cu in wrought products that do not fall into any other special categories. For cast alloys, copper content above 94% is included. The chief alloying elements are cadmium, beryllium, and chromium. Copper-zinc alloys (brasses) have zinc as the prime alloying element. Wrought alloys are subdivided into CuZn alloys, Cu-Zn-Pb (leaded brasses), and Cu-Zn-Sn alloys (tin brasses). Cast brasses have four subdivisions: Cu-Zn-Sn and Cu-Zn-Sn-Pb alloys (red and leaded red, semired and leaded semi-red, and yellow and leaded yellow brass); Cu-Mn-Zn and Cu-Mn-Zn-Pb (high-strength and leaded high-strength brass, also called manganese bronze and leaded manganese bronze); and Cu-Si (silicon brasses and bronzes); and Cu-Bi and CuBi-Se (copper-bismuth and copper-bismuth-selenium alloys). Bronzes include copper alloys that do not have zinc or nickel as the major alloying element. The four subgroups of wrought alloys are: Cu-Sn-P (phosphor bronze), Cu-Sn-P-Pb (leaded phosphor bronze), Cu-Al (aluminum bronze), and Cu-Si (silicon bronze). Although called bronzes, the manganese bronzes that have zinc as the major alloying element are classed with the brasses. The cast bronzes are called Cu-Sn (tin bronze), Cu-Sn-Pb (leaded and high-leaded tin bronze), Cu-Sn-Ni (nickel-tin bronze), and Cu-Al-Fe and Cu-Al-Fe-Ni (aluminum bronze). Copper-nickels are available as wrought and cast alloys. Copper-nickel-zinc alloys, wrought and cast, are known as nickel silvers. This name is based on their luster, not their composition, because they do not have silver as an intentional alloying element. Other alloys include specialty alloys, copper leads, and brazing alloys. Gas Solubility. Hydrogen and oxygen are quite soluble in liquid copper, but the solubility in solid copper is very small. The metal therefore rejects a considerable amount of these (and other) gases on solidification. Oxygen content must be carefully controlled so that detrimental quantities of Cu2O, which decreases workability, are not formed. In molten copper, oxygen can react with dissolved hydrogen to form water vapor, which evolves as voids during solidification, called hydrogen illness. The voids cause hairline cracks that can lead to fracture during hot rolling and produce a variety of defects on the surface of wire rods (Ref 2). Oxygen and hydrogen interfere with conductivity; however, small and controlled amounts of oxygen are actually beneficial to conductivity in that they combine with and remove from solution impurities such as iron that are far more detrimental. A copper-oxygen phase diagram would show a eutectic at 0.4 wt% O (or 3.4 wt% Cu2O). Figure 1, 2, 3, and 4 show hypoeutectic copper-oxygen alloys, where the primary dendrites (light color) are copper. Figure 5, 6, 7, and 8 are hypereutectic, where the structure consists of particles or dendrites of Cu2O (dark colored) and eutectic.

Fig. 1 The effect of oxygen content on the microstructure of a hypoeutectic as-cast copper-oxygen alloy. Oxygen content of 0.024% results in primary dendrites of copper (light) plus eutectic (mottled areas of small, round oxide in copper). As-polished. 100×

Fig. 2 The effect of oxygen content on the microstructure of a hypoeutectic as-cast copper-oxygen alloy. Oxygen content of 0.09% results in primary dendrites of copper (light) plus eutectic (mottled areas of small, round oxide in copper). As-polished. 100×

Fig. 3 The effect of oxygen content on the microstructure of a hypoeutectic as-cast copper-oxygen alloy. Oxygen content of 0.18% results in primary dendrites of copper (light) plus a more connected area of eutectic than in Figure 2 As-polished. 100×

Fig. 4 The effect of oxygen content on the microstructure of a hypoeutectic as-cast copper-oxygen alloy. Oxygen content of 0.23% results in less primary dendrites of copper (light) plus more connected areas of eutectic than in Figure 3 As-polished. 100×

Fig. 5 The effect of oxygen content on the microstructure of a hypereutectic as-cast copper-oxygen alloy. Oxygen content of 0.44% results in particles or dendrites of oxide (dark) and light eutectic. As-polished. 100×

Fig. 6 The effect of oxygen content on the microstructure of a hypereutectic as-cast copper-oxygen alloy. Oxygen content of 0.50% results in the increased amount of particles or dendrites of oxide (dark) in the light eutectic, as compared to Figure 5 As-polished. 100×

Fig. 7 The effect of oxygen content on the microstructure of a hypereutectic as-cast copper-oxygen alloy. Oxygen content of 0.70% results in the increase of dendritic structure of oxide (dark) in the light eutectic matrix, as compared to Figure 6 As-polished. 100×

Fig. 8 The effect of oxygen content on the microstructure of a hypereutectic as-cast copper-oxygen alloy. Oxygen content of 0.91% results in the increase of dendritic structure of oxide (dark) in the light eutectic matrix, as compared to Figure 7 As-polished. 100×

References cited in this section 1. G. Joseph, Copper, Its Trade, Manufacture, Use, and Environmental Status, ASM International, 1999 2. M. Tisza, Physical Metallurgy for Engineers, ASM International and Freund Publishing House, Ltd., 2001

Revised by R.N. Caron, R.G. Barth, and D.E. Tyler, Metallography and Microstructures of Copper and Its Alloys, Metallography and Microstructures, Vol 9, ASM Handbook, ASM International, 2004, p. 775–788 Metallography and Microstructures of Copper and Its Alloys >Revised by R.N. Caron, R.G. Barth, and D.E. Tyler, Olin Brass, Division of Olin Corporation

Microstructures of Copper and Copper Alloys Coppers and High-Copper Alloys. The pure coppers are commercially significant because they have the highest conductivity. The effect of hot and cold working and heat treatment of oxygen-free coppers is shown in Fig. 9, 10, and 11. Note in Fig. 11 that oxygen has been reintroduced into the previously oxygen-free metal by heating in a noninert atmosphere. The dendritic structure of electrolytic tough pitch copper is seen in the cast ingot of C11000 (Fig. 12). A macrograph of C12200, phosphorus-deoxidized copper (Fig. 13), reveals a typical

solidification pattern of grains in a 102 mm (4 in.) continuously cast ingot. As is typical with relatively pure copper, the grain boundaries are well defined. The transverse and longitudinal sections of the ingot show that the grains grow perpendicular to the wall. Grain boundaries are parallel to the direction of heat flow. At the core, a columnar grain is along the axis of the ingot. Sections taken normal to the direction of heat flow near the wall (Fig. 14) and near the center (Fig. 15) show how the grain structure gets finer near the center. These images (Fig. 13, 14, and 15 illustrate the significance that the orientation of a specimen section has in determining the appearance of the grain structure.

Fig. 9 Copper C10200 (oxygen-free copper), hot-rolled bar. Large, equiaxed, twinned grains. Etchant 1, Table 2. 100× Table 2 Etchants and procedures for microetching of coppers and copper alloys Composition(a) 1. 20 mL NH4OH, 0–20 mL H2O, 8–20 mL 3% H2O2

2.

1 g Fe(NO3)3 and 100 mL H2O

3.

25 mL NH4OH, 25 mL H2O, 50 mL 2.5% (NH4)2S2O8 2 g K2Cr2O7, 8 mL H2SO4, 4 mL NaCl (saturated solution), 100 mL H2O

4.

5. 6. 7.

CrO3 (saturated aqueous solution) 50 mL 10–15% CrO3 and 1–2 drops HCl 8 g CrO3, 10 mL HNO3, 10 mL H2SO4, 200 mL H2O

Procedure Immersion or swabbing 1 min; H2O2 content varies with copper content of alloy to be etched; use fresh H2O2 for best results(b) Immersion Immersion Immersion; NaCl replaceable by 1 drop HCl per 25 mL solution; add just before using; follow with FeCl3 or other contrast etch Immersion or swabbing Immersion; add HCl at time of use Immersion or swabbing

Copper or copper alloy Use fresh for coppers and copper alloys; film on etched aluminum bronze can be removed using weak Grard's solution, preferred for brasses Etching and attack polishing of coppers and alloys Attack polishing of coppers and some copper alloys Coppers; copper alloys of beryllium, manganese, and silicon; nickel silver, bronzes, chromium-copper; preferred for all coppers to reveal grain boundaries, grain contrast, and cold deformation Coppers, brasses, bronzes, nickel silver Same as above; color by electrolytic etching or with FeCl3 etchants Grain contrast etch for electrolytic tough pitch copper; does not dissolve Cu2O; use after etchant 3 when etching deoxidized high-phosphorus copper for

10 g (NH4)2S2O3 and 90 mL H2O 9. 10% aqueous copper ammonium chloride plus NH4OH to neutrality or alkalinity H2O, mL 10. FeCl3, g HCl, mL 5 50 100 20 5 100(c)(d) 25 25 100 1 20 100 8 25 100 5 10 100(e)(f) 11. 5 g FeCl3, 100 mL ethanol, 5– 30 mL HCl 12. HNO3 (various concentrations) 8.

13. NH4OH (dilute solutions) 14. 50 mL HNO3, 20 g CrO3, 75 mL H2O 15. 5 mL HNO3, 20 g CrO3, 75 mL H2O 16. 59 g FeCl3 and 96 mL ethanol

Immersion; use cold or boiling Immersion; wash specimen thoroughly

microstructure Coppers, brasses, bronzes, nickel silver, and aluminum bronze Coppers, brasses, nickel silver; darkens β in α-β brass.

Immersion or swabbing; etch lightly or by successive light etches to required results

Coppers, brasses, bronzes, aluminum bronze; darkens β phase in brass; gives contrast following dichromate and other etches

Immersion or swabbing for 1 s to several minutes Immersion or swabbing; 0.15–0.3% AgNO3 added to 1:1 solution gives a brilliant, deep etch Immersion Immersion

Coppers and copper alloys; darkens β phase in α-β brasses and aluminum brass Coppers and copper alloys

Immersion Immersion; heat sample first in hot H2O Immersion

Attack polishing of brasses and bronzes Aluminum bronze, free-cutting brass; film from polishing can be removed with 10% HF Same as above Macro- and microetch for annealed copper-nickel alloys Preferred etch for copper-nickel; preferential attack of copper-rich phase in castings Coppers, brasses

17. 16 g CrO3, 1.8 g NH4Cl (ammonium chloride), 10 mL HNO3, 200 mL H2O Immersion, 3 s 18. 5 parts HNO3, 5 parts acetic acid, 1 part H3PO4 Immersion Coppers and alloys 19. Equal parts NH4Cl and H2O Immersion Copper-nickel alloys 20. 60 g FeCl3, 20 g Fe(NO3)3, 2000 mL H2O Copper-nickel alloys 21. 1 part acetic acid, 1 part HNO3, Immersion 2 parts acetone (a) The use of concentrated etchants is intended unless otherwise specified. (b) This etchant may be alternated with FeCl3. (c) Grard's No. 1 etchant. (d) Plus 1 g CrO3. (e) Grard's No. 2 etchant. (f) (f) Plus 1 g CuCl2 and 0.03 g SnCl2 (tin chloride)

Fig. 10 Copper C10200 (oxygen-free copper), cold worked, annealed 30 min at 850 °C (1560 °F). Equiaxed, recrystallized grains containing twinned areas. Etchant 4, Table 2. 250×

Fig. 11 Copper C10200 (oxygen-free copper), hot-rolled bar, heated 1 h in air to 665 °C (1225 °F). Specimen taken from near the bar surface shows Cu2O (dark dots) caused by oxygen penetration during heating. Etchant 1, Table 2. 250×

Fig. 12 Alloy C11000 (electrolytic tough pitch copper), static cast. Excellent definition of dendritic structure. Etchant 10, Table 2. 5×

Fig. 13 Alloy C12200 (deoxidized high-phosphorus copper), continuously cast in a 102 mm (4 in.) diameter ingot. Top, transverse section showing radial grain growth. Bottom, longitudinal section. Dark center is columnar grains oriented along the axis of the ingot. Waterbury reagent was used, which has same constituents as etchant 7, Table 3. 0.6×

Fig. 14 The same C12200 (deoxidized high-phosphorus copper) continuously cast alloy in a 102 mm (4 in.) diameter ingot as in Figure 13 Section taken near the ingot surface normal to the radial grain growth. The structure is coarse, unbranched dendrites. Waterbury reagent was used, which has the same constituents as etchant 7, Table 3. 150×

Fig. 15 The same C12200 (deoxidized high-phosphorus copper) continuously cast alloy in a 102 mm (4 in.) diameter ingot as in Fig. 14 Section taken near the ingot core normal to the radial grain growth. The dendrite structure is much finer than in Fig. 14. Waterbury reagent was used, which has the same constituents as etchant 7, Table 3. 150× Alloy C12200 (deoxidized high-phosphorus copper) contains high-residual phosphorus, a common deoxidizer, that improves weldability. The micrograph (Fig. 16) shows the presence of P2O5. The addition of tellurium to copper-phosphorus alloy improves machinability. Figure 17, a micrograph of C14520, designated DPTE, shows the effect of hot working. The copper-telluride is present in the dark particles. The addition of sulfur likewise improves machinability. A cold-worked sample of C14700 is given in Fig. 18.

Fig. 16 Copper C12200 (deoxidized high-phosphorus copper). Internal oxidation (presence of dark dots of P2O5). Etchant 4, Table 2. 75×

Fig. 17 Copper C14520 (DPTE), hot-rolled and drawn rod. Dark particles elongated in the rolling direction are copper telluride, which improves machinability. Etchant 7, Table 3. 250×

Fig. 18 Copper C14700 (sulfur-bearing copper) rod, cold worked to 50% reduction. Transverse section shows dispersion of round particles of CuS, which improves machinability. Etchant 7, Table 3. 200× The greatest tonnage of copper alloys are those consisting of solid solutions. The high-copper alloys, copperberyllium, copper-chromium, and copper-zirconium, have limited solid solubility, however. These systems can be precipitation hardened. The phenomenon, which is also called precipitation strengthening and age hardening, is possible because the limit of solid solubility contracts with decreasing temperature, a condition known as retrograde solubility (Ref 1). Copper-beryllium alloys (Fig. 19, 20, and 21can be heat treated to remarkably high strengths, as evidenced by their hardnesses. The precipitation process increases the copper content of the surrounding matrix and improves the conductivity of the alloy. This combination of strength and conductivity makes these alloys useful as electrical contact components.

Fig. 19 Alloy C17200 (beryllium-copper), solution treated 10 min at 790 °C (1450 °F) and water quenched. Typical hardness is 62 HRB. Structure is equiaxed grains of supersaturated solid solution of beryllium in copper. Etchant 3, Table 2. 300×

Fig. 20 Same alloy (C17200) and processing as in Figure 19 but aged 3 h at 360 °C (680 °F) after solution treatment. Typical hardness is 37 HRC. Copper-beryllium precipitates at grain boundaries and within α grains. Etchant 3, Table 2. 300×

Fig. 21 Same alloy and processing as in Figure 19 except reduced 11% by cold rolling to quarter-hard temper. Typical hardness is 79 HRB. Alpha grains are elongated in the direction of rolling. Etchant 3, Table 2. 300× Brasses. The phase diagram of the copper-zinc system (Fig. 22) has the compositional ranges of five common brasses superimposed. It is seen that the α-solid-solution region extends to 32.5 wt% Zn and includes red brasses (C23000), low brass (C24000), and cartridge brass (C26000). These have formability similar to pure copper. The dendritic structure of C26000 (Fig. 23, 24) and the annealed grains of Fig. 24 are similar to the

coppers. The effect of grain size on formability is illustrated with C26000 in Fig. 25, 26, 27, and 28 Processing combinations of hot and cold working and various annealing temperatures alter the grain size and shape. Hotrolled and annealed, transverse and short longitudinal samples (Fig. 29, 30) are compared to the same alloy cold rolled (Fig. 31, 32). Cartridge brass that had been hot rolled, annealed, cold rolled, annealed, cold rolled to a 70% reduction, and then annealed at various temperatures yielded the grain sizes seen in Fig. 33, 34, 35, and 36

Fig. 22 A copper-zinc phase diagram with the compositional ranges of five common brasses (UNS designation) superimposed on it. Adapted from Ref 3

Fig. 23 Alloy C26000 (cartridge brass), cast, slowly cooled, and quenched. Primary dendrites aligned in 100! crystallographic directions. The fine, quenched structure has the same orientation as the coarse dendrites. Etchant 1, Table 2, then electropolished with electrolyte 1, Table 4. 30×

Fig. 24 Alloy C26000 (cartridge brass), same processing as in Fig. 23. Higher magnification shows that fine dendrites originate in the coarse ones and have the same orientation. Dendrites starting in directions that are not 100! do not grow very far. Same etchant and electrolyte as in Fig. 23. 85×

Fig. 25 Alloy C26000 (cartridge brass) drawn cup, showing “orange peel” (rough surface). See Figure 27 for grain structure. Etchant 1, Table 2. Actual size

Fig. 26 Alloy C26000 (cartridge brass) drawn cup, with a smooth surface. See Figure 28 for structural details. Etchant 1, Table 2. Actual size

Fig. 27 Grain structure of drawn cup in Figure 25 The rough surface of the cup was caused by the large grain size. Etchant 1, Table 2. 85×

Fig. 28 Structure of the drawn cup in Figure 26 Because grains are small, the cup has a smooth surface. Etchant 1, Table 2. 85×

Fig. 29 Alloy C26000 (cartridge brass), hot rolled to 10 mm (0.4 in.) thick, annealed to a grain size of 15 µm, cold rolled to 40% to 6 mm (0.24 in.) thick, and annealed to a grain size of 120 µm. Diagram in lower left corner of micrograph indicates the view relative to the rolling plane of the sheet. Nominal tensile strength of 296 MPa (43,000 psi). Etchant 1, Table 2. 75×

Fig. 30 Alloy C26000 (cartridge brass), hot rolled to 10 mm (0.4 in.) thick, annealed to a grain size of 15 µm, cold rolled to 40% to 6 mm (0.24 in.) thick, and annealed to a grain size of 120 µm. Diagram in lower left corner of micrograph indicates the view relative to the rolling plane of the sheet. Nominal tensile strength of 296 MPa (43,000 psi). Etchant 1, Table 2. 75×

Fig. 31 Alloy C26000 (cartridge brass), hot rolled to 10 mm (0.4 in.) thick, annealed to a grain size of 15 µm, cold rolled to 40% to 6.1 mm (0.24 in.) thick, and annealed to a grain size of 120 µm. Further reduced 37% by cold rolling from 6.1 to 3.8 mm (0.24 to 0.15 in.) thick, hard temper, nominal tensile strength of 524 MPa (76,000 psi). Diagram in lower left corner of micrograph indicates the view relative to the rolling plane of the sheet. Etchant 1, Table 2. 75×

Fig. 32 Alloy C26000 (cartridge brass), hot rolled to 10 mm (0.4 in.) thick, annealed to a grain size of 15 µm, cold rolled to 40% to 6.1 mm (0.24 in.) thick, and annealed to a grain size of 120 µm. Further reduced 37% by cold rolling from 6.1 to 3.8 mm (0.24 to 0.15 in.) thick, hard temper, nominal tensile strength of 524 MPa (76,000 psi). Diagram in lower left corner of micrograph indicates the view relative to the rolling plane of the sheet. Etchant 1, Table 2. 75×

Fig. 33 Alloy C26000 (cartridge brass), processed to obtain specific grain size. Preliminarily hot rolled, annealed, cold rolled, annealed to a grain size of 25 µm, cold rolled to 70% reduction. Final anneal at 330 °C (625 °F) for 5 µm grain size. Etchant 1, Table 2. 75×

Fig. 34 Alloy C26000 (cartridge brass), processed to obtain specific grain size. Preliminarily hot rolled, annealed, cold rolled, annealed to a grain size of 25 µm, cold rolled to 70% reduction. Final anneal at 370 °C (700 °F) for 10 µm grain size. Etchant 1, Table 2. 75×

Fig. 35 Alloy C26000 (cartridge brass), processed to obtain specific grain size. Preliminarily hot rolled, annealed, cold rolled, annealed to a grain size of 25 µm, cold rolled to 70% reduction. Final anneal at 405 °C (760 °F) for 15 µm grain size. Etchant 1, Table 2. 75×

Fig. 36 Alloy C26000 (cartridge brass), processed to obtain specific grain size. Preliminarily hot rolled, annealed, cold rolled, annealed to a grain size of 25 µm, cold rolled to 70% reduction. Final anneal at 425 °C (800 °F) for 20 µm grain size. Etchant 1, Table 2. 75× Above 37.5% Zn, the β phase crystallizes. Along the vertical solubility limit line of the α phase, note that the αphase solubility increases with a decrease in temperature. The structure of brass containing α and β phases, such as the C36000 free-cutting brass alloy, is seen in Fig. 37, 38, 39, and 40

Fig. 37 Alloy C36000 (free-cutting brass), with primary dendrites of α phase darkened. Lead appears as small spheroids. Etchant 1, Table 2. 50×

Fig. 38 Alloy C36000 (free-cutting brass), with β phase darkened by preferential attack of the etchant. In this case, α phase is formed in the solid state during cooling. Etchant 16, Table 2. 50×

Fig. 39 Alloy C36000 (free-cutting brass), semicontinuous cast. Alpha-phase dendrites in the columnar zone near the outside edge of the ingot. Etchant 1, Table 2. 30×. Source: Ref 4

Fig. 40 Alloy C36000 (free-cutting brass), mixed α- and β-phase dendrites near the center of the ingot. Etchant 1, Table 2. 30×. Source Ref 4 Muntz metal, C28000, has higher zinc content than the C36000 alloy. The α and β phases are again evident in the as-cast and processed samples (Fig. 41, 42, and 43). Alloys with higher zinc content have increased strength but are prone to dezincification corrosion, as seen in Fig. 43.

Fig. 41 Alloy C28000 (Muntz metal) ingot, as-cast. Structure is dendrites of α phase in a matrix of β phase. Etchant 1, Table 2. 210×

Fig. 42 Alloy C28000 (Muntz metal) ingot, as-cast, showing α feathers that formed at β grain boundaries during quenching of the all-β structure. Etchant 1, Table 2. 105×

Fig. 43 Alloy C28000 (Muntz metal) ingot, hot-rolled plate. Uniform (layer) dezincification. Alpha grains remain in the corroded area (top). Etchant 1, Table 2. 90× Bronzes. The copper-tin system equilibrium diagram (Ref 5) indicates a larger range of temperatures in the first sections of the liquidus and solidus curves than exists in the copper-zinc system. The tin bronzes, also called phosphor bronzes, form alpha solid solutions with copper extending to 15.8% Sn at temperatures between 520 and 586 °C (968 and 1087 °F). This extended freezing range causes tin bronzes to pass through a semisolid or “mushy” stage during solidification. Casting molds must be designed to take this weak structure into account. The extended liquidus-solidus gap is also responsible for the dendritic segregation, or coring, found in tin

bronze castings (Ref 1). This segregation is seen in Fig. 44 and 45. Figure 46 is C51000 containing 5% Sn, where processing has produced recrystallized α-phase grains with annealing twins.

Fig. 44 Horizontal cast strip of tin bronze (5% Sn), showing inverse segregation at the bottom surface of the casting. The bottom is nearly pure tin. Etchant: 40 mL HNO3, 25 g CrO3, 35 mL H2O. 100×. Source: Ref 6

Fig. 45 Phosphor bronze strip rolled from a static cast ingot, showing gross tin sweat on the top surface. This illustrates how segregation caused by exudation persists in the fabricated structure. Etchant not reported. 300×. Source: Ref 7

Fig. 46 Alloy C51000 (phosphor bronze, 5% Sn) rod, extruded, cold drawn, and annealed 30 min at 565 °C (1050 °F). Structure consists of recrystallized α grains with annealing twins. Etchant 4, Table 2. 500× Aluminum bronzes are copper-aluminum alloys that generally have aluminum between 5 and 15%. Cast alloys are show in Fig. 47, 48, and 49. Alloys less than 8.5 wt% Al are a single α-phase solid solution. Numerous intermetallic phases exist with higher aluminum content. At approximately 12% Al, the alloy will solidify as a eutectic and undergo eutectoid transformation on slow cooling (Fig. 50). Under faster cooling, a martensitic transformation occurs analogous to that in heat treated steels, and martensitic needles result (Fig. 47, 51, and 52) (Ref 8). It should be realized that the copper-aluminum bronzes are not just binary alloys but are multicomponent and multiphased systems that can contain combinations of iron, manganese, nickel, or silicon as well. In the Cu-Zn-Al and Cu-Al-Ni systems, the martensitic transformation is reversible and responsible for the shape memory effect (Ref 1).

Fig. 47 Alloy C95400 (aluminum bronze), solution treated 2 h at 900 °C (1650 °F), water quenched, tempered 2 h at 650 °C (1200 °F), and water quenched. Alpha grains (white martensitic needles) are smaller than in the as-cast condition. Etchant 4, Table 2. 200×

Fig. 48 Alloy C95500 (aluminum bronze with 11.5% Al), as sand cast. Small α grains (light gray, mottled) in matrix of retained β phase (white), with same eutectoid decomposed β phase (dark gray). Compare with Figure 49. Electrolytically etched in electrolyte 5, Table 5. 250×

Fig. 49 Alloy C95500 (aluminum bronze with 11.0% Al), with larger α grains and a greater amount of eutectoid decomposed β phase in the matrix than Figure 48. Electrolytically etched in electrolyte 5, Table 5. 250×

Fig. 50 Metal mold cast aluminum bronze casting. Alloy contains 5% Ni and 5% Fe (similar to C95500). Under slow cooling, the laminar Widmänstatten structure (light) is visible on a background of fine martensitic structure (dark). Etchant not reported. 100×. Source: Ref 8

Fig. 51 Cast aluminum bronze (11.8% Al). Under faster cooling than Figure 50 specimen, the structure has been transformed, with the formation of martensitic needles mixed with pearlite (trostite). Etchant not reported. 50×. Source: Ref 8

Fig. 52 Detail of cast Cu-Al-Ni alloy. Under fast cooling, the structure of martensitic needles forms. Etchant not reported. 100×. Source: Ref 8 Copper-nickel alloys form a continuous series of solid solutions over their entire binary composition range, because copper and nickel atoms differ by only 2.5% in volume and both exhibit the face-centered cubic structure. When atoms of different elements can occupy equivalent sites randomly on a crystal lattice, such elements form what are known as substitutional solid solutions (Ref 1). The C71500 wrought alloy seen in its as-cast state at the core of a billet is 30% Ni (Fig. 53). The nickel silver C74500 (65Cu-10Ni, balance zinc) exhibits the equiaxed grains and twinning found in pure copper (Fig. 54).

Fig. 53 Alloy C71500 (copper-nickel, 30% Ni), as-cast. Longitudinal section showing columnar structure near the surface of the billet. The grains are inclined upward from horizontal by up to 30° due to convection in the initial state of freezing. Etchant 18, then etchant 16, Table 2. 0.3×

Fig. 54 Alloy C74500 (nickel silver, 65-10) cold-rolled sheet, 2.5 mm (0.10 in.) thick, annealed at 650 to 700 °C (1200 to 1290 °F). Longitudinal section shows equiaxed crystallized grains of a solid solution containing twin bands. Etchant 20, Table 2. 100×

References cited in this section 1. G. Joseph, Copper, Its Trade, Manufacture, Use, and Environmental Status, ASM International, 1999 3. T.B. Malssalski, Ed., Binary Phase Diagrams, ASM International, 1990

4. T.F. Bower and D.A. Granger, “Copper and Copper Alloy Ingot Structure—A Preliminary Survey,” Report 70-34, The Casting Laboratory, Cleveland, 1970 5. Alloy Phase Diagrams, Vol 3, ASM Handbook, ASM International, 1992, p 2, 178 6. A. Cibula, “Review of Metallurgical Factors Influencing the Quality of Copper and Copper Alloy Casting,” BNFRMRA International Conference on the Control of the Composition and Quality of Copper and Copper Alloy Casting for Fabrication, Oct 1967 (Düsseldorf) 7. G.L. Bailey and W.A. Baker, “Melting and Casting of Non-Ferrous Metals,” Monograph and Report Series 6, Institute of Metals, London, 1949 8. A. Tomer, Structure of Metals through Optical Microscopy, ASM International, 1991

Revised by R.N. Caron, R.G. Barth, and D.E. Tyler, Metallography and Microstructures of Copper and Its Alloys, Metallography and Microstructures, Vol 9, ASM Handbook, ASM International, 2004, p. 775–788 Metallography and Microstructures of Copper and Its Alloys >Revised by R.N. Caron, R.G. Barth, and D.E. Tyler, Olin Brass, Division of Olin Corporation

Metallographic Examination Common applications of metallographic examination for quality control, material evaluation, and alloy development include (Ref 1): •

• •

•

Measurement of grain size: Grain size has a strong influence on properties and formability in coldworked and annealed products. Qualitative estimates of grain size in sheet and strip products also can be obtained by cup testing. Evaluation of the dispersion of second phases: This is particularly useful for lead and other additives that promote machinability. A check on heat treating conditions: Complex alloys, such as aluminum bronzes and high-tensile brasses, rely on the development of proper microstructures for mechanical properties and optimal corrosion resistance. Failure analysis: Metallographic examination readily reveals stress-corrosion cracking, dezincification, dealuminification, and other common corrosion mechanisms. The technique can also uncover defects in wrought, cast, and welded structures.

Both macrographic and micrographic examination are of use in these applications.

Macroscopic Examination Specimens for macroscopic examination are extracted from larger masses using common cutting tools. The tools must be kept sharp to minimize cold working of the specimens and cooled to avoid over-heating (recrystallizing) the samples. The article “Metallographic Sectioning and Specimen Extraction” in this Volume provides insight into the specimen selection method as well as processing advice. Surface Preparation. Surfaces suitable for macroetching usually can be obtained in two machining operations. In the first operation, a heavy cut is taken to remove the metal that was cold worked during sectioning; in the second, a light cut is taken, using a V-shaped tool, to remove the remaining effects of cold work.

The need for further surface preparation depends on the amount of detail required. The surface detail revealed by etching increases as the degree of surface irregularity decreases. The machined surface is often ground using 180-grit or finer abrasive and sometimes as fine as 600-grit, which is especially useful for revealing fine defects or cracks. Etching. Deep etching removes the effects of cold work but produces a rough surface; therefore, it is common practice to deep etch the machined or rough-ground surface and regrind it lightly, followed by a light etch. Selection of an etchant for a macrospecimen depends primarily on the alloy to be etched and the features to be examined. Because the capabilities of two or more etchants often overlap or are the same, selection of a specific etchant is arbitrary. Table 3 lists compositions of the more commonly used macroetchants, along with etching procedures, purposes of the etchants or characteristics revealed, and alloys for which they are ordinarily used. Any etchant used for macroetching should not remove second-phase particles, lest they appear as defects, such as porosity or grain-boundary cracks. Table 3 Etchants for macroscopic examination of coppers and copper alloys Procedure for use: Immerse at room temperature, rinse in warm water, dry Composition Copper or copper alloy All coppers and copper 1. 50 mL HNO3, 0.5 g AgNO3 (silver alloys nitrate), 50 mL H2O Coppers and all brasses 2. 10 mL HNO3 and 90 mL H2O (a) Coppers, all brasses, 3. 50 mL HNO3 and 50 mL H2O aluminum bronze(b) 4. 30 mL HCl, 10 mL FeCl3, 120 mL H2O or Coppers and all brasses methanol 5. 20 mL acetic acid, 10 mL 5% CrO3, 5 mL All brasses 10% FeCl3, 100 mL H2O(d) Coppers, high-copper 6. 2 g K2Cr2O7, 4 mL saturated solution of (e) alloys, phosphor bronze NaCl, 8 mL H2SO4, 100 mL H2O Silicon brass, silicon 7. 40 g CrO3, 7.5 g NH4Cl (ammonium bronze chloride), 50 mL HNO3, 8 mL H2SO4, 100 mL H2O Copper 8. 45 mL acetic acid and 45 mL HNO3

Comments Produces a brilliant, deep etch Grains, cracks, and other defects Same as above; reveals grain by contrast Same as etchant above(c) Produces a brilliant, deep etch Grain boundaries, oxide inclusions General macrostructure

Grain boundary and macroetch by polish attack Copper and copper Use after the acetic acid listed 9. Saturated (NH4)2S2O8 (ammonium alloys above; increases contrast of brass persulfate) Macroetch 90-10, 70-30, and 10. 40 mL HNO3, 20 mL acetic acid, 40 mL Copper and copper alloys leaded brass H 2O (a) Solution should be agitated during etching to prevent pitting of some alloys. (b) Aluminum bronzes may form smut, which can be removed by brief immersion in concentrated HNO3. (c) Excellent for grain contrast. (d) Amount of water can be varied as desired. (e) Immerse specimen 15–30 min, then swab with fresh solution.

Microscopic Examination Specimens of copper and copper alloys for microscopic examination are extracted from larger masses by sawing, shearing, filing, hollow boring, or abrasive-wheel and low-speed diamond saw cutting. Use sharp tools and care to avoid deep cold work, and adequate cooling to prevent recrystallizing of cold-worked lean alloys. The use of an abrasive cutoff wheel or precision low-speed diamond saw is recommended for sectioning, depending on the size and specimen material. The as-cut surface is generally free of damage and distortion and is ready for encapsulation with minimal grinding and polishing.

Mounting. In general, the procedures for mounting copper and copper alloy specimens are the same as those for other metals. Coppers and copper alloys are extremely susceptible to work hardening; therefore, when possible, the face used for examination should be the one that has been subjected to the least cutting damage. Phenolic is the mounting material most often used. Diallyl phthalate, glass or fiber filled, is a suitable alternative. The transparency offered by mounts made of acrylic thermoplastic mounting resins, such as methyl methacrylate, is often advantageous. However, these materials are softer than Bakelite (Georgia-Pacific Corp.) and are not as good for edge preservation of the sample. Heat generated during thermosetting can cause heavily cold-worked coppers to recrystallize. The combination of heat and pressure needed for compression-mounting materials will sometimes crush or adversely affect specimens, especially those of thin sheet or strip. Under these conditions, one of the epoxies or some other castable mounting material must be used. Edge preservation of copper and copper alloy specimens can be accomplished by the same methods used for specimens of other metals (see the article “Mounting of Specimens” in this Volume). It can also be done by plating with a hard metal or by the use of co-mounted samples of similar composition. Grinding. Wet grinding is preferred for all coppers and copper alloys. Common practice involves rough grinding the specimen surface to remove metal that has been cold worked, then finish grinding to obtain a suitable surface. Finish grinding is performed using flat wheels and silicon carbide papers of progressively finer grit—usually 240, 320, 400, and 600. Ultrafine 800- and 1200-grit papers are sometimes used. Rough Polishing. Most coppers and copper alloys are relatively soft and thus require cutting with minimum rubbing. Rough polishing should be performed using diamond-impregnated nylon cloth. Duck canvas, wool broadcloth, and cotton (listed in order of decreasing preference) are also used for polishing. The preferred abrasive for rough polishing on any of the cloths mentioned previously is 1 to 9 µm diamond paste. A wheel speed of approximately 200 rpm is generally recommended. Fine Polishing. Generally, napped cloths are preferred for fine polishing. The abrasive is usually 0.3 µm αAl2O3 or 0.05 µm γ-Al2O3; both abrasives are used with water as a vehicle. Other abrasives that have proved satisfactory for fine polishing are colloidal silica (SiO2) and fine diamond paste. Recommended wheel speed is 120 to 150 rpm. Specimen rotation during polishing elicits numerous opinions. Hand polishing necessitates developing a personal technique that may require a degree of manual dexterity; mechanical polishing gives more reproducible results and is preferred. Examples of different methods of hand polishing that produce artifact-free or nearly artifact-free results are given in Fig. 55(a) to (d) (Ref 9).

Fig. 55 Examples of artifact-free or nearly artifact-free surfaces produced by different manual final polishing methods on 30% Zn annealed brass (similar to C26000). (a) Polished manually using 0.1 µm grade polycrystalline diamond abrasive. Etched in a ferric chloride reagent. (b) Polished manually by skidding on a thick slurry of magnesium oxide abrasive. Etched in a ferric chloride reagent. (c) Polished as for (b); etched in a high-sensitivity sodium thiosulfate reagent. (d) Manually polished with colloidal silica (1% ferric nitrate solution added) using repeated polish-etch cycles. Etched in Klemm's No. 1 reagent. The arrows in (a) and (d) indicate scratch-trace etch markings. (d) Courtesy of G.F. Vander Voort, Buehler Ltd. Source: Ref 9 After polishing, the specimen is rinsed in water and dried with warm air. Automated polishing (usually vibratory) is efficient for polishing copper alloys, especially when a large number of specimens must be prepared. Attack polishing (combined polishing and etching) using ferric nitrate (Fe(NO3)3) or ammonium hydroxide/ammonium persulfate (NH4OH and (NH4)2S2O8) solution can be more safely performed using automatic equipment than by hand. An example of the grain relief that is created if the time duration of the etch-polish cycle is not properly rationed is given in Fig. 56 (Ref 9).

Fig. 56 An example of the manual polish-etch technique as used in Figure 55(d) on 30% Zn annealed brass (similar to C26000) but with inadequate time for polishing, as compared to etch time during each cycle. An excessive degree of relief has developed between the grains. This effect is enhanced by oblique illumination. 20×. Courtesy of G.F. Vander Voort, Buehler Ltd. Source: Ref 9 Electrolyte polishing of coppers and copper alloys alleviates many of the difficulties encountered in mechanical polishing. Additional information is available in the article “Chemical and Electrolytic Polishing” in this Volume. Apart from offering the usual advantages over mechanical polishing of saving time, minimizing the human variable, and minimizing artifacts resulting from disturbed metal, electrolytic polishing offers some advantages for copper and copper alloys: • • •

It is excellent for revealing grain size and shape on all sides of specimens. It is especially well adapted to use on single-phase copper alloys. It reveals true microstructure with less difficulty than mechanical polishing.

Disadvantages of electrolytic polishing for copper and copper alloys include: • • • • •

Different rates of attack cause some phases of multiphase alloys to stand out in relief. The edge effect of electrolytic polishing, whereby edges of specimens are attacked and polished more than other areas, limits application of the process to examination of surfaces in from the edges. It will round edges of cracks and pores. Attack around nonmetallic particles, voids, and inclusions in the specimen may occur at a more rapid rate than attack of the matrix, and so the size of voids or inclusions may be exaggerated. Because it changes surface topography, it should not be used for failure analysis or image analysis.

Table 4 lists compositions of some electropolishing solutions, together with electropolishing conditions that have proved satisfactory for the coppers and copper alloys shown in the last column in the table. The durations listed in Table 4 are generally based on conditions where electrolytic polishing completely replaces mechanical polishing. Useful results may be obtained when mechanical polishing is followed by electrolytic polishing. Durations for electrolytic polishing are then always under 1 min.

Table 4 Electrolytes and conditions for electrolytic polishing of copper and copper alloys Composition 1. 825 mL H3PO4 and 175 mL H2O 2. 250 mL H3PO4, 250 mL ethanol, 50 mL propanol, 500 mL distilled H2O, 3 g urea 3. 700 mL H3PO4 and 350 mL H2O

4. 580 g H4P2O7 and 1000 mL H2O 5. 300 mL HNO3 and 600 mL methanol

Current density A/cm2 A/in.2 0.02– 0.13– 0.1 0.65 0.4–0.8 2.6– 5.2

Cathode

Duration

Copper or copper alloy

Copper

10–40 min 50 s

Unalloyed copper

1.2–2.0

0.06– 0.1

0.39– 0.64

Copper

15–30 min

1.2–1.9

0.08– 0.12 0.65– 3.1 2.5–3.1

Copper

10–15 min 10–60 s

0.95– 2.2 0.06– 0.15

0.05– 0.77 4.2– 20.0 16.1– 51.0 6.1– 14.2 0.39– 0.97

Coppers; α, β, and α-β brasses; aluminum; silicon; tin; phosphor bronzes; beryllium; iron-lead; or chromium Coppers, brasses

Voltage, Vdc 1.0–1.5 3–6

20–70

Stainless steel

6. 170 g CrO3 and 830 mL H2O 7. 400 mL H3PO4 and 600 mL H2O

1.5–12

8. 30 mL HNO3, 900 mL methanol, 300 g Cu(NO3)2 (cupric nitrate) 9. 670 mL H3PO4, 100 mL H2SO4, 300 mL distilled H2O 10. 470 mL H3PO4, 200 mL H2SO4, 400 mL distilled H2O 11. 350 mL H3PO4 and 650 mL ethanol 12. 540 mL H3PO4 and 460 mL H2O

45–50

1.05– 1.25

6.77– 8.1

Stainless steel Stainless steel Stainless steel Copper or stainless steel Stainless steel

2–3

0.1

0.64

2–2.3

0.1

0.64

30–50

1.0–2.0

2–5

5–10 s

Coppers and copper alloys

Coppers, brasses

10–60 s

Silicon bronze, phosphor bronze Brasses

1–15 min

α, α-β brasses, copper-iron, copper-chromium

15 s

Bronzes (have tendency to etch)

Copper

15 min

Copper; copper-tin containing up to 6% Sn

Copper

15 min

Copper-tin up to 9% Sn

0.02– 0.13– Copper 10–15 Copper alloys with high 0.07 0.45 min lead (to 30%) 2 0.065– 0.4– Copper 5–15 Copper 0.075 0.5 min 2–2.2 0.1– 0.64– Copper 15 min Nickel silver 0.15 0.97 Reference 10 contains extensive appendixes of macroetchants, microetchants, and polishing solutions. Examination of As-Polished Specimens. As-polished specimens of coppers and copper alloys are frequently examined metallographically. Characteristics revealed include the presence of oxide in copper, lead particles in leaded brass and bronze, intermetallic phases in the precipitation-hardened alloys, oxides, phosphides, sulfides, and corrosion products. Specimens are also examined under polarized light to differentiate cuprous oxide (Cu2O) inclusions from other inclusions. Under polarized light, only the Cu2O inclusions appear ruby red; under white light, copper oxide and other inclusions appear blue-gray. Oxides of arsenic and antimony also are optically active under polarized light. As-polished specimens are used also for scanning electron microscopic examination. Chemical Etching. Table 2 lists chemical etchants that are used for coppers and copper alloys and includes etching procedures and the alloys to which each etchant is commonly applied. The ammonium

hydroxide/hydrogen peroxide/water solution (etchant 1, Table 2) is by far the most widely used etchant. It is probably optimal for routine work and applies to most coppers and copper alloys. This etchant was used for many of the specimens shown in the micrographs in this article. This etchant is also widely used for determining the inclusion content of brass and bronze strip. The potassium dichromate/sulfuric acid/sodium chloride/water etchant (usually referred to simply as potassium dichromate, K2Cr2O7; see etchant 4 in Table 2) is also used extensively, especially for revealing structures of welded and brazed joints. Chromic acid (H2CrO4, etchant 5 in Table 2) is also prevalent. It is formed when chromium trioxide (CrO3) is dissolved in water. The other etchants listed in Table 2 have limited uses, although some are used for the same alloys and structures as the etchants discussed previously. Electrolytic etching reveals cold-worked structures of brasses, gives contrast to β phase in brass, and, in coppernickel alloys, reduces the contrast due to coring that usually appears with chemical etching. It is also used to bring out the general structure of beryllium-copper, cartridge brass, free-cutting brass, aluminum bronze, nickel silver, and admiralty metal. Table 5 lists five electrolytes that have proved successful for electrolytic etching. Table 5 Electrolytes and operating conditions for electrolytic etching of copper and copper alloys Composition 1. 5–14% H3PO4 (8%) and bal H 2O 2. 250 mL 85% H3PO4, 250 mL 95% ethanol, 500 mL H2O, 2 mL wetting agent 3. 30 g FeSO4 (ferrous sulfate), 4 g NaOH, 100 mL H2SO4, 1900 mL H2O 4. 1 mL CrO3 and 99 mL H2O

Operating conditions(a) Voltage range, 1–8; etching time, 5–10 s Voltage range, 1–3; current density, 0.1–0.15 A/cm2 (0.64–0.97 A/in.2); etching time, 30–60 s 0.1 A at 8–10 V for 15 s; do not swab surface after etching

Copper or copper alloy Coppers, cartridge brass, free-cutting brass, admiralty, gilding metal Coppers

Darkens β phase in brasses and gives contrast after H2O2-NH4OH etch; also for nickel silver and bronzes 6 V; aluminum cathode; etching Beryllium-copper and aluminum time, 3–6 s bronze alloys; avoiding 5. 5 mL acetic acid (glacial), 10 Voltage range, 0.5–1; current Copper-nickel density, 0.2–0.5 A/cm2 (1.3–1.9 contrast associated with coring mL HNO3, 30 mL H2O A/in.2); etching time, 5–15 s (a) Voltages are direct current Examination for Inclusions. Microscopic examination has become increasingly valuable for evaluating the fabrication characteristics of certain copper alloys, particularly brass and bronze sheet and strip. A correlation exists between the number of inclusions present, as well as their length and distribution, and fabrication characteristics, especially formability. Inclusions are best revealed by swabbing the specimen quickly with NH4OH/H2O2 (etchant 1, Table 2), then washing it in running water and drying with an air blast. In Fig. 57, the C26800 yellow brass with abnormally high sulfur content (0.02%) in (a) is lightly etched to reveal the zinc sulfide stringers. The image of a normally etched specimen is seen in (b), which is appropriate to develop the contrast required for grain size and constituent volume fraction measurements.

Fig. 57 Alloy C26800 with abnormally high sulfur content (0.02%). (a) Specimen is lightly etched with NH4OH/H2O2 (etchant 1, Table 2) to expose the zinc sulfide stringers. (b) Same specimen is etched in the normal manner to reveal grain contrast, but the zinc sulfide stringers are then concealed. Etchant 1, Table 2. 75× Sources of Micrographs. The Copper Development Association has a collection of micrographs of copper alloys at their Web site. The ASM Micrograph Center Web site contains a compendium of micrographs that supplements those illustrated in this article. The Copper Development Association has provided a number of micrographs in this collection.

References cited in this section 1. G. Joseph, Copper, Its Trade, Manufacture, Use, and Environmental Status, ASM International, 1999 9. L.E. Samuels, Metallographic Polishing by Mechanical Methods, 4th ed., ASM International, 2003 10. G.F. Vander Voort, Metallography: Principles and Practice, McGraw-Hill, 1984, reprinted by ASM International, 1999

Revised by R.N. Caron, R.G. Barth, and D.E. Tyler, Metallography and Microstructures of Copper and Its Alloys, Metallography and Microstructures, Vol 9, ASM Handbook, ASM International, 2004, p. 775–788 Metallography and Microstructures of Copper and Its Alloys >Revised by R.N. Caron, R.G. Barth, and D.E. Tyler, Olin Brass, Division of Olin Corporation

References 1. G. Joseph, Copper, Its Trade, Manufacture, Use, and Environmental Status, ASM International, 1999

2. M. Tisza, Physical Metallurgy for Engineers, ASM International and Freund Publishing House, Ltd., 2001 3. T.B. Malssalski, Ed., Binary Phase Diagrams, ASM International, 1990 4. T.F. Bower and D.A. Granger, “Copper and Copper Alloy Ingot Structure—A Preliminary Survey,” Report 70-34, The Casting Laboratory, Cleveland, 1970 5. Alloy Phase Diagrams, Vol 3, ASM Handbook, ASM International, 1992, p 2, 178 6. A. Cibula, “Review of Metallurgical Factors Influencing the Quality of Copper and Copper Alloy Casting,” BNFRMRA International Conference on the Control of the Composition and Quality of Copper and Copper Alloy Casting for Fabrication, Oct 1967 (Düsseldorf) 7. G.L. Bailey and W.A. Baker, “Melting and Casting of Non-Ferrous Metals,” Monograph and Report Series 6, Institute of Metals, London, 1949 8. A. Tomer, Structure of Metals through Optical Microscopy, ASM International, 1991 9. L.E. Samuels, Metallographic Polishing by Mechanical Methods, 4th ed., ASM International, 2003 10. G.F. Vander Voort, Metallography: Principles and Practice, McGraw-Hill, 1984, reprinted by ASM International, 1999

Metallography and Microstructures of Lead and Its Alloys, Metallography and Microstructures, Vol 9, ASM Handbook, ASM International, 2004, p. 789–800

Metallography and Microstructures of Lead and Its Alloys Introduction LEAD AND LEAD ALLOY specimens are prepared for metallographic examination in a similar fashion as other metals (see the Section “Metallographic Techniques” in this Volume), but the softness of the material presents challenges. Lead and lead alloys are so soft (25 to 40 HV hardness) that considerable surface flow or distortion occurs during grinding and polishing. If not removed, the distorted layer obscures the true structure of the specimen. In addition, the deformation and heating caused by the preparation can be sufficient to develop a pseudostructure (recrystallization) in the specimen. Abrasives used for polishing are easily embedded into the lead. It is essential, therefore, to employ techniques that minimize these effects in sample preparation.