Specimen Preparation

Metallography and Microstructures of Zinc and Its Alloys

Page 1 of 2 Metallography and Microstructures of Zinc and Its Alloys, Metallography and Microstructures, Vol 9, ASM Handbook, ASM International, 2004, p. 933–941

Metallography and Microstructures of Zinc and Its Alloys

Specimen Preparation Sectioning. The initial sample can be removed from a larger mass of material by sawing, breaking, or shearing (see the article “Metallographic Sectioning and Specimen

Extraction” in this Volume). Because zinc alloys are comparatively soft, final sectioning of specimens is sometimes performed using special techniques. Either abrasive or diamond wheels are employed, because they produce less metal flow than sawing or shearing. Mounting. Most specimens are mounted using conventional cold resin materials (see the article “Mounting of Specimens” in this Volume). Hot compression is not used,

because the zinc will deform and recrystallize. Specimens of rolled zinc and zinc alloys can be secured by clamping. Several specimens are mounted together in a screw clamp using thin spacers of soft zinc between specimens and heavy strips of zinc between the clamp plates and the outermost specimens. The assembly is tightly clamped to prevent seepage of etchants between specimens. The zinc spacers are of known structure and serve as convenient standards of comparison for determining if the specimens have been prepared correctly. Grinding and polishing of cast zinc can cause distortion to a depth 20 to 100 times as great as the deepest scratch. Therefore, in each stage of grinding and polishing,

considerably more metal should be removed than the amount required for eliminating the scratches that remain from the previous stage. It is easier to prepare a distortionfree surface on specimens of fine-grained zinc than on specimens of coarse-grained, soft zinc. Wet grinding on a belt grinding machine using 60- and 180-grit silicon carbide abrasives is suitable for zinc and zinc alloys. Local heating from grinding must be minimized using water cooling, because heat can cause structural changes too deep to remove by polishing. Rough polishing is performed using 240-, 320-, 400-, and then 600-grit (65, 45, 35, and 20 µm) silicon carbide papers. These papers are less susceptible to loading than emery papers. Some authorities recommend applying wax to the polishing papers so that they do not become embedded with zinc particles. Specimens should be rotated 90° relative to the direction of polishing after each polishing step. For soft (pure) zinc, polishing can be carried out by hand on papers supported on a flat surface. Zinc alloys are polished on a wheel using the same grades of paper. A low wheel speed (250 rpm maximum) during polishing will minimize overheating of the specimen, as will application of water to the silicon carbide papers and polishing in intervals of a few seconds, allowing the specimen to cool before polishing resumes. Fine polishing is performed using magnesium oxide (MgO) or alumina (Al2O3). A method for preparing specially graded wet-polishing abrasives is described in Ref 6. In fine polishing, the first two wheels are covered with smooth canvas, the third wheel with felt or billiard cloth. A soft-nap polishing cloth is used for fine polishing. Hands as well as the specimen must be washed between polishing steps to prevent carryover of coarser grit from previous steps. Overpolishing and its consequences can be avoided by etching between polishing steps. Zinc alloys that have intermetallic phases etch differently than unalloyed zinc. Because the intermetallic phases remain in relief, excessive polishing and etching should be avoided. Some of the steps listed previously can often be eliminated for alloys with intermetallic phases. Polishing through all four wheels is necessary only for specimens with microstructures requiring high magnifications for resolution. Most low-magnification examinations can proceed after polishing on the third wheel. An ethanol powder mixture instead of water should be used when polishing galvanized steel. The specimen is then cleaned ultrasonically with alcohol and blown dry with warm air. Vibratory polishing of zinc and zinc alloys will produce surfaces with improved response to polarized light. Macroetching. Use of concentrated hydrochloric acid (HCl) at room temperature, followed by rinsing and wiping off the resulting black deposit, produces satisfactory grain contrast on copper-free zinc and zinc alloys. Etchant 1 in Table 1 may be used for zinc containing 1% Cu or less, but with this etchant, grain contrast is not well defined. An etchant equal to HCl for producing grain contrast has not been found for the zinc alloys containing copper. Microetching. The most useful etchants for microscopic examination of zinc and zinc alloys are aqueous solutions of chromic acid (CrO3) to which sodium sulfate

(Na2SO4) has been added. The grades of CrO3 used for chromium plating are adequate. The compositions of etchants commonly used are given in Table 1. Any use of chromic acid and the other etchants must done with strict adherance to the safety and regulatory requirements. Chromic acid is toxic, may cause cancer by inhalation, and must not enter ground water, bodies of water, or sewage systems. See the Material Safety Data Sheets available from the supplier for conditions of use. Etching should follow soon after final polishing. The specimen should be cleaned in alcohol, then running water, and etched while wet. To avoid staining, the use of etchant 1 or 2 in Table 1 should be followed immediately by a rinse in etchant 3. The specimen is then thoroughly washed in running water, dipped in alcohol, and dried with a stream of warm, clean air. Table 2 lists recommendations for etchants and etching times for zinc and zinc die-casting alloys. The etching time may be longer or shorter for specific etching conditions; a minor difference in solution temperature may affect etching time. In addition, as indicated in Table 2 for cast or rolled zinc, etching time is often decreased as the magnification to be used is increased. Etchant 4 in Table 1 may also be used for etching zinc pressure die cast and galvanized specimens. Etching should proceed for 4 to 5 s, followed by rinsing in water and drying in warm air. Table 2

Etchants and etching times for zinc and zinc die-casting alloys

Time, s, for examination at Specimen metal(a)

Etchant (from Table 1)

250×

1000×

Cast or rolled zinc

1

5

1

Die cast alloy 3 (Z33520), 5 (Z35531), or 7 (Z33523)

2

1

1

Zinc-coated steel

7

1–30

1–30

(a) Selected die cast alloys given by common name (UNS)

Electrolytic etching has been used to differentiate two intermediate phases of the zinc-copper system (γ phase and ε phase). The electrolyte is a 17% aqueous solution of

CrO3. The polished specimen is the anode, and a small coil of platinum wire in the bottom of the dish or beaker holding the electrolyte serves as the cathode. The specimen is connected to the current source before immersion in the electrolyte. At a current density of 0.2 A/cm2 (1.5 A/in.2), γ and ε phases are approximately equally attacked. At higher current densities, γ phase is preferentially attacked; at lower current densities, ε phase is preferentially attacked. In a common procedure, the specimen is first etched at 0.78 A/cm2 (5 A/in.2). Gamma, if present, will be attacked; ε phase will not be attacked. The specimen is then repolished and etched at 0.15 A/cm2 (1 A/in.2), which will reveal any ε phase present. Further details on electrolytic etching of zinc alloys are available in Ref 7.

References cited in this section

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Metallography and Microstructures of Zinc and Its Alloys

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6.

J.L. Rodda, Preparation of Graded Abrasives for Metallographic Polishing, Trans. AIME, Vol 99, 1932, p 149–158

7.

J.L. Rodda, Notes on Etching and Microscopical Identification of the Phases Present in the Copper-Zinc System, Trans. AIME, Vol 124, 1937, p 189–193

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