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Lecture Series2a-L7-8: Metallographic Characterisation of Materials FUNCTIONS OF A METALLURGICAL LABORATORY Determination of Microstructure – Microstructure is evaluated by using standard metallographic techniques including sectioning, mounting, polishing, chemical etching, and inverted optical microscopy in bright field or polarized light as necessary. When required, the analysis is supplemented by scanning electron microscopy.

Heat Treatment – Heat treatment is checked by coupled microstructural evaluation and hardness measurements, and supplemented by electrical conductivity measurements when necessary.

Type and level of Inclusions – Inclusions are checked by using appropriate metallographic sections to analyze inclusion type and amount (volume fraction or length). When required, composition of the inclusions is checked by EDS.

Plating/Coating – Characterisation and thickness are checked using an appropriate metallographic section that includes a representative plated or coated surface. Plating or coating thickness is measured on the metallographically prepared section using a calibrated digital measurement system. Composition is analyzed by a combination of scanning electron microscopy and energy dispersive spectroscopy.

Grain Size – Grain structure is revealed by appropriate chemical etchants and measured by comparison using a microscope eyepiece equipped with the standard ASTM grain size grid. When necessary (e.g. for very fine or coarse grain sizes outside the range of the grid), standard stereological measurements are performed.

Grain Flow – Grain flow is determined by using a section(s) of appropriate size and orientation. The section is prepared using standard metallographic techniques, and chemically etched to reveal the grain structure and deformation patterns. The grain flow is

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analysed with respect to the component geometry and photographically recorded using either an inverted optical metallograph or macro level high-resolution digital imaging. Decarburization – Decarburization is checked by using appropriate metallographic sections that include a representative portion of the component surface. Chemical etching is used to reveal the presence of decarburization. If decarburization is detected, the depth is measured using a calibrated digital image analysis system and compared with the requirements. In addition, if the decarburized layer is of sufficient depth, microhardness measurements are used as a confirmatory test. Retained Austenite – Retained austenite is revealed by special chemical etching of standard metallographic sections. Microstructural estimation of retained austenite content is performed by standard stereological volume fraction counts. If the retained austenite content is found to exceed 3% by volume, X-ray diffraction is used as a confirmatory test and the results provided for comparison. Case Hardening – Case hardening is checked by using an appropriate metallographic section that includes a representative portion of the component surface. The “case” is revealed by an appropriate chemical etchant and measured by a microhardness scan from the surface to the interior. Typically, the case depth is reported as the distance from the surface at which the hardness is HRC 50.

Metallographic Analysis required a careful sample preparation!

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Metallographic Sample Preparation

To view the structural features a careful sample preparation is required. Goals of Metallographic Sample Preparation

The preparation procedure and the prepared specimen should have the following characteristics to reveal the true microsctructure:

• Deformation induced by sectioning, grinding and polishing must be removed or be shallow enough to be removed by the etchant.

• Coarse grinding scratches must be removed; very fine polishing scratches may be tolerable for routine production work.

• Pullout, pitting, cracking of hard particles, smear, and other preparation artifacts, must be avoided.

• Relief (i.e., excessive surface height variations between structural features of different hardness) must be minimized; otherwise portions of the image will be out of focus at high magnifications.

• The sur face must be flat, particularly at edges (if they are of interest) or they cannot be imaged.

• Coated or plated surfaces must be kept flat if they are to be examined, measured or photographed.

• Specimens must be cleaned adequately between preparation steps, after preparation, and after etching.

• The etchant chosen must be either general or selective in its action (reveal only the phase or constituent of interest, or at least produce strong contrast or color differences between two or more phases present), depending upon the purpose of the investigation, and must produce crisp, clear phase or grain boundaries, and strong contrast.

Preparation of metallographic specimens generally requires five major operations: (a) sectioning, (b) mounting (optional), (c) grinding, (d) polishing and (e) etching (optional).

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Metallographic Specimen Preparation Sample Selection: The first step in metallographic analysis is to select a sample that is representative of the material to be evaluated. This step is critical to the success of any subsequent study. The second, equally important step is to correctly prepare a metallographic specimen. The region of the sample that is of interest must be sectioned from the component.

For example, if a failure occurred because a steel pipe leaked during service, the metallographic analysis would probably involve at least three samples: one removed from the pipe such that a portion of the leak is contained in the sample, another removed near the leak, and a third taken far from the leak.

Sectioning of a metallographic sample must be performed carefully to avoid altering or destroying the structure of interest otherwise the results would be misleading as shown in Fig. 2.

Alterations in the microstructure of the specimen can be produced by deformation and the creation and further development of cracks and breakouts. Due to heat generation, recrystallization, local tempering, and in extreme cases, partial melting may occur. These problems can be minimized by the use of generous amounts of inert lubricants and coolants (water, oil, compressed air, etc.). The sectioning techniques are summarized in Fig. 1.1 and arranged according to the different sectioning mechanisms. When sectioning with a torch or by normal mechanical sawing, cutting, sand blasting, or cleaving, care must be taken to cut sufficiently far from the area of interest to avoid harmful effects.

Figure 2. Cutting damage (top) and a “burr” after sectioning of an annealed titanium specimen (mod. Weck’s reagent, 100X, polarized light plus sensitive tint).

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The most widely used sectioning device is the abrasive cutoff machine, ranging from units using thin diamond-rimmed wafering blades to those using wheels that are more than 1.5 mm ( in.) thick, 30 to 45 cm (12 to 18 in.) in diameter, containing silicon carbide particles

Depending on the type of material to be sectioned, cutting wheels of different compositions should be used, and their selection is dictated by hardness and ductility of the material to be cut. Table 1.1

Materials Cutting wheel: abrasive/binder Steel, ferrous materials, hardened steels

Al2O3 (corundum)/bakelite

High-alloy steels Cubic boron nitride (CBN)/bakelite

Nonferrous metals, hardmetals Silicon carbide (SiC)/bakelite Hard and tough materials, cermets, ceramics

Diamond/bakelite

Hard and brittle materials, ceramics, minerals

Diamond/metal

Fig. 1.1 Methods of metallographic specimen sectioning

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Mounting facilitates handling of the specimen during subsequent grinding and polishing operation.. A procedure that does not damage the specimen should be selected.. Mounting mediums should be compatible with the specimen regarding hardness and abrasion resistance.

Mounting involves placing the specimen in a mold and surrounding it with the appropriate powders. The mold and its contents are then heated under pressure to the thermal setting or the softening temperature. Once the powder sets, thermosetting mounts can be removed from the mold without lowering the temperature; thermoplastic mounts must be cooled to ambient temperature before removal. Two common mounting materials are thermosetting phenolics, such as Bakelite, and thermoplastic materials, such as methyl methacrylate (Lucite). A thermosetting polymer develops a rigid three-dimensional structure upon being heated and held at 200 to 300 °C.

Fig. 1.2 Methods of metallographic mounting

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Table 1.3 Properties of some important mounting materials Material Property Hot mounting materials Phenol resins (bakelite) Low hardness, poor adhesion (may be improved during

cooling under pressure). Poor chemical resistance to aggressive chemicals and hot etchants. Easy to use, low cost

Epoxy resins Only little shrinkage during curing, good edge retention. Resistant to etchants

Diallyl phthalate Suitable for hard materials. No shrinkage during curing. Resistant to aggressive chemicals and hot etchants. Mounting conditions must be strictly followed.

Acrylics Care must be taken during grinding; material may crack due to imposed stresses. Poor adhesion. Not resistant to aggressive chemicals. Transparent. Sample should be well cooled during curing. Suitable for pressure-sensitive specimens; pressure is only to be applied during the cooling cycle.

Cold mounting materials Epoxy resins Good adhesion. High viscosity, fills cracks, gaps, and pores

easily and is therefore well suited for infiltration. Resistant to etchants and solvents. Nearly transparent. Mold material should be made of silicon rubber, polyethylene, or Bakelite. Curing time at least 8 h. Work under a fumehood because poisonous fumes are being generated. Skin irritant

Polyester resins Good abrasion resistance, therefore well suited for hard materials. Shrinkage. Chemical resistance varies with the product.

Acrylics Shrinkage. Short curing times. Poor resistance to alcohol and Chlorohydrocarbon

Conducting Mould: If analysis requires using SEM during characterisation, then a copper containing thermosetting material used. Grinding is generally considered the most important step in specimen preparation. Care must be taken to minimize mechanical surface damage. Grinding is performed by the abrasion of the specimen surface against water-lubricated abrasive wheels (assuming water does not adversely affect the metal).

Grinding develops a flat surface with a minimum depth of deformed metal and usually is accomplished by using progressively finer abrasive grits on the grinding wheels.

A typical sequence might begin with 120- or 180-grit papers and proceed to 240, 320, 400, and 600 grits. Scratches and damage to the specimen surface from each grit must be removed by the next finer grinding step.

The surface damage remaining on the specimen after grinding must be removed by polishing. If this disturbed or deformed metal at the surface is not removed, microstructural observations may be obscured (Fig. 2).

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Polishing of the metallographic specimen generally involves rough polishing and fine polishing. In rough polishing, the cloth covering on a wheel is impregnated with a fine (often as small as 1 μm) diamond paste or a slurry of powdered α- Al2O3 in water, and the specimen is held against the rotating wheel. The cloth for rough polishing is frequently napless, providing easy access of the polishing abrasive to the specimen surface. Fine polishing is conducted similarly, but with finer abrasives (down to 0.05 μm in diameter) on a napped cloth.

Polishing should yield a scratch-free specimen surface, in which inclusions and other second-phase articles may be visible. Polishing damage, such as that illustrated in Fig. 3, should be recognized and avoided when preparing metallographic specimens.

Fig. 2 The effect of disturbed metal on the metallographicappearance of a plain carbon steel. (a) A layer of disturbed metal--an artifact structure caused by grinding damage--covers the polished surface. (b) The layer of disturbed metal is removed, and the structure is revealed to be lamellar pearlite. Etched using picral. 1000×

Fig. 3 The effect of improper polishing on AISI 1010 steel. (a) "Comet tails" from improper polishing. (b) The same material polished correctly, exhibiting small manganese sulfide inclusions

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Grinding and Polishing Artefacts

Figure 14. SiC grit particle (arrow) embedded in a 6061-T6 aluminum weldment (500X, aqueous 0.5% HF).

Figure 15. 6μm diamond particles (arrows) embedded in lead (1000X).

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Grinding and Polishing Media

Figure:The abrasive size ranges applicable to thevarious stages of preparation.

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Etching includes any process used to reveal the microstructural features such as grains, grain boundaries, twins, slip lines, and phase boundaries of a metal or alloy which are not observable on an as-polished specimen,. Etchants attack at different rates areas of different crystal orientation, crystalline imperfections, or different composition. The resulting surface irregularities differentially reflect the incident light, producing contrast, coloration, polarization, etc. Various etching techniques are available, including chemical attack, electrochemical attack, thermal treatments, vacuum cathodic etching, and mechanical treatments

Chemical etching selectively attacks specific microstructural features. It generally consists of a mixture of acids or bases with oxidizing or reducing agents. Table XI lists some of the more common etchants.

Table XI. Common Chemical Etchants

CAUTION: Safety is very important when etching. Be sure to wear the appropriate protective clothing and observe all WARNINGS on chemical manufacturers MSDS. Also review COMMENTS Section for each etchant. Etchant Composition Application Conditions Com Etchant Composition Application Conditions Comments

Kellers Etch

190 ml Distilled water 5 ml Nitric acid 3 ml Hydrochloric acid 2 ml Hydrofluoric acid

Aluminum alloys

10-30 second immersion. Use only fresh etchant

Kroll's Reagent

92 ml Distilled water 6 ml Nitric acid 2 ml Hydrofluoric acid

Titanium 15 seconds

Nital 100 ml Ethanol 1-10 ml Nitric acid

Carbon steels, tin, and

Seconds to minutes

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nickel alloys

Kallings Reagent

40 ml Distilled water 2 grams Copper chloride (CuCl2) 40 ml Hydrochloric acid 40-80 ml Ethanol (85%) or Methanol (95%)

Wrought stainless steel, Fe-Ni-Cr alloys

Immerse or swab for few seconds to a few minutes

Lepito's Reagent

50 ml Acetic acid 50 ml Nitric acid

High temperature steels

Swab

Marble's Reagent

50 ml Distilled Water 50 ml Hydrochloric acid 10 grams Copper sulfate

Stainless steels, Nickel alloys

Immersion or swab etching for a few seconds

Murakami Reagent

100 ml Distilled Water10 grams K3Fe(CN)8 10 grams NaOH or KOH

Wrought Stainless steel, tungsten alloys, silver alloys,SiC, B4C

Immerse or swab for seconds to minutes

Use fresh

Picral 100 ml Ethanol 2-4 grams Picric acid

Iron and steel, tin alloys

Seconds to minutes

Do not let etchant crystallize or dry - explosive

Vilella's Reagent

45 ml Glycerol 15 ml Nitric acid 30 ml Hydrochloric acid

Stainless steel, carbon steel,cast iron

Seconds to minutes

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Electrolytic etching is another fairly common etching technique. It is similar to chemical etching in that acids and bases are used for modifying the pH, however the electrochemical potential is controlled electrically by varying the voltage and current externally as opposed to chemically.

Electrolytic etching is often used for harder to etch specimens that do not respond well to basic chemical etching techniques. Electrolytic techniques require that the specimen be conductive and are therefore limited primarily to metals.

The most common electrolytic etching equipment uses a two-electrode design (anode and cathode) with acids or bases for the electrolyte. Procedures for this type of electrolytic etching are fairly common and can be found in the literature.

Molten Salt etching is a combination of thermal and chemical etching techniques.

Molten salt etching is useful for grain size analysis for hard to etch materials such as ceramics. The technique takes advantage of the higher internal energy associated at a materials grain boundaries.

At the elevated temperature of molten salts, the higher energy at the grain boundaries is relieved, producing a rounded grain boundary edge, which can be observed by optical, or electron microscope techniques. Thermal etching/Heat Tinting Etxhing is a useful etching technique for ceramic materials. Thermal etching is technique that relieves the higher energy associated at the grain boundaries of a material. By heating the ceramic material to temperatures below the sintering temperature of the material and holding for a period of minutes to hours the grain boundaries will seek a level

Figure 38. Grain structure of annealed CP titaniumrevealed by heat tinting on a hot plate (100X, polarized light plus sensitive tint).

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of lower energy. The result is that the grain boundary edge will become rounded so that it can be evaluated by optical or electron microscope techniques. Depending upon the ceramic material, the atmospheric condition of the furnace may need to be controlled. For example, etching silicon nitride will require either a vacuum or an inert atmosphere of nitrogen or argon to prevent oxidation of the surface to silicon dioxide. Plasma etching is a lesser-known technique that has been used to enhance the phase structure of high strength ceramics such as silicon nitride. For silicon nitride, the plasma is a high temperature fluoride gas, which reacts with the silicon nitride surface producing a silicon fluoride gas. This etching technique reveals the intragrain microstructure of silicon nitride.

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Tint Etching

Tint etchants have been developed to color etch many metals and alloys -- cast irons, steels, stainless steels, nickel base alloys, copper base alloys, molybdenum, tungsten, lead, tin, and zinc. Limited success has been obtained with tint etching of aluminum and titanium alloys.

A selected list of etchants is given in Table I; additional information can be obtained in Ref 1 and 2.

The most widely applicable tint etchant is that developed by Klemm which colors ferrite in steels, reveals overheating or burning in steels, and develops the grain structure of copper and many copper alloys, as well as those of lead, tin, and zinc.

Fig. 1-- Examples of brasses tint etched using Klemm's I reagent. A: cold worked and annealed alpha brass (70 Cu-30 Zn). B, C, and D are alpha-beta brass (60 Cu-40 Zn) heat treated via three different methods. B: 940 F (505 C), water quench. C: 1200 F (650 C), water quench. D: 1550 F (845 C), air cool. Original magnification: 100X

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Table I - Selected Tint Etchants 1. 200 g CrO3

20 g sodium sulfate 17 mL HCI 1OOO mL water

Beraha's tint etch for Al alloys. Pre-etch with10% aq NaOH followed by 50% aq HNO3. Rinse in water, dip immediately into tint etch for 1-5 s. Rinse and dry. Colors matrix grains, outlines second phase particles.

2. 1 g ammonium molybdate 6 g ammonium chloride 200 mL water

Tint etch of Lienard and Pacque: colors CuAl2 violet. Immerse about 2 min.

3. (a) Stock solution: 1:2, 1:1, or 1:0.5 HCI-water

(b) 100 mL stock solution plus 0.6-1.0 g potassium metabisulfite

(c) Optional additions: 1-3 g FeCI3, or lg CUCI2, or 2-10 g NH4FHF

Beraha`s tint etch for Fe, Ni, or Co base heat resistant alloys. Colors the matrix, carbides and nitrides unaffected. Immerse sample in solution at room temperature for 60-150 s. Move sample during etching. Start with lowest HCI concentration; if coloration does not result, increase HCI or etch longer.

4. 50 mL saturated aqueous sodium thiosulfate 3 1 g potassium metabisulfite

Klemm's I tint etch. Good for many alloys. Immerse 3 min or more for beta brass, alpha-beta brass, and bronzes. Use 10-60 min for alpha brass. Use 40-100 s for coloring ferrite in steels, reveals P segregation and overheating. Longer time produces line etching of ferrite. Etch 30 s for zinc alloys.

5. 50 mL saturated aqueous sodium thiosulfate 5 g potassium metabisulfite

Klemm s II tint etch. Immerse 6 min or more for alpha brass. lmmerse 30-90 s for steels Reveals P segregation. Good for austenitic Mn alloys. Immerse 60-90 s for tin and its alloys.

6. 5 mL saturated aqueous sodium thiosulfate 45 mL water 20 g potassium metabisulfite

Klemms III tint etch. Immerse 3-5 min for bronze. Immerse 6-8 min for Monels.

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240 g sodium thiosulfate 30 g citric acid 24 g lead acetate 1000 mL water

Beraha`s lead sulfide tint etch. Dissolve in order given. Allow each to dissolve before adding next (cannot get complete dissolution). Age in dark bottle at least 24 hr before using. Do not remove precipitate. When stock solution turns gray after prolonged storage, discard. Immerse in solution until surface is colored violet or blue. Excellent for copper and its alloys. To color MnS in steels, add 200 mg sodium nitrate (optional) to 100 mL solution; good for 30 min. Colors MnS white. Pre-etch with nital or picral.

8. 21-28% aqueous sodium bisulfite

Beaujard and Tordeux`s tint etch for steels. Immerse 10-25 s Reveals grain boundaries and ferrite orientations. darkens as-quenched martensite.

9. 1 g sodium metabisulfite 100 mL water

Tint etch for lath or plate martensite. lmmerse 2 min (Benscoter et al).

10. 8-15 g sodium metabisulfite 100 mL water

Darkens as-quenched martensite. Immerse about 20 s.

11. 3-10 g potassium metabisulfite 100 mL water

Darkens as-quenched martensite. Immerse 1-15 s.

12. 1 g sodium molybdate 100 mL water

Beraha`s tint etch for cast iron and steels. Add HNO3 to pH 2.5-4.0 (about 0.4 mL). Immerse 20-30 s for cast iron. Fe3P and Fe3C,

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yellow-orange; ferrite, white. For low carbon steei add 0.1 g NH4FHF, immerse 45-60 s. For medium carbon steel add 0.2 g NH4FHF. For high carbon steel add 0.3-0.4 g NH4FHF. Carbides, yellow-orange to violet; ferrite, white or yellow

13. 3 g potassium metabisulfite 10 g anhydrous sodium thiosulfate 100 mL water

Beraha's tint etch for iron and steel. Immerse 1-15 min. Colors ferrite, martensite, pearlite, and bainite: sulfides are brightened

14. 0.5-1.0 mL HCI 100 mL water 1 g potassium metabisulfite

Beraha's tint etch for irons, steels, tool steels. Agitate strongly during etching. then hold motionless until surface is colored; 10-60 s total time. Colors ferrite, martensite, pearlite, bainite. Reveals grain boundaries.

15. 20 mL HCI 100 mL water 0.5-1 g potassium metabisulfite

Beraha`s tint etch for stainless steels. Immerse 30-120 s with agitation. Colors austenite.

16. Stock solution: 20 mL HCI 100 mL water 2.4 g NH4FHF

Beraha's tint etch for stainless steels. Before use, add 0.6-0.8 g potassium metabisultite (0.1-0.2 g for martensitic grades). Afer mixing, reagent is good for 2 hr. Use plastic tongs and beaker. Immerse 20-90 s, shake gently during etching. Colors matrix phases.

17. 40-60 mL FeCI3, solution (1300 g/L water) 25 mL HCI 75 mL ethanol

Hasson's tint etch for Mo. Immerse without agitation for 40-50 s (do not exceed 70 s). FeCI3 can be dissolved in ethanol but etch is slower, 2-3 min. Colors vary with grain orientation.

18. 70 mL water 20 mL H2O2 (30%) 10 mL H2SO4,

Tint etch for Mo alloys (Oak Ridge National Laboratory). Immerse 2 min, wash and dry. Swab removes colors, produces grain boundary attack.

19. 5 g NH4FHF 100 mL water

Weck's tint etch for alpha Ti. For pure Ti, immerse a few seconds, longer times for Ti alloys. Colors vary with grain orientation.

20. 39 g NH4FHF 4 mL HCI 100 mL water

Weck's tint etch for alpha Ti alloys. Immerse for a few seconds. Colors vary with grain orientation.

21. 94 mL 10% aqueous HCI 20 g Cr03

Tint etch for W (Lehwald et al). Immerse at 55 C (130 F). Use 2 or 3 stages (view between etches) of 15, 10, and 10 min. Pre-etch with grain boundary etch.

Footnotes 1. Additional tint etches are listed in Ref 1 and 2. 2. Whenever water is specified, use distilled water. 3. Maximum solubility of anhydrous sodium thiosulfate (Na2S2O3) is 50 g/100mL water at 20 C (70 F), while that of the crystal form (Na2S2O3. 5H2O) is 79.4 g/100mL water at 0 C (32 F) or 291.1g/100 mL water at 45 C (115 F).

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Applications Of Tint Etching Tint etching is particularly well suited for copper and copper alloys. Klemm's I reagent works very well with most of these compositions. Beraha's lead sulfide tint etch is also useful. All of the copper samples to be shown were attack polished using 1% aqueous ferric nitrate added to colloidal silica. Coloration is generally somewhat different, and sometimes less sharp, when other polishing abrasives are used.

Figure 1 (above) shows the microstructure of cold worked and annealed alpha brass (Cu-30% Zn) and three examples of alpha-beta brass (Cu-40% Zn) heat treated in various ways to alter the grain size and amount and distribution of the phases. For the alpha brass sample (A), etching is very slow -- requiring nearly an hour. Tint etching has revealed all of the grains and the annealing twins. With ordinary chemical etchants it is very difficult to get such complete revelation of the grain structure.9

For the alpha-beta brass samples, the anodic beta phase is colored rather quickly, generally less than 5 min is required. The sample (B) heated to 940 F (505 C) and water quenched contains close to the minimum amount of beta (actually, ordered beta) that will form. Tint etching produces only a light yellowish color in all of the beta grains.

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Samples heated higher in the two phase region -- 1200 F (650 C) and water quenched -- contain more beta which is tinted with a variety of colors depending on orientation.

The third example -- heated into the all-beta region and air cooled -- shows three prior beta grains where the beta phase within each of these regions has the same color. Note that the coloring is very precise, no enlargement of the beta occurs, and all of the beta is revealed. None of these samples was pre-etched -- the attack polish produced enough relief to eliminate need for one.

Figure 2 shows the microstructure of eutectic Al-33% Cu tint etched with the aqueous ammonium molybdate-ammonium chloride solution suggested by Lienard and Pacque.10 This sample was attack polished using a few mL of 0.5% aqueous HF added to colloidal silica in the Fini-Pol device. The CuAl2 phase is colored violet.

Figure 3 illustrates the use of Beraha's acidified aqueous sodium molybdate reagent to color the cathodic cementite in an Fe- 1% C, high purity alloy where the cementite in the pearlite is blue but the grain boundary cementite is violet.

Figure 4 shows the microstructure of solution annealed, austenitic Hadfield manganese steel. After polishing with colloidal silica, the sample was given a light pre-etch with 2% nital (3 s) and then etched 20 s with 20% aqueous sodium metabisulfite. This procedure is also effective for revealing the a + e structure of decarburized surfaces of the alloy.11

Tint etched samples can also be photographed in black and white and still produce superb results (see Fig. 5 and 6).

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Figure 5 (above)-- Tint etching of nickel base superalloys such as X750 using Beraha's reagent (right) can produce excellent color contrast development of twin and grain structures compared with that produced using standard reagents such as Kalling's No.2 etch (left). B&W photos clearly reveal color differences. 100X. Composition of Beraha's tint etchant: 100mL HCL, 50 mL H2O,1 g potassium metabisulfite, and 1 g ferric chloride.

Figure 6 (above)-- Beraha's lead sulfide etch will color sulfide inclusions white. Shown here is quenched and tempered AISI 41S50, pre-etched with 2% nital. Photomicrograph at left is of as-polished steel; one at right is of the tint etched sample. 500X

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Metallographic Analysis

Metallurgical analysis (metallography) of the microstructural provides the Material Engineers information varying from phase structure, grain size, solidification structure, casting voids, etc.

Figure 2 shows an example of two cast iron structures. o Figure 2a shows a cast iron microstructure which has graphite

flakes. Over time this materials will most likely fail under load. o On the other hand by adding some solidifying agents the cast iron

can be made to form the more durable graphite nodules (Figure 2b).

Analysis of a materials grain size provides valuable information regarding a materials physical hardness and ductility. Figure 3 shows the grain size of a tough pitch copper.

Microstructural analysis can also provide very useful information about the types of phases that occur during cooling.

Figure 4 shows the

dendritic growth of the microstructure for an aluminum-silicon alloy which formed during solidification. The direction and size of these dendrites again relate to the materials strength and durability.

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Macroanalysis

Macrostructural characterization of metals and alloys is the detailed evaluation of large-scale inhomogeneities in composition, morphology, and/or density.

These inhomogeneities may develop during processing such as casting,

extrusion, forging, rolling, and welding or during service.

Materials characterization by optical macrostructural examination can be divided into three categories.

First, examination of the macrostructure of metallographically prepared sections of interest is used to evaluate such structural parameters as:

� Flow lines in wrought products � Solidification structures in cast products � Weld characteristics, including depth of penetration, fusion-zone size

and number of passes, size of heat-affected zone and type and density of weld defects

� Size and distribution of large inclusions. � Gas and shrinkage porosity in cast products � Depth and uniformity of a hardened layer in a case-hardened product

Second, characterization of the macrostructural features of fracture surfaces is used to identify such features as:

� Fracture initiation sites and changes in crack propagation process

Third, characterization of surfaces and surface defects on parts and coupons such as: � Estimations of surface roughness, grinding patterns, and honing angles � Evaluation of coating integrity and uniformity � Determination of extent and location of wear � Estimation of plastic deformation associated with various mechanical processes � Determination of the extent and form of corrosive attack; readily

distinguishable types of attack include pitting, uniform, crevice, and erosion corrosion

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Example of Macro Examinations

Exampl-1: Casting

Figure 5 shows the macrostructure of a small relatively pure aluminum ingot exhibiting typical cast grain structure.

Transverse section shows outer chill zone and columnar grains that have grown perpendicularly to the mold faces

To obtain the macrograph, the aluminum ingot was sectioned, then ground and polished to produce a flat reflective surface. The polished section was then etched by immersion in a solution that attacked the various grain orientations at different rates.

The etched structure was examined using a low-power microscope.

The structural elements visible in this macrograph are grains.

The small grains near the bottom of the ingot appear relatively equiaxed. This region of small equiaxed grains is the chill zone.

During casting, such macrostructural defects as gas or shrinkage porosity and center cracks can develop.

Many of these defects can be

characterized using macrostructural evaluation.

Fig. 5 Macrostructure of as-cast aluminum ingot. Transverse section shows outer chill zone and columnar grains that have grown perpendicularly to the mold faces. Etched using Tucker's

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Exampl-2: Flow Lines Figure 7 shows the flow lines show the direction of metal flow during processing and frequently represent paths for easy fracture.

Example 3: Case Hardening Figure 8 shows the use of similar macroscopic techniques to illustrate the depth of case hardening in a tool steel;

Lecture Series on Quantitative Metallography

Fig. 8 Case-hardened layer in W1 tool steel. Specimens were austenitized at 800 °C (1475 °F), brine quenched, and tempered 2 h at150 °C (300 °F). Black rings arehardened zones. Etched using 50% hotHCl. Approximately 0.5×

Fig. 7 Flow lines in a forged 4140 steel hook. Specimen was etched using 50% HCl. 0.5×