Micrographic Preparation and Observation · Micrographic Preparation and Observation 1 Content ... x. Such microscopic studies, in the hands of an experienced observer, provide abundant

Mechanical Engineering Department Materials Engineering 1 Laboratory

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Micrographic Preparation and Observation 1

Content 1 Introduction ............................................................................................................................................. 2

2 Sampling ................................................................................................................................................... 2

3 Manual Grinding ....................................................................................................................................... 3

4 Polishing ................................................................................................................................................... 4

5 Characteristics of the Mechanical Preparation Process ........................................................................... 7

6 Mounting of Small Samples...................................................................................................................... 9

7 Etching of the Samples for Microscopic Examination ............................................................................ 10

8 Reagents for Chemical Etching ............................................................................................................... 10

9 Etching Mechanisms .............................................................................................................................. 11

10 Etching Methods .................................................................................................................................... 11

11 Microscope Observation of Material Microstructures .......................................................................... 13

12 Preparing a Sample for use in an Upright Microscope .......................................................................... 14

13 Operation of an Optical Metallurgical Microscope ................................................................................ 15

1 Adapted from: Kehl, George; The Principles of Metallographic Laboratory Practice, McGraw-Hill, 1949; and other sources.


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1 Introduction Metallography is, essentially, the study of the microstructural characteristics of a material or an alloy to relate this with its physical and mechanical properties.

Undoubtedly, the most important part of the micrograph is the microscopic examination of a properly prepared specimen; using magnifications that, with the optical microscope, oscillate between 50 and 1,000 x. Such microscopic studies, in the hands of an experienced observer, provide abundant information on the constitution of the investigated material or alloy. Through them you can define structural characteristics, such as grain size, very clearly; we can know the size, shape and distribution of the phases that comprise the alloy, and their inclusions and impurities, as well as the presence of segregations and other heterogeneities that can so profoundly modify the mechanical properties and the general behavior of a material. Once the microscopic examination has allowed the determination of these and other microstructural characteristics, it is possible to predict with great accuracy the behavior of the material when it is used for a specific purpose. Similar importance has the fact that, with certain limitations, the microstructure reflects the history of thermo-mechanical processing and treatment that the material has undergone.

Experience shows that little to nothing can be obtained from microscopic examination if the specimen is not prepared to obtain a satisfactory surface. A defective preparation can tear out interesting inclusions, destroy grain edges, temper a hardened steel or alter the surface by plastic deformation. In summary, a poorly prepared sample does not bear any relation to the microstructure and true characteristics of the material. It is clear that the examination of such a surface will lead to erroneous interpretations and inadmissible conclusions.

The preparation of a sample consists, in general, of first obtaining a flat and semi-polished surface by using abrasive papers of increasing grit. Then, samples are finished on a polishing discs provided with cloths embedded with very fine abrasives. The aim is to obtaining a specular surface that is required prior to a chemical etching which will reveal the desired characteristic of the microstructure to be observed.

One of the most essential factors that influence the technique of specimen preparation is the care with which it is handled through all phases of operation. Of equal importance is cleanliness, as a particle of abrasive or foreign matter can render a test sample unusable. At frequent intervals, during the course of preparation, both the sample and the hands of the operator should be washed with running water and soap.

2 Sampling The choice of the sample to be examined under the microscope is of great importance, since it should be representative of the material as a whole. The specimens selected must be characteristic of the material studied and serve the purpose for which such study is intended; for example, if a component has failed in service and the object of the study is to determine the causes of the fracture, the specimen should be obtained from that particular region of the fracture. To be able to make comparisons, a test sample of must be taken from a healthy section of the component in question. The examination of both types of specimen is desirable because the inclusions and other characteristics may not be satisfactorily observed on specimens taken in only one from one position.

If the section to be taken as a sample is relatively soft, the separation can be done by a mechanical or manual saw. When the materials are fragile, as is the case with castings and some tin rich bronzes, you can break the part with a hammer and select a suitable fragment.

Samples from hard materials, which cannot be easily sawn, such as hardened steels and non-ferrous alloys hardened by aging, can be safely cut using abrasive wheels. Such wheels are generally thin and are a


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conglomerate of an abrasive such as carborundum or diamond powder. To prevent heating of the material, the specimen can be kept completely submerged in water or other cooling liquids, or by projecting an uninterrupted stream of cooling liquid over it.

If the cutting disc is not carefully selected and the specimen is not cooled sufficiently during cutting, the original structure can be radically altered, at least on the surface obtained by the cut.

Whenever possible, the specimens should be conveniently sized and comfortable to handle. Very large surface specimens may require excessively long polishing times, while those too small tend to be rounded during etching, resulting in curved surfaces with marred edges. Therefore, small specimens must be mounted or encapsulated, as described below.

3 Manual Grinding

3.1 Rough Grinding

The surface to be observed must be flat, and this is achieved by a rough grinding. Sample pressure during roughing, and during intermediate and final grinding, has to be controlled. Excessive pressure not only produces very deep scratches, which are difficult to remove later, but also intensely distort the material on the surface of the specimen.

Surface distortion cannot be avoided, but it can be greatly reduced by appropriate grinding and polishing techniques; the contact pressure in the operations must be kept low, and in any case, the final deformed material can be eliminated by several cycles of polishing and etching. Beveling the outer edges of the specimen during roughing will prevent breakage and tearing of papers and cloths in subsequent operations.

The roughing step is finished when the surface to be observed is completely flat, and its irregularities have been eliminated.

3.2 Intermediate Grinding

For the intermediate grinding the abrasive paper is placed on a flat and clean surface. The specimen is moved longitudinally from one side of the paper to the other pressing it gently. The direction of movement remains constant so that all the lines produced are parallel. The end of the operation on a paper is determined by the disappearance of the stripes produced by the previous paper. To be able to recognize the pattern easily, we operate in such a way that the new lines are perpendicular to the previous ones; it is easier to see when these new lines completely replace the previous ones.

To grind many heat-treated alloy test specimens, and in particular many of the soft metals, it is convenient to impregnate the abrasive papers with a suitable lubricant. For this purpose, various liquids can be used, such as oils, gasoline, paraffin solutions in kerosene, liquid soaps, glycerin, and mixtures of glycerin and water. These lubricants reduce the superficial deformation of soft metals and avoid the superficial structural modification of the thermally treated ones, since they act not only as lubricants, but also as coolants.

3.3 Final Grinding The final or fine grinding is done in the same way as the intermediate grinding, but ensuring adequate cooling and lubrication; in general, the grit sizes used include 600, 800, 1,200 and even 2,000.

Each time the grit is changed, the sample is rotated 90⁰ in order to obtain new lines perpendicular to the previous ones. When the visual observation shows that the lines produced by the previous steps have been totally eliminated, the specimen is in a condition to be polished.


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3.4 Mechanical Grinding

3.4.1 Abrasive Paper Grinding Machines

A more efficient grinding can be performed mechanically using a rotating disc that is covered with abrasive papers, held by an appropriate fixing ring. The disc should rotate at about 600 rpm for the rougher paper and preferably at smaller speeds with the finer once.

In mechanical grinding, extreme care must be taken so that the excessive pressure does not cause overheating of the specimens, with consequent structural alterations in the thermally treated specimens and exaggerated distortion of the surface material. It is common to use a constant stream of water to wash away grinding debris, lubricate and cool the specimen.

3.4.2 Paraffin Discs

A paraffin disc is simply a disc covered with a high-melting temperature paraffin layer or with a canvas cloth that has been impregnated with paraffin. For grinding, the discs are loaded, before and during operation, with suspensions of abrasive powders in an aqueous soap solution.

4 Polishing

4.1 Introduction

The purpose of polishing a specimen is to eliminate the fine lines produced in the last grinding operation from its surface and achieve a surface with a high level of gloss.

Success in polishing, and the time spent in the operation, depends to a large extent on the care with which the grinding has been carried out. If a specimen has deep and thick scratches, which have not been removed in the finish grinding, time and work are wasted if they are to be eliminated with polishing.

Precautions should be taken so that the operation is carried out in a clean, dust free environment so as not to introduce new lines and deformations on the sample surface.

4.2 Metallographic Polishers

The initial and final polishing of a specimen for micrographic observation is done on one or more discs on mechanical polishers. Such discs are, essentially, bronze plates of 8 to 10” in diameter, covered with a cloth of appropriate quality. The discs rotate in a horizontal plane, and it is convenient that each disc possesses its individual motor to facilitate the control and adjustment of the speed of rotation.

The automatic grinding and polishing equipment saves, undoubtedly, a lot of time and work of the laboratory technician in the routine operations of sample preparation. However, many experts say that in automatic equipment, contrary to what happens in manual techniques, it is difficult to observe the progress of the preparation of the specimen and, especially, to achieve the control of the final degree of polishing that is necessary in high quality work on difficult to polish materials.

4.3 Abrasives for Metallographic Polishing

Physically, an ideal abrasive must possess a relatively high hardness; the external shape of the particles must be such that they have numerous sharp edges and sharp corners. If the particles break during use, they should generate new sharp edges and corners. Finally, the nature of the abrasive must be adequate


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to allow a good separation of the particles by size by means of colloidal dispersion 2 , after a good pulverization has been carried out.

Diamond Powder: The abrasive that most closely approximates the ideal is the unadulterated diamond powder. It is widely used to prepare specimens of very hard alloys, such as sintered tungsten or boron carbides, and has recently been widely used, with great success, for polishing the most common alloys and metals.

Alundum: For the initial and intermediate polishing of the metallographic specimens, alundum (molten aluminum oxide) and, sometimes, carborundum (silicon carbide) or boron carbide, are used as abrasives, all in a fineness of 500 to 600 mesh. They are used in the form of an aqueous suspension, which are added to the cloth that covers the disc of the polisher.

Magnesium oxide: It is the abrasive that is usually recommended for the final polishing of soft metals, such as aluminum, magnesium and others, or to replace alumina in the polishing of cast irons and other relatively hard materials.

The proper technique for the use of magnesium oxide in the final polishing is to put a small amount of fresh and dry powder on the polishing cloth, add distilled water in sufficient quantity to form a clear paste, and then work this paste with the fingertip extending it and imbibing it in the fibers of the cloth. After this loading, and during the subsequent polishing, the cloth is kept wet by adding more distilled water.

Alumina: Alumina (aluminum oxide) is probably the most satisfactory and universal polishing abrasive. Commercially alumina is available in form of powder, paste or aqueous suspension.

Alumina exists in three distinct crystallographic forms: alpha, beta and gamma. Of these, alpha and gamma are the most used as abrasives.

Colloidal alumina is slightly acidic, aiding in the removal of the highly deformed outer layer (skin) of the metallographic sample.

Silica: Silica (silicon oxide) is also very popular when high quality metallographic preparation are at stake. Mechanically, colloidal silica works in the same way as colloidal alumina; however, silica is slightly alkaline. It is for this reason that silica is preferred for the polishing of non-ferrous metals.

Some types of dry alumina powder, although already sized, can be colloidal dispersed to refine the abrasive particle size. The process simply consists in suspending a small amount of alumina in clean water, using a tall glass. After stirring well let it settle for about 10 minutes, this will separate the coarse and fine particles. Finally the liquid, consisting of a suspension of the finer abrasive, is siphoned out. The sediment can be re-dispersed to obtain slightly coarser alumina suspensions, or discarded.

Other Abrasives: In addition to the aforementioned abrasives, chromium oxide and iron oxide (jewelers’ red) have also been successfully used in the polishing of various specimens. The jewelers’ red, however, has a propensity to make the surface metal flow, and although it provides an extraordinarily polished surface, such a surface is not suitable for microstructural observation.

Colloid Alumina (Acidic) and Colloidal Silica (Alkaline),

4.4 Polishing Cloths In general, the surface texture of the polishing cloths varies from those without hair, such as natural silk and the fabric used to cover the wings of airplanes, to those with relatively long hair, such as velvet and corduroy. In the intermediate case there are billiard table cloths, wool cloths of different finishes, and tarpaulins of different weights.

2 Colloidal system or colloidal dispersion is a heterogeneous system which is made up of a very fine particles

dispersed phase and a dispersion medium. [http://www.chemistrylearning.com/colloidal-dispersions/]


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The best quality polishing cloths do not usually require preliminary treatment before use. However, cheaper materials must be boiled in water to soften hard fibers, and washed with a green soap dye to remove foreign matter they may contain.

When a cloth is not going to be used for some time, it is removed from the polishing disc, soaped and washed thoroughly with running water. Afterwards it can be dried or, better, it is kept submerged in water in a glass. The washing eliminates practically all the debris adhered to the cloth, and keeping it in a wet condition prevents the remaining abrasive residues from forming crusts that are difficult to dissolve again.

4.5 Initial Polishing The purpose of the initial polishing (also called preliminary) is to remove the fine lines produced in the last grinding operation. The disc used in this operation is covered, usually with wool cloth, billiard cloth or a light weight canvas, and rotated at 400 to 500 rpm. As an abrasive, alundum or 600 mesh carborundum, or equivalent products, is used.

To perform a preliminary polishing the specimen is held firmly on the rotating disc, and during the operation it moves continuously from the center to the edge of the disc, and inverse. If necessary, suspension of the abrasive is added from time to time, containing about 15 g per 100 cc of water. If the amount of abrasive on the disc is sufficient, but the cloth is dry, water is added in the necessary amount. For the operation to go well, it is necessary to carefully observe the humidity of the cloth. If it gets too wet, the polishing action of the cloth-abrasive combination is retarded, and if it dries excessively, the specimen will stain and even scratch again.

The preliminary polishing lasts from 2 to 5 min. When the operation is finished, the test piece is washed with running water, and it can even be introduced to an ultrasonic bath to completely eliminate all debris and the adhered abrasive. Finally, the sample is rinsed with ethyl or isopropyl alcohol, then dried in hot air. The alcohol displaces the water and evaporates without leaving any residue.

The well-prepared specimens, after grinding and preliminary polishing, shows only the characteristic stripes of the alundum or carborundum, and the surface is of dull brightness.

4.6 Final Polishing

This operation has the purpose of eliminating the scratches produced in the preliminary polishing and finally produce a uniform polished surface, free of scratches and superficial plastic deformation. According to the material or alloy that is polished, one of the abrasives mentioned above is used - colloidal alumina, magnesium oxide, silica or chromic oxide. For most of the specimens alumina gives an excellent result, and is therefore recognized by all as the most universal abrasive in the final polishing step.

During polishing, a moderate pressure is applied to the specimen, and it is continuously moved from the center to the periphery of the disc. Eventually, and in particular at the end of the operation, the specimen is rotated in the opposite direction to that of the disc rotation. This operation continuously modifies the polishing direction and prevents the formation of "comet tails". Such formations are unavoidable when polishing in only one direction, because there is a tendency to tear off hard surface particles, such as non-metallic inclusions in metals. Once the hard particles are removed, the material adjacent to the hole wears unidirectional leaving said marks.

To avoid surface deformation of the material, fine polishing should be suspended as soon as the stripes are no longer observable at x100 magnifications. If the fine lines persist, the final polishing can be continued; it is, however, more likely that better results will be obtained by repeating the preliminary polishing before finishing the final polishing.

Finally, the polished sample, cleaned and rinsed with alcohol, can be examined without etching, be etched immediately, or stored for later use. In any case, the surface of the specimen must be protected from oxidation and other harmful atmospheric effects.


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4.7 Electrolytic Polishing

Electrolytic polishing reduces many of the difficulties encountered in mechanical polishing, since this method avoids the formation of scratches, as well as deformed surface layers in the specimen. It is ideal for the preparation of many soft metals, single phasic alloys and alloys that harden easily by deformation, such as austenitic stainless steels.

The main disadvantage of electrolytic polishing is the partial or total destruction of non-metallic inclusions by chemical reactions with the electrolyte. Another disadvantage is the possibility of obtaining wavy surfaces.

4.7.1 Fundamentals and Devices

Electrolytic polishing is relatively simple and requires very little experience from the laboratory technician.

The specific details of electrolytic polishing are not completely understood at present. However, the general mechanism is associated with anodic dissolution. The protruding ridges of the rough-cut specimen are removed by selective dissolution, while the valleys between the protrusions are protected from the dissolution by the reaction products formed.

In the electrolytic cell, the sample serves as an anode, and as a cathode another appropriate metal; a direct current is passed through the electrolyte, which is transported from anode to cathode by the metal ions of the polished specimen. The most important factor for success of electrolytic polishing is the relationship between current density and voltage, the electrolyte used, and the general arrangement of the electrolytic cell.

The choice of electrolyte depends on the composition and structural characteristics (number and type of phases present) of the specimen. Although electrolytic polishing does not produce distortion of the surface metal, sometimes a wavy final surface may be observed.

5 Characteristics of the Mechanical Preparation Process The metallographic preparation of soft metals, such as aluminum, copper and lead, among other materials; that of hard materials, such as cemented tungsten and boron carbides, and the preservation of inclusions in steel or graphite in the iron castings, require more or less specific techniques. Relatively soft metals and their alloys flow very easily during grinding and polishing, and, if great care is not taken, deformed layers of considerable thickness originate, which are impossible to remove with the usual method. Here some general recommendations.

5.1 Aluminum

The preparation of aluminum alloy specimens is difficult because the surface metal flows easily and distortion occurs during grinding and polishing. Once a flat surface has been obtained and the edges of the specimen have been rounded, it is ground with the usual three grades of abrasive paper rotating the specimen 90° each time it is passed from one paper to the next.

During the first stage of polishing, it is necessary to keep the cloth always moist using distilled water. Equally important is the pressure with which the specimen is worked against the cloth. The optimum pressure is determined by previous tests and depends on the chemical composition and heat treatment of the specimen.

When all the stripes have practically disappeared, the operation is continued adding water, in such a way that in the end the cloth is completely free of abrasive. In this phase it is advisable to turn the specimen in the opposite direction to the rotation of the disk to eliminate marks and comet tails caused by polishing in only one direction.


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5.2 Copper and its Alloys

The surface of copper specimens and their alloys is initially ground by filing. Rough grinding with abrasive papers is not necessary in these materials, going directly to fine grinding. The polishing stage is carried out in two or three passes. The first pass of polishing is carried out on canvas, using as 500 mesh carborundum or alundum abrasive. The second operation is carried out on a disc covered with wool cloth and using pulverized tripoli abrasive. The third phase is done on a disk covered with a fine wool cloth, using as an abrasive a suspension in water of alumina or magnesium oxide. Some alternative polishing strategies suggest diamond in mineral oil suspension.

After polishing, these alloys are directly rinsed with alcohol and dried quickly before etching. Pure copper is difficult to polish, as it is extremely soft and ductile. However, it is necessary to obtain an almost perfect surface to check for the presence of cuprous oxide when observing it without etching.

5.3 Lead and its Alloys The preparation of these materials is very difficult due to its inherent softness. There is a lot of plastic flow and superficial distortion of the metal, with which the real structure is completely masked. It is generally accepted that it is almost impossible to obtain a surface free of cold deformation by polishing.

Electrochemical methods are regularly used, among which a variety of options can be chosen depending on the alloy, shape and size of the specimen, and also the equipment available. In any case, you always need to remove a lot of surface metal to eliminate the deformation effects of the saw, shear or element used to obtain the specimen.

5.4 Cast Irons

It is very difficult to preserve the graphite particles in gray and malleable castings if the specimens are prepared by usual methods, and special techniques must be used. The best way to prepare them is to grind them into the usual three grades of abrasive paper (220, 400, 800), prolonging the time spent on the 400 grit paper. The final grinding is usually done on an 800 grit paper, which is previously smoothed with talc or graphite.

Graphite extraction may initiate during grinding, but it is most frequent occurs during polishing. For this reason it is convenient to carry out the final polishing on a damp hairless cloth; but not excessively wet, and it is necessary to polish only in one direction.

5.5 Inclusions Conservation

The manual preparation is done on the three usual grits (400, 600, and 800) ending on a very worn or smoothed sheet of 800 grit paper. The last grinding is continued until the thin scratches are almost invisible to the naked eye.

The polishing is done on two discs. The first is covered with a hairless cloth and the second with a cloth with more hair. The abrasive can be aluminum oxide or colloidal alumina.

The fine stripes produced in the first polishing are eliminated by a second and careful polishing on the second disc. In this second polishing it is essential that the pressure with which the specimen is processed is light. Excessive pressure will cause inclusions extraction. Finely colloidal alumina is a good abrasive for this operation.

5.6 Formation of the Distorted Surface Material Layer

The production of this distortion is a natural consequence of grinding and polishing, and is unavoidable, to a greater or lesser degree, no matter how much care is put into operation. The intimate contact between the surfaces of the specimen and the particles of the abrasive used in the grinding and polishing operations generates mechanical stresses, as well as the induced thermal effects. These mechanical forces are large enough to make the superficial material flow plastically.


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The majority of the studies carried out by means of electron or X ray diffraction on metallographic surfaces show that this plastic flow completely alters the crystalline structure of the surface material. This completely disturbed surface, with a depth of the order of some interatomic distances, has a higher free energy than the only plastically deformed metal that remains under it. For this reason, it is easily and effectively eliminated by chemical dissolution during the etching process. Therefore, the appearance of the metallographic structure, observed after etching, is not influenced by the presence of this totally disorganized surface.

The amount of distorted metal produced by grinding and polishing depends on several circumstances, among which the most important are the chemical and structural composition of the specimen, the methods used for grinding and polishing, the care taken during preparation, and the nature of the abrasives used in polishing.

Normally, a single etching is not able to eliminate all the distorted metal; therefore, it is necessary to etch and polish alternately several times. For most metals and alloys etching time, must not exceed that necessary in the final etching; and polishing should be done with care, using very light pressures to avoid the formation of new distorted metal.

6 Mounting of Small Samples When samples are small or in a shape that does not allow easy handling for the grinding and polishing operations, as it happens, for example with shavings, wires, rods and small tubes, sheets and thin sections, among others, it is necessary to assemble them in a suitable material to make the preparation possible.

6.1 Fusible Mountings

There are many fusible materials that are suitable for specimen mounting, such as sulfur, sealing wax, dental plastics and low melting point alloys. The melting points of these materials differ greatly and must be selected so that the required heating does not alter the material structure of the specimen.

6.2 Mounting Polymers

The mounting of small specimens in polymeric materials such as Bakelite, Lucite, and others is one of the most satisfactory methods used today to facilitate the handling of small specimens. The manipulation is simple; but since heat and pressure must be applied simultaneously, a special mounting press is required.

6.2.1 Thermoset Resins

Thermosetting resins such as Bakelite, aniline and formaldehyde compounds are the most popular among those used to mount metallographic specimens. Bakelite molding powders are commercially available in a variety of colors, and this circumstance is of interest, because it simplifies the identification and filing of diverse specimens.

Thermosetting resins, unlike thermoplastic resins, harden during molding at the proper temperature and pressure because they undergo an irreversible curing reaction. The hardened state is no longer altered by temperature, even if it is close to that which can cause the resin to carbonize. For most Bakelite molding powders, the maximum temperature required for curing ranges from 135° C to 150° C, in conjunction with a pressure of 2,500 to 3,500 psi.

6.2.2 Thermoplastic Resins

Resins of this type, such as polystyrene, methyl methacrylate (Lucite) and cellulose-based materials, have the property of being transparent like glass if they are molded correctly.


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Thermoplastic resins do not harden during molding, but instead soften and flow each time a suitable combination of temperature and pressure is applied. The mounting is performed satisfactorily at a pressure of between 2,500 to 3,500 psi, and at temperatures ranging from 140° C to 165° C.

7 Etching of the Samples for Microscopic Examination The goal of most metallographic investigations is to determine the true microstructural characteristics of the specimen. For this it is necessary that the different structural constituents are defined with precision and clarity. This is achieved by subjecting the specimen to the chemical action of an appropriate reagent under carefully controlled conditions.

We will now deal with the chemical etching necessary to make the desired microstructural characteristics visible. Also, we will discussing the principles on which etching is based, and the techniques by which it is carried out. A non-etched specimen reveals few or no structural details, despite which it is of great interest because it allows to observe details that are already visible such as surface defects and non-metallic inclusions.

In alloys composed of several phases, the constituents are made visible by the differential etching or the staining of one or several of said constituents, as a consequence, mainly of the differences in chemical composition, which bring about differences in dissolution rates.

8 Reagents for Chemical Etching In general, the reagents suitable for metallographic etching are composed of dissolved organic acids in water, alcohol, glycerin, glycol or mixtures of several solvents. The activity and general behavior of the different reagents is related to the concentration of hydrogen ions or hydroxyl ions, and their ability to react preferentially with one or more of the structural constituents.

Typical reagents used to examine metallic microstructures under a microscope

Reagent Composition Uses

Nital Nitric acid 5.0 ml Reveal general microstructure in

iron and steel Ethyl alcohol 95.0 ml

Picral Picric acid 4.0 g Reveal general microstructure in low

alloy steels Ethyl alcohol 100.0 ml

Ferric Chloride

Ferric Chloride 5.0 g Reveal general microstructure in austenitic stainless steels

Hydrochloric acid 50.0 ml

Water 100.0 ml

Ammonium Persulfate

Ammonium persulfate 10.0 g Reveal general microstructure in copper, nickel, silver and some aluminum alloys Water 90.0 ml

Hydrofluoric Acid Hydrofluoric acid 0.5 ml Reveal general microstructure in

aluminum and most of its alloys Water 99.5 ml

For the etching of the metal or alloy be perfect, and clearly show the desired structural details, it is necessary that the composition of the reagent used is appropriate to the composition of the specimen, and the different phases that constitute it. For example, a reagent composed of ammonium hydroxide


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and hydrogen peroxide is ideal for copper and alpha brass, but is completely unsuitable for etching iron and steel. On the contrary, a diluted solution of nitric acid or sodium picrate in alcohol is recommended for steels and irons, but does not work on copper.

9 Etching Mechanisms When a reagent is applied to the polished surface of a specimen, the structural details are manifested, in part, by selective destruction of the surface. This is because the different phases, in a poly-phase alloy, or the crystalline planes with different orientation of each grain have different inherent speeds of dissolution. Additionally, many reagents stain or color different phases in different ways.

Poly-phase Alloys: The etching mechanism in poly-phase alloys is essentially of electrochemical nature, consequence of the potential differences that occur in the different constituents when the sample is put in contact with the reagent.

The potential difference between the anodic and cathodic constituents is, in general, large enough for the more electropositive phase to dissolve relatively quickly in ordinary reagents. It is then necessary to control the process to avoid over etching. Because of this potential difference, poly-phase alloys are etched more rapidly than single phase alloys or pure metals.

As a consequence of the preferential dissolution of the anodic phase during the etching, such constituents in the poly-phase alloys become rougher and deeper relative to the plane of the polished surface, at least at the anode-cathode interface, therefore appear, microscopically, as darker. Stopping the process at the precise moment achieves an optimal contrast between the different phases.

Single phase Alloys and Pure Metals: As a consequence of the existence of a single phase it is more difficult to explain the phenomenon of differential etching in terms of potential differences. In a pure metal or single phase alloy the reagent attack is the result of the different ease of dissolution presented by each grain as a consequence of its different crystallographic orientation in relation to the polished metallographic surface. For this reason, the etch produces in each one of the grains some well-defined facets with are different to those in the neighboring grains.

When an illuminating beam hits the specimen, the direction of light reflected by each grain depends on the system of facets that the attack has developed on it. The microscopic observation, and in some cases to the naked eye (if the grains are sufficiently large), makes the grains appear to be brighter whose facets are properly oriented and darker to the less advantageous orientation. The adjacent schematic illustration outlines these effects.

10 Etching Methods

10.1 Conventional Etching

The most important preliminary consideration is the successful choice of the reagent to be used. Making a selection requires judging and knowing the behavior of the different reagents when they are

Schematic of Contrast Formation in the Microscope

The light beam of the microscope is reflected in different directions, therefore different levels of illumination are generated, which we detect with contrast.

Grains with different orientation that were etched

differently.

Grain boundaries are etched with a at a faster rate than the matrix

that surrounds it.


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used under the recommended conditions. Each reagent must be used for the specific use for which it is intended, and exactly according to the instructions for its use.

The attack reagents are generally applied by immersing the specimen in the reagent or by contacting the surface with a reagent impregnated cotton swap. The surface of the specimen must be clean to ensure that the reagent will wet it evenly. To do this, it is first washed thoroughly with running water, shaken, rinsed with ethyl alcohol, and finally dried in a hot air jet. It is important to note that the surface to be observed must never be touched with any object, especially finger tips.

When the etching is done by immersion, the sample should be suspended in the reagent, with its polished surface downwards. Holding the sample con be done by hand or with tweezers. To dislodge air bubbles retained on the surface and ensure that there is always fresh reagent under it, the specimen is shaken moderately, taking care that the polished surface does not come into contact with the container.

Immediately after the immersion, it can be observed that the polished surface becomes more or less matt. This indicates that the etching is progressing and, with some practice, the observation of the appearance of the surface indicates when the reaction should be suspended.

When the time elapsed for the etching is considered sufficient, the specimen is removed from the reagent, and it is quickly introduced into a stream of water. In this way, the action of the reagent is stopped immediately and all traces of it are eliminated on the surface of the specimen. It is then rinsed with ethyl alcohol to displace the water and dried in a hot air jet. Thus, the sample is ready for its microscopic examination. It may be necessary to attack again if the images are not clear or the etching was so extensive that contrast was lost.

Etching time is a key factor. The visual appearance of an etched structure and the quality of a photomicrograph depend to a large extent on the precision with which the most delicate details of the microstructure have been revealed. Thus, the selection of the reagent to be used is as important as the allowed reaction time. Depending on the metal attacked and the reagent used, the etching times can vary from a few seconds to 30 or more minutes.

Many reagents have a composition that ensures low activity, therefore long working times; these formulations allow for a close control of process times.

Over-attack will hide many fine details and broaden grain boundaries rendering an untrue image. When the specimen is over-etched it is always necessary to polish it again, at least on the final disc and then repeat the process for a shorter time.

10.2 Electrolytic Etching

The electrolytic etching is particularly useful to highlight the structure in difficult to work materials, such as nickel and chromium alloys, heavily cold worked metals, corrosion and heat resistant alloys, and alloys that exhibit surface passivity during usual etching.

Electrolytic etching consists of passing a DC current, which may vary from a fraction of ampere to several amperes, through an electrolytic cell. The cell will consist of the sample, which will be the anode, and of some insoluble material, such as platinum or graphite, which will be the cathode. In addition, an appropriate electrolyte will be required for the material to be etched, and for the constituents of the microstructure to be revealed.

10.3 Polished Relief

Polishing in relief is used successfully when the alloy contains constituents with very different relative harnesses. By means of suitable techniques of hand polishing, and by using an abrasive suspended in some dilute acid solution, it is possible to cause a more rapid abrasion of the softer constituent, leaving the


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hardest phase in relief at the end of the operation. This difference in abrasion speeds may be sufficient to generate the contrast differences required for microscopic observation.

11 Microscope Observation of Material Microstructures

11.1 The Metallurgical Microscope

Optical metallurgical microscopes are reflection type (light is shined through the objective onto the sample and then reflected back into the microscope tube) and are categorized according to their configuration. An inverted microscope (left image) observes a specimen from beneath and is used for observing mineralogy and metallurgy specimens. An upright microscope (right image) observes a specimen from above and is widely known as the most common type with a multitude of uses, ranging from metallurgy to biology (when equip for transmitted light as well as reflected light).

Inverted Metallurgical Microscope Upright Metallurgical Microscope

11.2 Microscope Parts and Functions

1. Eyepiece: The eyepiece (sometimes called the 'ocular') is the lens of the microscope closest to the eye that you look through. It is half of the magnification equation (eyepiece power multiplied by objective power equals magnification), and magnifies the image made by the objective lens, called the virtual image. Eyepieces come in many different powers. One can identify which power any given eyepiece is by the inscription on the eyecup of the lens, such as "5x", "10x", or "15X". Oculars are also designed with different angles of view; the most common is the wide field (W.F.).

1

2

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5

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8

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2. Eyepiece Holder: This simply connects the eyepiece to the microscope body, usually with a setscrew to allow the user to easily change the eyepiece to vary magnifying power. Most microscopes allow a certain angular adjustment of the distance between eyepieces so as to accommodate different users.

3. Body: The main structural support of the microscope which connects the lens apparatus to the base.

4. Nose Piece: This connects the objective lens to the microscope body. With a turret, or rotating nose piece as many as five objectives can be attached to create different powers of magnification when rotated into position and used with the existing eyepiece.

5. Objective: The lens closest to the object being viewed which creates a magnified image in an area called the "primary image plane". This is the other half of the microscope magnification equation (eyepiece power times objective power equals magnification). Objective lenses have many designs and qualities which differ with each manufacturer. Usually inscribed on the barrel of the objective lens is the magnification power and the numerical aperture (a measure of the limit of resolution of the lens).

6. Focusing Mechanism: Adjustment knobs to allow coarse or fine (hundredths of a millimeter) variations in the focusing of the stage or objective lens of the microscope.

7. Stage: The platform on which the prepared slide or object to be viewed is placed. A slide is usually held in place by spring-loaded metal stage clips. More sophisticated high-powered microscopes have mechanical stages which allow the viewer to smoothly move the stage along the X (horizontal path) and Y (vertical path) axis. A mechanical stage is a must for high-power observing.

8. Illumination Source: The means employed to light the object to be viewed. The simplest is the illuminating mirror which reflects an ambient light source to the object. Many microscopes have an electrical light source for easier and more consistent lighting. Generally electrical light sources are either tungsten or fluorescent, the fluorescent being preferred because it operates at a cooler temperature. Most microscopes illuminate from underneath, through the object, to the objective lens (transmitted light). On the other hand, metallurgical microscopes use both top and bottom illumination.

9. Base: The bottom or stand upon which the entire microscope rests or is connected. 10. Camera: Photography unit with CMOS or CCD sensor able to make pictures via the microscope

tube. 11. Filter Set: Typical microscopes us polarized filters (or other light modifying filters) to alter the

nature of the light. This function aids in the examination of certain specific microstructural features.

12. Light Pad Levers: The light pad levers are use to divert light from the eye piece to the camera and vice versa.

12 Preparing a Sample for use in an Upright Microscope Ideally, the surface to be examined optically should be flat and level. If it is not, the image will pass in and out of focus as the viewing area is moved across the surface. In addition, it will make it difficult to have the whole field of view in focus - while the center is focused, the sides will be out of focus. By using a specimen levelling press (shown below) this problem can be avoided, as it presses the mounted specimen into plasticine on a microscope slide, making it level. A small piece of paper or cloth covers the surface of the specimen to avoid scratching.


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13 Operation of an Optical Metallurgical Microscope 0. The adjustment of an optical microscope is "personal" because the instrument needs to be

coupled to the characteristics of the user's vision. 1. Remove the protective cover from the instrument. 2. Mount a sample with good contrast level and start adjusting the instrument.

a. Verify that the stage is more or less centered. b. Place a well-prepared, clean sample in the center of the platen. c. Rotate the nose piece until you find the 10x marked objective. d. Turn on the light source at 50% intensity; move the lever of the light path to obtain

maximum intensity of light in the binocular. e. Adjust the intraocular distance of the binocular until you are able to see a single circle of

light. f. Release the coarse focus brake (if provided) and roughly focus the sample taking care not

to hit the objective lens. 3. Adjustment of the eyepieces.

a. With your left eye closed (looking with the right eye) operate the focus controls (coarse and fine) until you find the optimal setting.

b. Close the right eye (looking with the left eye) and rotate the focus ring of the left eyepiece until the optimal image is obtained.

4. Lighting adjustment. a. Open the two diaphragms of the light source completely. b. Close the field diaphragm until you can see its circumference coinciding with that of the

observation area (the field diaphragm is the one that can be seen focused in the visual field).

c. Remove the right eyepiece carefully (do not alter the setting you already have!). d. Take a look down the binocular tube and fix your eyes on the illuminated plane that you

see in the background. e. Close the opening diaphragm until you see that the illuminated area is of the order of 50

to 70% of the total area (of the circle seen at the bottom of the tube). f. Reinsert the eyepiece in its original position.

Important notes: I. Some microscopes have a scale on the opening diaphragm ring that matches the

powers of the objectives; if you wish, you can use this scale instead of steps 4.c) to 4.f).

II. This procedure must be repeated each time the objective is changed. Therefore, it is worthwhile to plan your session so that you do not have to readjust the microscope constantly.

5. Adjustment of the intensity of the light source. a. If available, insert the filters of the lighting train required for the job:

Observation Filter

Maximum resolution green

Photography Filter

"Black and white" film green

"Daylight" film dark blue

"Artificial light" film pale blue


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b. Reduce or increase the voltage of the lamp by means of the potentiometer until reaching a "comfortable" light intensity (you should never modify the intensity of the light with the help of the opening diaphragm).

6. End of the session. a. Turn off the microscope. b. Remove your samples and raise the stage a couple of turns (from the thick focus). c. Return the turret to the 10x objective. d. Remove all filters used. e. Record the operation in the diary. f. Put the protective cover over the instrument.

7. Other controls: a. DO NOT OPERATE UNLESS YOU HAVE BEEN TRAINED FOR IT!! b. The column magnification ring and other camera related controls and adjustments need

to be arranged for the specific camera used. Ask the lab staff for help. c. Especially the controls of the alignment of the light source should NOT even be touched. d. The polarized filters completely change the interaction of light with the sample and you

will not see anything interpretable; in fact, they are auxiliary for other filters that require polarized light.

e. The filter for Nomarski interferometry (if provided), in addition to being very delicate, produces color effects that are difficult to interpret because they generates an "apparent relief" that is used for the identification of hard precipitates.

f. The filter of red reflections of first order (if provided) is only used in very specific situations where the contrast of light is the information you are looking for, as for example in determining the degree of recrystallization of a sample.

8. Taking Photographs. a. First obtain a high quality image in the microscope binocular using the filter corresponding

to the observation. b. Start the computer and the digital camera (ask the lab staff for help). c. Change the light path towards the camera (light level in the eye piece will dim partially or

totally, depending on the microscope). d. Adjust the amount of light, only by operating the potentiometer, until an acceptable image

(contrast and brightness) is obtained on the computer monitor. e. You may have to fine-tune the focus of the image to correct for the camera focusing

distance. f. Take the photo by pressing "the button". Stand back from the microscope table and avoid

all types of vibration. g. Make sure you save the image taken and that you label the file so that you recognize the

image later. h. Make a note in your lab book the record the sample, the microscope condition and the

file name. i. If you want to test for a better image, alter the light intensity and the diaphragm opening

slightly. This will modify the contrast/brightness of the image. j. When finished, make sure you copy your image files on to your memory device, then ask

the staff if you are t leave the computer on or shut it down. k. Before leaving, turn off the microscope according to item 6.

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Micrographic Preparation and Observation · Micrographic Preparation and Observation 1 Content ... x. Such microscopic studies, in the hands of an experienced observer, provide abundant