Microstructural Characterisation of Materials

Transcription

1 Department of Materials and Metallurgical Engineering Bangladesh University of Engineering and Technology, Dhaka MME298 Structure and Properties of Biomaterials Sessional 1.50 Credits 3.00 Hours/Week July 2017 Term Laboratory 2 Microstructural Characterisation of Materials 1. Objective After completion of this experiment, students should be able to 1. 1 understand the techniques of micro-specimen selection, polishing, and etching, 1.2 have hands on practice in the techniques of micro-specimen selection, polishing, and etching, 1.3 have initial training in the use of the metallurgical microscope, and 1.4 identify, draw and label the microstructures of plain carbon steels. 2. Materials and Equipment 2.1 Metallographic sample 2.2 Grinding and polishing machines 2.3 Grinding papers, polishing papers and polishing powder 2.4 Etching reagents 2.5 Metallurgical microscope 3. Experimental Procedure 3.1 Preparation of metallographic sample (a) Take one sample from your instructor and polish the specimen manually by grinding on a series of emery papers of progressively finer grade. To polish at each paper, hold the paper on top of a glass sheet with one hand while rubbing the specimen with the other hand using moderate pressure, back and forth across the abrasive surface in one direction only. This creates a series of parallel scratches or grind marks on the specimen. (b) It is naturally very important to avoid the transfer of loose abrasive particles from one paper to another. Therefore, before proceeding to the next finer paper clean thoroughly the loose abrasive particles from the specimen and from the hands. Clean each paper carefully before use to protect the quality of the polishing job. (c) During grinding on the next finer paper, hold the specimen in such a way that the new, finer set of scratches will be approximately perpendicular to the existing set of scratches. MME298/July 17 Term/Expt. 02: Microstructural Characterisation of Materials Page 1 of 8

2 (d) After completing the paper polishing, clean the sample thoroughly with soap water and show it to the instructor. He will check its finish and indicate whether you may proceed to the final polishing step. (e) (f) Place some alumina powder on top of the wet polishing cloth of the grinding wheel. Hold the specimen face down on the wheel under a moderate pressure and slowly move in a direction opposite to the direction of the wheel. Continue final polishing until a mirror-finish is attained. Hold the specimen quite stationary during most of the polishing operation. Before leaving the wheel at the end of the final fine polishing, rotate the specimen counter to the direction of wheel rotation to eliminate streaks caused by draught of inclusions. After a mirror-finish is attained, wash the specimen and the hands and dry the specimen surface. (g) Never touch the polished surfaces with fingers at any time because skin oil and salt will deposit a film or cause tarnish, either of which will hide the structure to be observed. 3.2 Etching of the specimen (a) Freshly prepare 20 ml 2% nital solution in a beaker by mixing about 2% HNO 2 in 98% water. (b) Take about 5 ml of the above etching reagent in a watch glass. Hold the specimen using a tong and etch the specimen by dipping it on to the solution for about 5 seconds. (c) Avoid over-etching. Under-etching is preferable to over-etching. Re-polish the over-etched sample till the cloudy film has been removed and the surface is again showing a mirror finish. Reetch carefully. Ask the instructor to check your etched specimen and advise you whether to proceed to examine it on the microscope or whether you need to polish and etch again. 3.3 Observation of Specimen using Metallurgical Microscope (a) Examine the microscope. Locate each of its components. Observe the magnifying power of the microscope by noting the magnifying power of each objective lens and each eye-piece. (b) Obtain three specimens of plain carbon steels provided by the instructor. (c) Study the structure by focusing it by 'going away' from the lens. Using the mechanical stage movements, explore the etched surface, adjusting the focus, if necessary, as you go. Practice using both the stage control knobs simultaneously to move the specimen in various directions. Locate a good clear area to sketch. If your specimen does not show a good, clear structure, ask the instructor to advise you whether you should re-polish and re-etch. (d) Examine the specimens using a magnification of x200. Reproduce the microstructures in a piece of paper and label all microconstituents carefully and neatly. (e) Estimate the approximate amount of the microconstituents present in each sample and then calculate its carbon content. 3.4 Complete the data sheet, Table Discussion 4.1 Answer the following questions: (a) What are the precautions one should take while cutting a metallographic sample from an object? (b) What is the purpose of etching? What happens to the metallographic sample during etching? (c) What is the purpose of mounting the specimen? (d) Discuss why observation of sample under microscope using high magnification is not always helpful. MME298/July 17 Term/Expt. 02: Microstructural Characterisation of Materials Page 2 of 8

3 Table 1.1: Data Sheet Microstructure of showing grains of and. Microstructure of showing grains of and. Microstructure of showing grains of and. MME298/July 17 Term/Expt. 02: Microstructural Characterisation of Materials Page 3 of 8

4 5. Theoretical Background 5.1 Introduction The properties of a material are a direct consequence of the microstructural features of that material. These structural patterns vary both with the materials themselves, and with their manufacturing processes. The identification of the microstructural features of a material is therefore of paramount importance. The general microstructural features of most of the common materials can be evaluated with relatively simple and inexpensive apparatus, known as the metallurgical microscope. To reveal other special features with intricate details, require more sophisticated and very expensive microscopes, such as scanning electron microscope, transmission electron microscope, etc. A representative area from the material to be investigated is cut and specially prepared to reveal its structural features and to be observed under these microscopes. The science of preparation of samples for investigation under microscope is commonly known as metallography, although a term ceramography is often used for preparation of ceramic materials only. The ideal specimen for metallographic examination is a representative block of handy size, having a plane mirror-like surface with good edge retention, on which the structure of the material can be easily seen and evaluated, and which is practically free from any changes due to the preparation. These changes include deformation, loss of inclusions, scratches, reaction products or smearing of the individual phases. The preparation of specimens for observation under a microscope, often regarded as a tedious and frustrating process, is of great importance, since the true microstructure may be partially or fully obscured by poor technique or execution. 5.2 Sample Preparation Sample preparation consists of many rather simple-appearing operations. However, these operations require great care, precise techniques, extreme cleanliness, and much practice before a professional level of results can be obtained. While the microscope is one of the most sophisticated equipment items used in materials technology, its value is entirely dependent on the quality of the job done in preparing the specimen for observation Preparation of samples for microstructural examination involves a process consisting of many steps. These steps include: 1. Selection of sample or specimen 2. Sectioning or cutting 3. Polishing the specimen to make it flat and mirror-smooth 4. Etching to create relief on the surface to be observed Selection of sample Only a small piece of material can be placed on the platform of a metallurgical microscope and only a plane or flat section of it can be observed under it. The intent of metallographic examination usually dictates the location of the specimens to be studied, its orientation, etc. Sometimes more than one specimen may be necessary to adequately represent the material. For example, for the examination of the structure of a casting, specimens should be taken from the zones wherein maximum segregation might be expected to occur as well from the zones where segregation should be at a minimum. To examine the structure of fibrous materials or other similar materials having anisotropic properties, both longitudinal and transverse sections of material are necessary. For the identification of the causes of premature failures, the test specimens should be taken as closely as possible to the fracture or to the initiation of the failure. Sampling for research studies is usually more extensive than for routine examinations. MME298/July 17 Term/Expt. 02: Microstructural Characterisation of Materials Page 4 of 8

5 5.2.2 Sectioning or cutting The object of sectioning is to extract specimen of suitable size from the parent material. Sectioning is not a prerequisite of subsequent preparation, but if done wrongly, the original structure of the material can be changed. This is usually associated with heat and can be readily demonstrated on a hardened steel component where the heat generated during sectioning can temper the hardened structure. Therefore, in cutting metallographic specimens from the main body of the material, care must be exercised to ensure proper heat dissipation so that the structure of the metal is not changed. Hacksaw or abrasive cut-off wheel are ideally suitable for removing a sample from an object. Flame cutting completely alters the structure of the metal at the flame cut edge and, therefore, should not be used for sectioning Polishing of Specimen Abrasive particles are used in successively finer steps to remove material from the surface of the specimen to produce in it a surface that is perfectly flat and scratch-free when viewed under a microscope. The first step towards obtaining a perfectly flat and scratch-free surface is often required to rough grind the face of the specimen on a grinding wheel, a coarse file or a belt sander. The sample is then ground manually by grinding on a series of emery papers of progressively finer grade. Emery is a natural abrasive containing 55-75% Al 2O 3 (corundum) the balance is iron oxide (magnetite) and has Mohs hardness of 8.0. The emery papers are made by attaching hard abrasive particles onto papers by suitable glue. These must be of the very best quality, particularly in respect of uniformity of the particle size. Various nomenclatures are used to indicate the fineness of the hard particles glued on to the emery papers. In some cases, the paper containing the coarsest paper is designated as No. 3 and the subsequent papers are designated as No. 2, 1, 1/0, 2/0, 3/0 and 4/0. Before polishing, the specimen may often require mounting for the following reasons: 1. the specimen is too small or of awkward shape for ease of handling in subsequent stages of preparation. 2. to support the outermost edge of the specimen s surface to prevent damage or rounding during the subsequent grinding/polishing operations. 3. the specimen is of delicate or of friable nature. Specimens may be either mechanically mounted by binding or clamping, mounted in plastics by using a thermosetting material such as Bakelite, or phenolic, or a combination of the two to obtain optimum results. To polish at each station, the paper should be held taut with one hand while the specimen is rubbed, with moderate pressure, back and forth across the abrasive surface in one direction only. This creates a series of parallel scratches or grind marks on the specimen. Of course, the specimen itself must remain in the same position and must not be turned while working on one paper. When all the marks on the surface being polished are running in one direction and all others have been removed, the operator should clean the specimen and his hands and then proceed to the next finer paper. During grinding on the next finer paper, the operator should hold the specimen in such a way that the new, finer set of scratches will be approximately perpendicular to the existing set of scratches (Fig. 1). Rubbing is continued again in a single direction until the previous set of coarser scratches is gone and a new fresh set of scratches along the new direction is produced. The process is continued until grinding on the finest paper (the 4/0 grade) is completed. During grinding, the specimen should not be pressed too hard against the abrasive paper, because the heat generated due to friction may change the original structure of the specimen. Moreover, the specimen should be held flat against the abrasive paper throughout the polishing action. Although considerable time and energy may have been spent during grinding and polishing the specimen, the failure to observe simple precautions will result in complete obscurity when the specimen is placed on the microscope stage. This is due to the crisscross lines that will show up under the microscope, signifying the MME298/July 17 Term/Expt. 02: Microstructural Characterisation of Materials Page 5 of 8

6 presence of un-removed scratches (Fig. 2.). Should this be the case, it will be necessary to go back to the coarser grades of emery paper, and start all over again. FIGURE 1: Appearance of specimen surface at successive stages of grinding. FIGURE 2: Criss-cross lines due to improper grinding. After fine grinding the sample using emery papers, final polishing is done. Polishing makes the surface of the specimen smooth than grinding. As opposed to grinding, several techniques of polishing are used, which can be divided into chemical, mechanical or electrolytic methods. The mechanical polishing process is the most commonly used methods of polishing where the sample is pressed on to a rotating wheel, covered with a cloth such as billiard felt, and a slurry of a finely divided polishing powder. Many abrasives have been used in metallography. Silicon carbide (SiC), iron oxide (Fe 2O 3), chromium oxide (Cr 2O 3), cerium oxide (CeO), silica (SiO 2), alumina (Al 2O 3), magnesia (MgO) and diamond powders are most common abrasives in polishing Etching Ordinarily a polished metal surface reflects light so equally that the details of the structure cannot be distinguished. Thus, it is necessary to develop contrast in the structure. The most common method to give contrast is etching, a chemical method where the surface of the sample is corroded by dipping it to a selected corrosive media for a pre-determined time. Control of the etch time is important in obtaining a sharp, crisp image that permits resolution of fine details. Most etched specimens are prepared by dissolution of the specimen in selected solutions. This is known as chemical etching and depends on electrochemical processes, namely oxidation-reduction processes. The MME298/July 17 Term/Expt. 02: Microstructural Characterisation of Materials Page 6 of 8

7 increase in contrast depends on differences in electrochemical potential. For pure metals and single-phase alloys, a potential difference exists between grains of different orientations, between grain boundaries and grain interiors, between impurity phases and the matrix, or at concentration gradients in single phase alloys. For multiphase alloys, a potential difference also exists between the various phases present. These potential differences alter the rate of attack, thus revealing the microstructure when chemical etchants are used. Common etching reagents used are 2% nital (2 ml nitric acid solution in 98 ml methyl alcohol) for steels, 4% picral (4 g picric acid solution in 96 ml methyl alcohol) for cast irons, ammonia-hydrogen peroxide (50 ml ammonia, 20 ml hydrogen peroxide and 50 ml water) for copper-base alloys, and dilute hydrofluoric acid (0.5 ml hydrofluoric acid, 99.5 ml water) for aluminium-base alloys. It must be remembered, however, that there are many features that are best examined on unetched specimens. Indeed, it is a good practice always to look at a specimen in the unetched condition in the first place, particularly when the specimen has been taken from a component which has failed in service or when it is being used to sample the quality of the material. Non-metallic inclusions, cracks and cavities are obvious cases of features that should be viewed before etching. 5.3 Metallurgical Microscope The metallurgical microscope, also known as optical microscope, is a major tool used for the identification of common microstructural features of most of the common metals and alloys and non-metallic materials. A metallurgical microscope is similar in optical principles to any other microscope, but it differs from some of them in the method by which the specimen is illuminated. For example, most of the biological specimens can be prepared as thin, transparent slices mounted between sheets of thin glass and illumination can be arranged simply by having a source of light behind the specimen. Metals, on the other hand, are opaque substances and visible radiation cannot penetrate even a very thin metallic object. Therefore, the study of structures of metals and alloys with a metallurgical microscope is carried out by using reflected light. In a metallurgical microscope (Fig. 3), the image produced by the objective lens system is further magnified by the eye-piece. The total magnification is, therefore, the product of the magnification of the objective lens and the magnification of the eye-piece. In a metallurgical microscope a magnification of 1000 (usually written as 1000X) is the practical upper limit. Most work, however, is done at magnification of a few hundred. To examine a specimen under the microscope, the specimen must first be mounted so that its surface becomes normal to the axis of the instrument. In any study, one should always begin the examination by visual observation followed by application of progressively higher magnification. The specimen is brought into focus by using first the coarse adjustment and then the fine adjustment knobs. Accidental jamming against the specimen surface may damage the objective. It is a good practice to bring the specimen slightly closer to the objective than necessary for true focus, using the coarse focusing knob and observing this directly by eye. Then whilst viewing the image through the eye-piece, true focus is approached by using the fine focusing knob to remove the specimen away from the objective. This reduces the risk of damaging the objective lens by running it into the specimen. Slight adjustment can then be made to suit the individual eye. Finally, the iris in the illumination system should be closed to a point where illumination just begins to decrease. This will limit the glare due to internal reflections in the tube. It is a mistake to assume that high magnifications, in the region of 500 or 1000, are always most useful. In fact, they may give a completely meaningless impression of the structure, since the field of observation will be so small. Directional properties in wrought structures or dendritic formation in cast structures are best seen using low magnifications of 40X to 100X. Even at 40X a single crystal of say cast brass may completely fill the field of view and the dendritic pattern will be clearly apparent, whereas at 500X only a small area between two dendrite arms would fill the field of view and the nature of the dendritic structure will not be apparent. Thus, as a matter of routine, a low power objective should always be used first to gain a general impression of the structure before it is examined at high magnification. MME298/July 17 Term/Expt. 02: Microstructural Characterisation of Materials Page 7 of 8

8 Eye Eye-piece Coarse Draw Tube Body Tube Iris Lamp Fine Adjustment knobs Limb Glass Clip Objective Specimen Stage Foot FIGURE 3: Schematic Representation of a Metallurgical Microscope. (a) (b) (c) FIGURE 4: Photomicrographs of (a) hypoeutectoid steel (C < 0.76%) showing mostly ferrite (white) grains with small pearlite (black), (b) eutectoid steel (C = 0.76%) containing 100% pearlite grains (appeared as finger prints), and (c) hypereutectoid steel (C > 0.76%) containing mostly pearlite grains surrounded by thin cementite (white) network. MME298/July 17 Term/Expt. 02: Microstructural Characterisation of Materials Page 8 of 8