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Microstructure is the very smallscale structure of a material, defined as the structure of a prepared surface of material as revealed by an optical microscope above 25× magnification. Microstructures form through a variety of different processes. Microstructures are almost always generated when a material undergoes a phase transformation brought about by changing temperature or pressure, by deformation or processing of the material and by combining different materials to form a composite material. The term microstructure is used to describe the appearance of the material on the nmcm length scale. A reasonable working definition of microstructure is:The arreangement of phase and defects within a material
BẮC Hi every one Today our group wanna give you a presentation about observation microstructure Our group includes Nguyễn Hữu Bắc, Đặng Tiến Đạt, Phùng Thị Hương, Đào Duy Khánh, Đặng Bùi Nhật Lê, Vũ Thị Thùy, Phạm Bá Tuấn The content of the presentation consists of parts I will present to you the first part: microstructure The next part will be presented by Lê, Tuấn, Hương Then, Thùy anh Khánh will take responsibility for sample preparation And the last one is Đạt He will talk about the procedure for the metallographic preparation First of all, we would like to introduce the microstructure Microstructure is the very small-scale structure of a material, defined as the structure of a prepared surface of material as revealed by an optical microscope above 25× magnification Microstructures form through a variety of different processes Microstructures are almost always generated when a material undergoes a phase transformation brought about by changing temperature or pressure, by deformation or processing of the material and by combining different materials to form a composite material The term ' microstructure' is used to describe the appearance of the material on the nm-cm length scale A reasonable working definition of microstructure is:"The arreangement of phase and defects within a material" A ‘phase’ is taken to be any part of a material with a distinct crystal structure and/or chemical composition Different phases in a material are separated from one another by distinct boundaries A ‘defect’ is taken to mean any disruption to the perfect periodicity of the crystal structure This includes point defects such as vacancies and interstitials, planar defects such as surfaces, twin boundaries, grain boundaries, and dislocations And what is microstructural characterisation? Microstructural characterization involves qualitative and quantitative analysis of surface topography, porosity, crystal defects, and interfaces Crystal defects mostly form either as a result of imperfections during the crystal growth process or as a consequence of structural phase transitions Microstructural characterization is usually achieved by allowing some form of probe to interact with a carefully prepared specimen The most commonly used probes are visible light, X-ray radiation, a high-energy electron beam, or a sharp, flexible needle These four types of probe form the basis for optical microscopy, X-ray diffraction, electron microscopy, and scanning probe microscopy LÊ Now, we move on to the structure observation technique In this part, we will mention techniques They are light microscopy, hardness testing and quantitative microscopy First, I wanna talk about the light microscopy The light microscopy is an instrument for visualizing fine detail of an object It does this by creating a magnified image through the use of a series of glass lenses, which first focus a beam of light onto or through an object, and convex objective lenses to enlarge the image formed Light microscopy has numerous applications The most important application is the determination of the structural phases present and the constitution of the bulk of the metal Although numerous sophisticated electron metallographic tools are now available to an investigator, the light microscope remains the single most important device Now, let's go into detail about this instrument First, about the illumination, various light sources are available to the metallographer The most common light sources are Low-voltage tungsten filament lamp, Carbon arc, Xenon arc, Quartz-iodine lamp, Zirconium arc lamp In condenser system, the condenser is a lens designed to focus light from the illumination source onto the sample The condenser may also include other features, such as a diaphragm and filters, to manage the quality and intensity of the illumination Filters are often required to modify the light for optimum visual examination or photomicrograph Selective filters, either absorption or interference, alter the light to provide wavelengths for which the objective lenses are corrected Selective filters are often used to match the color temperature of the light source to that required by the film These filters can also be used to increase contrast between phases of different colors The next component is objective lens The objective lens forms the primary image of the specimen and, thus, is the most critical item of the microscope The objective collects as much light as possible coming from any point on the specimen and combines this light to form the image The working distance means the distance between the front surface of the objective lens and the sample, can be an important lens parameter As a rule, as the magnifying power of the objective increases, the working distance decreases The objectives are usually mounted on a rotating nosepiece turret that can hold from four to six objectives Next, the next one is eyepieces The major function of the eyepiece is to magnify the primary image produced by the objective so that the eye can use the full resolution of the objective A virtual image of the specimen is formed at the point of most distinctvision, approximately “two hundred fifty” mm from the eye About the stage, it must be sturdy so that vibrations are not encountered Stage movement should be smooth and precise The stage surface is usually fitted with an X and Y graduated scale for making measurements or locating features Special stages are available with a micrometer screw control for precise measurement In microscopy, the term 'resolution' is used to describe the ability of a microscope to distinguish detail Resolving power is the ability to produce separated images of multiple structures under the best conditions and is usually expressed as the number of uniformly spaced similar parallel black lines per unit length on a white background that can be separated in the image Besides, magnification also plays an important role in image visibility, since the degree of visual perception of the human eye varies from person to person The depth of field is the distance along the optical axis over which details of the object can be observed with adequate sharpness Factors that affect resolving power influence depth of field as well but in the opposite direction, i.e., increasing the resolving power decreases the depth of field TUẤN Now, we continue with the working principle The compound microscopes shown here magnifies an object in two stages Light from a mirror is reflected up through the specimen, or object to be viewed, into the powerful objective lens, which produces the first magnification The image produced by the objective lens is then magnified again by the eyepiece lens, which acts as a simple magnifying glass The magnified image can be seen by looking into the eyepiece lens The secound is formed by the ocular To obtaine good photomicrographs, three separate effects are used to create the visual impression of good focus over the field of view: The depth of field of the objective The adjustment of the fine-focus control while viewing The slight change in focus due to eye accomodation In obtaining sharp photomicrographs, the microscopist must control the following variables: • Eliminate vibrations • Align illumination • Match illumination color to objective corrections • Maintain cleanliness of optics • Correct adjustment of field and aperture diaphragms • Focus precisely, generally with the aid of a focusing telescope There are some examination modes light microscopy, such as bright-field illumination, dark-field illumination, Polarized light, Phase-contrast illumination, Interference techniques, Light-section microscopy, Fluorescence microscopy In this part, we will focus on the bright-field illumination Bright Field (B.F.) illumination is the most common illumination technique for metallographic analysis The light path for B.F illumination is from the source, through the objective, reflected off the surface and returning through the objective and back to the eyepiece or camera This type of illumination produces a bright background for flat surfaces with the non-flat features (pores, edges, etched grain boundaries) being darker as light is reflected back atan angle HƯƠNG Now, we turn to the other technique: hardness In its most general sense, hardness implies resistance to deformation As applied to metals, hardness is a measure of resistance to permanent deformation (i e plastic) Hard materials exhibit high strengths Hardness also has other connotations—resistance to scratching, resistance to cutting, ability to cut softer materials, brittleness, lack of elastic damping, wear resistance, lack of malleability, magnetic retention, and so forth These pictures are the system of Vicker and Rockwell’s hardness Next, we continue with quantitative microscopy To determine the quantitative morphometric (i.e., number, size, orientation) of biological structure (i.e., cells nuclei, collagen fibers) in an automated unbiased fashion The use of a light microscope for the quantitative analysis of specimens requires an understanding of: light sources, the interaction of light with the desired specimen, the characteristics of modern microscope optics, the characteristics of modern electro-optical sensors (in particular, CCD cameras), and the proper use of algorithms for the restoration, segmentation, and analysis of digital images THÙY Let’s move on the sample preparation First of all, we need to select the sample Specimens should be chosen from locations that are most likely to show the maximumvarieties within the material being studied Specimens should be taken as closely as possible to the fracture or to the initiation of the failure Before taking the specimens, study of the fracture surface should be complete, or, at the very least, the fracture surface should be documented In many cases, specimens should be taken from a sound area for a comparison of structures and properties Next, in the sample preparation, there are steps: sectioning, mounting, grinding, polishing, etching Samples need to be cut accordingly to the area of interest and for handling convenience Depending upon the material, the sectioning operation can be obtained by Fracturing, Shearing, Sawing, Cutting or Wire Saws Proper sectioning is required to minimize damage, which may alter the microstructure and produce false metallographic characterization Proper cutting requires the correct selection of abrasive type, bonding, and size; as well as proper cutting speed, load and coolant The step of this process, we need to define the intended sectional plane and mark it on the bar Then, the bar to the abrasive saw to cut out the small sample; fix the bar securely, prepare the machine, select the cutting parameter and start the first abrasive cut Intensive water cooling and appropriate parameter are important to keep the material as cool as possible Next step is mounting When working with bulk samples, mounting may not be required; however, if the sample is small or oddly shaped, mounting may be necessary The purpose of mounting is protecting the material’s surface and edges, filling spots that have pores on the material and handling irregular-shaped samples, conveniently Mounting process consists of cleaning, adjusting specimen, adhensive mounting and vacuum impregnation The majority of metallographic specimen mounting is done by encapsulating the specimen into a compression mounting compound (thermosets or thermoplastics), casting into ambient castable mounting resins (acrylic resins and polyester resins) KHÁNH The next step is grinding Grinding is a very important phase of the sample preparation sequence because damage introduced by sectioning must be removed at this phase The grinding step is accomplished by decreasing the particle size sequentially to obtain surface finishes that are ready for polishing Care must be taken to avoid being too abrasive in this step, and actually creating greater specimen damage than produced during cutting We need to grind the sample to reduce the damage created by sectioning, creat a flat, reflective, smooth and scratch-free surface and handle irregular-shaped samples, conveniently After grinding, the specimen is thoroughly washed with water, followed by alcohol and then allowed to dry The drying can be made quicker using a hot air dryer Then, the sample is polished to produce a flat, reasonably scratch-free surface with high reflectivity When an unpolished surface is magnified thousands of times, it usually looks like a succession of mountains and valleys By repeated abrasion, those "mountains" are worn down until they are flat or just small "hills." The process of polishing with abrasives starts with a coarse grain size and gradually proceeds to the finer ones to efficiently flatten the surface imperfections and to obtain optimal results Cleaning between polishing stages is more critical than between grinding stages because carryover is a bigger problem Carryover can also be caused by abrasive on the operator's hands; thus, both the sample and the operator's hands should be washed between steps If automatic devices are used, the fixture must also be washed The mount or fixture can be held under running water and swabbed with cotton, followed by similar treatment with alcohol With porous samples or when gaps are present between the sample and the mount, ultrasonic cleaning should be used Careful cleaning is important if good results are to be obtained The last step is etching The metallographic etching is a chemical technique used to highlight features of metals at microscopic levels The specimen is covered or dipped into a protective layer of etching liquid (this varies from the electrochemical, chemical, physical or cathodic vacuum) The sample must be thoroughly cleaned before etching, a satisfactory etchant must be selected and prepared, and the etching technique must be carefully controlled Following etching, the sample must be washed free of any residue and dried ĐẠT Let’s us give a detailed sample preparation As a typical example, we are going to investigate a bar made from steel The plane carbon steel C45E with 0,45% of carbon The material tester define the intended sectional plane and mark it on the bar In this case, it's the longitudinal section then, takes the bar to the abrasive saw to cut out the small sample we need to fix the bar securely, prepare the machine, select the cutting parameter and start the first abrasive cut Intensive water cooling and appropriate parameter are important to keep the material as cool as possible for further processing, the specimen has to be mounted in resin To ensure that the resin will adhere well to the specimen surface, the specimen is placed in an ultrasonic cleaner bar for a few seconds alcoho and ultrasonic wave have to remove fat and lose particle from the surface To mount the specimen, small plastic mold are suitable the material tester picks up the clean specimen and carefully place it in to one of the mold the intended plane of examination is at the bottom then he posly put the resin in to the mold a thin layer of grese at the end surface of the mold act as the release agent this ensure that the cure resin can lay to be release easily from the mold now the mold containing the specimen goes in to the lightcuring unit under the action of the intense blue light, the resin polimerize within half an hour the resin has cured and embedded the specimen well Because the top surface is still an even and slightly sticky, the material tested grinds it there until it is even The material tester uses a rotating water lubricated disk equipped with coarse grain silicon carbide paper Now comes the most important part of the preparation of the intended plane of examination, all materials has that begins with comparatively abrasive paper to make sure to press the specimen commonly and evenly until the abrasive paper compensating the tendency for the top of the Tilt Now having achieved an even relatively rough surface, the specimen is ready for the next step with final paper, insert a new abrasive paper onto the disc rotating the specimen by 90 degrees leads to new grinding groups perpendicular to the old ones In this way the material test can easily check whether the old grinding grooves have been removed completely it is important to use sufficient water flushing during grinding the specimen is then ready for polishing An absolute prerequisite for polishing is a thoraly clean specimen the ultrasonic bath and running water help to remove any residue from a ground surface, otherwise hard particle might be press in to the polishing cluff and scratch the surface Hand should be washed as well so that no unwanted particle find their way to the sample and the polishing cloth Now we can start with the first polishing operation he chosed the polishing disk equip with hard cloth of low resilient, moisten the cloth with suitable lubricant and add splashes of diamond suspension of particle size micrometer on the counter rotating motion, he press it with high pressure on to the polishing cloth the diamond particle have settled on the cloth and now abrading the surface after about a minute, all grinding gruel have been removed, and the specimen surface already has a shiny appearance but still there are many find scratched to be found resulting from the 6micrometer diamond abrasive to remove them, material tester washed specimen and his hand again very carefully and polishes the specimen for a second time now he uses a softer polishing pad, final diamond suspension of micrometer particle size and less pressure after the the second polishing operation, the specimen surface has an almost mirror like appearance No more scratches can be seen with naked eyes Never the less, the specimen has to be clean again and go to a third and final polishing operation this time, fifteen nanometer aluminum oxide abrasive is used only then is the surface prepared to a sufficient quality at the end of the mechanical preparations, special care has to be taken to clean the specimen properly on the one hand, all abrasive particle has to be removed On the other hand, the freshly surface must not be scratch or damage under running water, the material tester gently wipes the specimen with cotton wool, carefully rinses it with alcohol from now on, the sample is called a metallic graphic specimen or micro section THÙY It is now alow to go under the microscope for the first time now on the monitor, in the polish stage, there isn't much to be seen in this material only some elongated nonmetalic inclusion can be observe but if these inclusions are a special interest, then the polish stage is the perfect one Inclusion may best be seeing here However, if one wants to see the crystal, the grain and grainboudary, then the micro section has to be etched To this, material tester protect himself with a lab coat, put on safety glasses and pour the agent into a glass bowl The agent consists of solution of 10% of concentrated nitric acid in alcohol Now he turns on the water tap, pick up the freshly polish microsection with the gripping tongs and emerges it for a few seconds into the agent Then he rinses microsection thoroughly the water and afterward with alcohol Now, this sample is well- prepared for the next step: observation After observation, we have this image As you can see, the darker region is perlite Pearlite is a two-phased, layered structure composed of alternating layers of ferrite and cementite The white region is ferrite crystal They consist of almost the pure iron