CHAPTER TWO METALLOGRAPHY & MICROSCOPY

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1 CHAPTER TWO METALLOGRAPHY & MICROSCOPY 1. INTRODUCTION: Materials characterisation has two main aspects: Accurately measuring the physical, mechanical and chemical properties of materials Accurately measuring (determining) the structure of a material Atomic level structures Microscopic level structures A critical part of materials science & technology is to seek relationships between: 1

2 STRUCTURE PROPERTIES PERFORMANCE PROCESSING Performance is the ultimate end use function of the material and is the result from a proper set of properties achieved by optimising both atomic level and microstructural levels of the structure of the material prepared and fabricated using carefully controlled and optimised processing techniques. What is metallography (Materiallography)? The study of structure of materials It includes the techniques used to: Prepare specimens for examination, Examine the specimens and Interpreting the structures. What is a microstructure? Most engineering materials of great importance are based on metals, they are crystalline in the solid form. 2

3 Metals also are normally polycrystalline, meaning they consist of many small crystals, called grains. In some cases, these grains can be viewed with the naked eye and these structures are called MACROSTRUCTURES The structural features of small grains are observed using an optical microscope, or an electron microscope. These structures are called MICROSTRUCTURES. The structural features present in a material are a function of the composition and form of the starting material, and any subsequent heat treatment and /or processing treatment the material receives. Microstructural analysis is used to provide information on how the material was produced and the quality of the resulting material. Microstructural features, which are of great concern to us include: grain size, phase volume fraction, precipitate size, defects (porosity and cracks) 3

4 What is Microstructural Analysis used for? Macrostructural and Microstructural examination techniques are used in areas such as: Quality Control: Analysis is used to determine whether the structural parameters are within specifications: a criteria for ACCEPTANCE or REJECTION of products Failure Analysis: to determine the cause of failure. Failure occur due to several factors (incorrect material selection, improper processing treatment, poor quality control).failure analysis provides information about the cause of failure Research Studies: is used to determine the microstructural changes that occur as a result of varying parameters such as composition, heat treatment or processing. The research studies develop the PROCESSING - STRUCTURE - PROPERTIES relationships. What Information can be Observed Quality of specimen preparation is a determining factor in the value of examination Effect of cold work 4

5 SPECIMEN PREPARATION Specimen preparation is an important part of metallography A specimen must be appropriately prepared to ensure correct observation and interpretation of the microstructure. Specimen preparation requirements Deformation-freefree specimen Flat specimen No thermal damage No scratches Specimen preparation consists of : Sample Selection The number, location and orientation of the samples examined are important parameters in selection samples. Sectioning Mounting Grinding Polishing Etching Sample Examination Specimens are examined using optical and /or electron microscopes 5

6 Sectioning Abrasive cutting is the most common cutting method The cutting tool (disc) is made of silicon carbide (SiC) of Diamond particles use coolant fluid (lubrication) to avoid overheating of specimen and possible change in material structure Electric Discharge Machining (EDM) cutting Mounting: Provides a safe and efficient mean of handling samples (particularly small and irregular specimens). There are two types of mounting: 1. Hot mounting: requires compression pressure and heat and is done using mounting machines 6

7 Mounting 2. Cold mounting: uses epoxy resin hardened at room temperature Grinding and Polishing Grinding removes the damage on the specimen surface produced by sectioning Grinding is done using abrasive paper with different grit size After grinding is complete, the specimen is polished 7

8 Grinding and Polishing Grinding Water rinse Water rinse Water rinse Polishing 0.25 µm 1 µm 6 µm 8

9 Grinding and Polishing Etching Techniques Swab Immersion Electrolytic Purpose Grain boundaries Flow lines Constituents or phases present in structure 9

10 QUANTITATIVE METALLOGRAPHY An image is quantified by describing: Size, Shape, Distribution, and Quantity The measurements are made manually (linear intercept method, point counting), or by computerised automated methods on digitally acquired images (Image analyser) 10

11 XRD TEM SEM OM Grain 1 Grain 2 Atomic Optical Microscopy (OM) Old & Modern Optical Microscopes 11

12 Inverted Microscope Upright Microscope u v a Object f Image Object b Image v u c Object u v Image Ray diagrams illustrating the formation of an image by a single lens of focal length, f. 12

13 MAGNIFICATION IN LIGHT MICROSCOPE Using the thin lens equation: 1 f 1 1 = + u v Figures above show that, by similar triangles, the magnification M produced by the single lens is given by v/u M f = u f v = f f From the formula, larger M requires smaller focal length, f, However, lenses with smaller focal lengths are difficult to make Instead, higher magnifications are achieved by combining lenses For example, when using two lenses: M = ( v f )( v f ) 1 1 f 1 f

14 RESOLUTION IN THE LIGHT MICROSCOPE Resolution is the closest spacing of two points that are visible as distinct entities through the microscope In the light microscope, the quality of the objective lens plays a major role in determining the resolving power of the apparatus. However, the resolution in a light microscope is limited by a diffraction of light effect Diffraction: when light passes through an object the intensity is reduced depending upon the color absorbed. Thus the selective absorption of white light produces colored light Diffraction in a microscope occurs when a light wave passes very close to the edge of an object or through a tiny opening, such as a slit or aperture Short wavelengths are bent more than long wavelengths Dispersion 14

15 In the microscope, diffraction of light can occur at the specimen plane due to interaction of the light with small particles or features, and again at the margins of the objective front lens or at the edges of a circular aperture within or near the rear of the objective. It is this diffraction of light that makes it possible to observe magnified images of specimens in the microscope. However, it is also diffraction that limits the size of objects that can be resolved (limit the resolution) micro.magnet.fsu.edu/primer/lightandcolor/diffraction.htm 15

16 micro.magnet.fsu.edu/primer/lightandcolor/diffraction.htm 16

17 α r = d/2 Aperture Rayleigh s criterion: when the maximum of intensity of an Airy disc coincides with the first minimum of the second, then the two points can be distinguished RESOLUTION IN THE LIGHT MICROSCOPE From the diffraction theory, the resolution is given by Abbe s equation: d r = 2 = 0.61λ 0. 61λ = µ sin α NA λ is the wavelength of light, µ is the refractive index for the medium through which the light passes (air = 1, water = 1.33, oil = 1.4) The refractive index is = ratio of the speed of light in a vacuum to that in a second medium of greater density 17

18 µ sinα is called the numerical aperture (NA) In order to obtain higher resolution (smallest r), it is possible to decrease λ or increase µ or α. The higher the NA the greater the resolution The limits of the objective lens are that α cannot be greater than 90 o, and that the object space can only reach an NA = 1.4 Material Air Water Immersion oil Glass Zircon Diamond NA

19 N.A D - separation distance 19

20 DEPTH OF FIELD The depth of filed is defined as the distance from the nearest part of the specimen to the farthest part of the specimen that is in focus when the picture is being taken DEPTH OF FIELD The depth of field can be estimated from the Figure, which shows rays converging at the specimen. Simple geometry gives: 0.61λ h = µ sin α tan α h d A α Aperture Plane of optimum focus 20

21 Light Microscope vs Electron Microscope As λ increases, r increases (low resolution) Best resolution is when λ is lowest Since electron microscopes use electron as illumination, λ is shorter, thus resolution is higher the source of OM SEM Depth of Field CONSIDERATIONS IN MICROSCOPY Magnification Resolution High magnification without high resolution is EMPTY EMPTY magnification RESOLVING LIMIT Eye 10 5 nm (0.1 mm) Light Microscope 200 nm (0.2 µm) SEM TEM 3 nm 0.2 nm This data is outdated!!! Check the latest machines 21

22 LENS ABERRATIONS Image quality, in all optical instruments, is limited by distortions which arise from optical defects called aberrations Lens aberrations include: Monochromatic (spherical) aberration Chromatic aberration Astigmatism Diffraction SPHERICAL ABERRATION Ideal lens These aberrations occur when light waves passing through the periphery of a lens are not brought into identical focus with those passing closer to the centre Electrons further from axis are focused closer than the electrons closer to axis 22

23 CHROMATIC ABERRATION Ideal lens These aberrations are the result of the fact that white light is composed of numerous wavelengths. When white light passes through a convex lens, the component wavelengths are refracted according to their frequency. ASTIGMATISM The off-axis image of a specimen point appears as a disc or blurred lines instead of a point Depending on the angle of the off- axis rays entering the lens, the line image may be oriented either tangentially or radially 23