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 isis to seek relationships between: 1 STRUCTURE PROPERTIES PERFORMANCE PROCESSING Performance isis thethe ultimate end use function ofof thethe material and isis thethe result from aaproperproper set ofof properties achieved byby optimising both atomic level and microstructural levels ofof thethe structure ofof thethe material prepared and fabricated using carefully controlled and optimised processing techniquestechniques.. 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 araree based on metals, they are crystalline in the solid form. 2 •• Metals also are normally polycrystalline, meaning theythey consist of many small crystals, called grains. •• In some cases, these grains can be viewed with ththee naked eye and these structures are called MACROSTRUCTURES •• The structural features of small grains are obserobservevedd using an optical microscope, or an electron microscope. These structures are called MICROSTRUCTURES .. •• The structural features present inin aamaterialmaterial aarreeaafunctionfunction ooff tthhee composition and form oofftthheestartingstarting material, and any subsequent heat treatment and //oorrprocessingprocessing treatment tthheematerialmaterial receivesreceives.. •• Microstructural analysis isis used toto provide information oonnhowhow tthhee material was produced and tthheequalityquality oofftthheeresultingresulting materialmaterial.. •• Microstructural features, which aarreeooffgreatgreat concern ttoouussincludeinclude:: grain size ,, phase volume fraction ,, precipitate size ,, defects (porosity and cracks) 3 What isis Microstructural Analysis used for? Macrostructural and Microstructural examination techniques aarreeusedused inin areas such aass:: Quality Control :: Analysis isis used toto determine whether tthheestructuralstructural parameters aarreewithinwithin specificationsspecifications:: aacriteriacriteria ffoorr ACCEPTANCE oror REJECTION ofof products Failure Analysis ::toto determine tthheecausecause ofof failurefailure..FailureFailure occur due toto several factors (incorrect material selection, improper processing treatment, poor quality control)control)..FailureFailure analysis provides information about tthheecausecause ofof failure Research Studies :: isis used toto determine tthheemicrostructuralmicrostructural changes that occur aassaaresultresult ofof varying parameters such aasscomposition,composition, heat treatment oror processingprocessing.. The research studies develop tthhee PROCESSING --STRUCTURESTRUCTURE --PROPERTIESPROPERTIES relationshipsrelationships.. What Information can bbeeObservedObserved •• Quality ofof specimen preparation isis aadeterminingdetermining factor iinntthheevaluevalue ofof examination Effect of cold work 4 SPECIMEN PREPARATION •• Specimen preparation isis anan important part ofof metallography •• AA specimen must bebe appropriately prepared toto ensure correct observation and interpretation ofof thethe microstructuremicrostructure.. •• Specimen preparation requirements DeformationDeformation--freefree specimen Flat specimen NoNo thermal damage NoNo 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 Specimens are examined using optical Sample Examination and /or electron microscopes 5 Sectioning •• Abrasive cutting is the most common cutting method ••TheThe cutting tool (disc) is made of silicon carbidcarbidee (SiC) of Diamond particles ••useuse coolant fluid (lubrication) to avoid overheatoverheatiningg 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)specimens).. There are two types of mounting: 1.1. Hot mounting: requires compression pressure and heat and is done using mounting machines 6 Mounting 2.2. Cold mounting: uses epoxy resin hardened at room temperature Grinding and Polishing •• Grinding removes the damage on the specimen surfasurfacece produced by sectioning •• Grinding is done using abrasive paper with differdifferenentt grit size •• After grinding is complete, the specimen is polispolishehedd 7 Grinding and Polishing Grinding Water rinse Water rinse Water rinse Water Polishing 0.25 µm 1 µm 6 µm 8 Grinding and Polishing Etching Techniques Swab Immersion Electrolytic Purpose Grain boundaries Flow lines Constituents or phases present in structure 9 QUANTITATIVE METALLOGRAPHY AAnnimageimage isis quantified bbyydescribingdescribing:: Size, Shape, Distribution, and Quantity The measurements aarreemademade manually ((linear intercept method ,, point counting )),, oorr bbyycomputerisedcomputerised automated methods oonndigitallydigitally acquired images ((Image analyser )) 10 XRD TEM SEM OMOM Grain 1 Grain 2 Atomic Optical Microscopy (OM) Old & Modern Optical Microscopes 11 Inverted Microscope Upright Microscope u v Image a •• Object f Object b •• Image u v v c •• Object u Image Ray diagrams illustrating the formation of an image by a single lens of focal length, f. 12 MAGNIFICATION IN LIGHT MICROSCOPE •• Using the thin lens equation: 1 1 1 = + f u v •• Figures above show that, by similar triangles, ththee magnification M produced by the single lens is given by v/u f v − f M = = u − f f From tthheeformula,formula, larger MMrequiresrequires smaller focal length, f,f, However, lenses with smaller focal lengths aarreedifficultdifficult toto make Instead, higher magnifications aarreeachievedachieved bbyycombiningcombining lenses For example, when using ttwwoolenseslenses:: (v − f )(v − f ) M = 1 1 2 2 f1 f2 13 RESOLUTION ININ THE LIGHT MICROSCOPE •• Resolution iisstthheeclosestclosest spacing ooff ttwwoopointspoints that aarreevisiblevisible aass distinct entities through tthheemicroscopemicroscope •• IInntthheelightlight microscope, tthheequalityquality oofftthheeobjectiveobjective lens plays aamajormajor role inin determining tthheeresolvingresolving power oofftthheeapparatusapparatus.. •• However, tthhee resolution inin aa light microscope isis limited bbyy aa diffraction ofof 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 inin aamicroscopemicroscope occurs when aalightlight wave passes very close ttootthheeedgeedge ooff aannobjectobject oror through aatinytiny opening, such aassaasslliitt oorr aperture Short wavelengths are “bent” more than long wavelengths Dispersion 14 •• IInntthheemicroscope,microscope, diffraction ofof light can occur atat tthheespecimenspecimen plane due toto interaction ooff tthheelightlight with small particles oror features, and again aatt tthhee margins oofftthheeobjectiveobjective front lens oorraatttthheeedgesedges ofof aacircularcircular aperture within oror near tthheerearrear oofftthhee objectiveobjective.. •• ItIt isis this diffraction ofof light that makes itit possible toto observe magnified images ofof specimens iinntthhee microscopemicroscope.. •• However, itit isis also diffraction that limits tthheesizesize ofof objects that can bbee resolved (limit tthhee resolution) micro.magnet.fsu.edu/primer/lightandcolor/diffraction.htm 15 micro.magnet.fsu.edu/primer/lightandcolor/diffraction.htm 16 α r = d/2 Aperture Rayleigh’s criterion :: when tthheemaximummaximum ofof intensity ooff aannAiryAiry disc coincides with tthhee first minimum ooff tthheesecond,second, then tthheetwotwo points can bbeedistinguisheddistinguished RESOLUTION ININ THE LIGHT MICROSCOPE •• From tthheediffractiondiffraction theory, tthheeresolutionresolution isis given bbyy Abbe’s equationequation:: d 0.61λ 0.61λ r = = = 2 µ sin α NA •• λλ iisstthheewavelengthwavelength ofof light, µµ iisstthheerefractiverefractive index ffoorr tthheemediummedium through which tthheelightlight passes (air ==11,,waterwater ==11..3333,,ooiill ==11..44)) •• The refractive index isis ==ratioratio ooff tthheespeedspeed ofof light inin aavacuumvacuum toto that inin aa second medium ofof greater density 17 •• µµ ssiinnαα isis called tthhee numerical aperture (NA) •• InIn order toto obtain higher resolution (smallest r),r), itit isis possible toto decrease λλ oror increase µµ oror αα.. •• The higher tthheeNNAAtthheegreatergreater tthheeresolutionresolution •• The limits ooff tthheeobjectiveobjective lens aarreethatthat αα cannot bbeegreatergreater than 9900oo,, and that tthheeobjectobject space can only reach aannNNAA==11..44 Material NNAA Air 1.0003 Water 1.33 Immersion oil 1.4 Glass 1.52 Zircon 1.92 Diamond 2.42 18 N.A. 0.13 0.51 0.95 D --separationseparation distance 19 DEPTH OOFFFIELDFIELD •• The depth ofof filed isis defined aass tthhee distance from tthhee nearest part ooff tthhee specimen ttoo tthhee farthest part ooff tthhee specimen that isis inin focus when tthhee picture