Metallography and Microscopy PDF

# Metallography and Microscopy ## Praveen Kumara J ## Introduction - **Microstructure:** It is the geometric arrangement of grains and the different phases present in a material. - **Grain Boundaries:** It is the interface between two grains in a polycrystalline material where the crystal is dis...

# Metallography and Microscopy ## Praveen Kumara J ## Introduction - **Microstructure:** It is the geometric arrangement of grains and the different phases present in a material. - **Grain Boundaries:** It is the interface between two grains in a polycrystalline material where the crystal is disordered due to rapid change in crystallographic directions. ## Why is Microstructural Analysis Used For? - **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. ## Application - Study and characterization of materials. - Ensure that the associations between properties and structure are properly understood. - Predict properties of materials. - Design alloys with new properties. - Check if the material has been correctly heat treated. ## Safety Instructions - Optical Metallography involves the use of etchants (standard solutions containing a variety of chemicals such as strong acids and solvents) which can be very corrosive and poisonous. ## 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-free specimen - Flat specimen - No thermal damage - No scratches ## Specimen Preparation Consists Of: - **Sample Selection:** The number, location orientation of the samples examined are important parameters in selection samples. ### Process Steps: - SECTIONING - MOUNTING - GRINDING - POLISHING - ETCHING ## Sectioning - Sectioning is the first step in the overall process of sample preparation. - Sectioning of the test sample is performed carefully to avoid altering the structure of the material. - Abrassive cutting is the most common cutting method. - The cutting tool is made of Silicon carbide (Sic) of diamond particles. - Use coolant fluid to avoid overheating of specimen and possible change in material structure. ### Sectioning Methods: - **Sawing:** is perhaps the oldest met lab method of sectioning that is still used today. It can be accomplished with a hand-held hacksaw, but it can alter the microstructure. - **Abrasive Wheel Sectioning:** is The most popular method of sectioning is with abrasive cut-off wheels. Wheels made of silicon carbide, aluminum oxide, and diamonds are used in the sectioning process. With this type of sectioning, the metallographer has more control over the conditions used. ### Machines Used For Sectioning: - EDM ## Mounting - Mounting. Small samples can be difficult to hold safely during grinding and polishing operations, and their shape may not be suitable for observation on a flat surface. They are therefore mounted inside a polymer block or mount. ### Mounting Types: - **Cold mounting:** - It can be done using two components resins (epoxies) which are liquid to start with but which set solid shortly after mixing. - It requires very simple equipment consisting of a cylindrical ring which serves as a mould and a flat piece which serves as the base of the mould. The sample is placed on the flat piece within the mould and the mixture poured in and allowed to set. Cold mounting takes few hours to complete. - **Hot-mounting:** - The sample is surrounded by an organic polymeric powder which melts under the influence of heat (about 200 C). - Pressure is also applied by a piston, ensuring a high quality mould free of porosity and with intimate contact between the sample and the polymer. ## Grinding - Grinding is done using rotating discs covered with silicon carbide paper and water. - There are a number of grades of paper, with 180, 240, 400, 1200, grains of silicon carbide per square inch. 180 grade therefore represents the coarsest particles and this is the grade to begin the grinding operation. Always use light pressure applied at the centre of the sample. - Wash the sample in water and move to the next grade, orienting the scratches from the previous grade normal to the rotation direction. This makes it easy to see when the coarser scratches have all been removed. - After the final grinding operation on 1200 paper, wash the sample in water followed by alcohol and dry it before moving to the polishers. ## Polishing - The polishers consist of rotating discs covered with soft cloth impregnated with a pre-prepared slurry of hard powdery alumina particles (Al2O3, the size ranges from 0.5 to 0.03 µm). - Begin with the coarse slurry and continue polishing until the grinding scratches have been removed. It is of vital importance that the sample is thoroughly cleaned using soapy water, followed by alcohol, and dried before moving onto the final stage. Any contamination of the final polishing disc will make it impossible to achieve a satisfactory polish. - Examining the specimen in the microscope after polishing should reveal mirror like surface. ## Etching - The purpose of etching is two-fold. - 1. Grinding and polishing operations produce a highly deformed, thin layer on the surface which is removed chemically during etching. - 2. Attacks the surface with preference for those sites with the highest energy, leading to surface relief which allows different crystal orientations, grain boundaries, precipitates, phases and defects to be distinguished in reflected light microscopy. ### Etching Process Notes: - Etching should always be done in stages, beginning with light attack, an examination in the microscope and further etching only if required. - If you overetch a sample on the first step then the polishing procedure will have to be repeated. - The table below gives the etchants for alloys that will be examined in this experiment. | Sample | Etchant | |---|---| | Al alloys | Keller's (2 ml HF +3 ml HCL + 5 ml NO3+ 190 ml water) | | Cu-Zn alloy (brass) | 10 ml HNO3+90 ml water | | Steel and cast irons | Nital (2% HNO3 + 98% ethanol) | ## Industrial Etching - The surface is than exposed to chemical attack or ETCHING, with grain boundaries being attacked more aggressively than reminder of the grain to reveal the microstructure. - Light from an optical microscope is reflected or scattered from the sample surface depending how the surface is etched. ## 3D Image Illustration - The micrograph in the figure below was created by taking optical micrographs from a specimen, after polishing (& etching) to various depths (sectioning). This gives a '3D' view of the sample. - The image is a 3D diagram depicting grain boundaries, a grain and a triple line.