Microscopic Examination of Metals

MICROSCOPIC EXAMINATION OF METALS

In this experiment, our aim is examining the microstructure of metals. By studying microscopic structures of metals, we determine which material fits best to a given application. We used the most common method, optical technique, to examine the microstructure.

We used a small specimen cut from the metal to be examined. To be able to see the structure clearly, we first cleaned and polished the specimen. First we start polishing with emery paper no: 1 and some finer grades. One should be careful about the coarse abrasive particles and striations from them. Cleaning and rotating the specimen 90Ð'o during the transfer can prevent these. The next step is polishing, yet washing the sample before polishing gives a more successful result. Finally, we polished the specimen on a rotating cloth covered with an effective abrasive like Al2O3-Water suspension. We kept polishing until we obtained a mirror like face.

After we finished polishing, the crystalline structure of the specimen, any cracks, seams, non-metallic inclusions and inhomogenities, could be revealed. Before start etching we first applied mounting process. In this step we used a matched die set. We placed our sample into the die set in the way that the rough face of the specimen was the lower surface and the polished face looked upward. We filled the die cavity with Bakelite and then we transferred our die to a mounpress. Mounting not only protects our sample but also by making its base flat and stable helps us while we are examining the sample under the microscope.

In etching process, depending upon chemical composition, energy content and grain orientation, we determine the grain boundaries and the presence of chemically different phases. To reveal these micro structural details of the polished mount we used an etchant like 1% Nital. We immersed the mount with the polished face upward in a small dish of etching solution. As soon as a bloom appeared we removed the mount. At this stage, the first appearance of microstructure can be observed. After etching, we rinsed the mount in water and sprayed alcohol over it. Finally we dried it in a stream of hot air to get it ready to be examined under microscope.