Theory (Microstructure and Macrostructure)
Metallography is a branch of metallurgy where the structure of metals and other materials are under consideration using optical and electron microscopes. For metallography, sample preparation is extremely important. Better the sample is prepared, more information that you can extract from microstructural examinations. In sample preparation, the Scaling of microstructure is very important and most people get it wrong.
For sample preparation, follow the guide in this article. Important functions related to microstructure are:
- To observe defects, grain morphologies and several secondary phases
- Linking those important structural phases with materials properties
The linkage between structural features with properties of materials helps engineers in designing a combination of materials for advanced applications. Further can be studied at WIKIPEDIA.
Difference between microstructure and macrostructure
For generating complete history for every material, structural characterization is always started from the naked eye. At start features like any inclusions, porosity and beach marks are observed in a sample. For revealing these features, we can also use a stereomicroscope and magnifying glass.
All those structural features which can be visualized up to 50X fall under the category of Macroscope. While, in microstructure, features that are not visible with the naked eye like grain morphology, secondary phases, and grain boundaries come under consideration.
Fractography can be best studied using macroscopic features while structure-property relationships require more in-depth study i.e. microstructural characterizations.
Microscopy can give information concerning a material’s composition, previous treatment, and properties. Significant features which might be on an interest are;
- Grains, grain boundary and grain size
- Chemical phases and homogeneity
- phases distribution in microstructure
- deformed grains and elongated microstructure in result of cold and hot working
Use of Optical microscope with a digital camera For scaling of microstructure
Nowadays, digital cameras are attached to the optical microscope for transfer and better study of optical microstructures. Digital cameras have a term called megapixels i.e. 12 megapixels means it can capture 12 million pixels in one image. Now, depending upon camera’s pixels, picture magnification is different.
Having a clear understanding of the picture’s magnification is very important. If you are studying the optical microstructure and you want to find a grain size of microstructure, right magnification is very important. A little variation is magnification can give large divergence in grain size which can give poor depiction about materials processing and properties.
That’s why, while taking the picture, scaling of microstructure pictures according to the right magnification is very important.
Procedure for Scaling of Microstructure
- Take the micron(image) of unknown magnification
- Open the image of micron in the Image editing software (Powerpoint).
- Draw a line segment of 40mm.
- In the Micron
At 0 X, 1 small division= 0.01mm At 200X, 1 small division= 2mm
So 40mm line segment will cover 2 big divisions.
- Reduce the image size (diagonally) up to a size at which the line segment covers 2 Big divisions completely.
- Note the dimensions of the image (micron).
- The length of the line segment will be calculated as:
- Label the line segment with its length (In our case 200 µm)
- Group the Image (Micron), line segment and its label together.
- Lock the aspect ratio for scaling of microstructure
- Open the micrograph in the software (PowerPoint)
- Set the micrograph in the dimensions of
- Copy the line segment of 40 mm on
- Calculate the length of the line segment by the formula:
- Label the line segment with the calculated length.
- Group the micrograph, line segment, and its label together.
- Lock the aspect ratio
- Repeat the same procedure for each micrograph.