Metallic alloys are important for a variety of products in many industries. They especially play a critical role for automobiles, trucks, trains, aircraft, and other forms of transportation. This report explains the importance of alloy characterization, specifically grain size, for the automotive and transportation industries and a practical, efficient solution for its analysis using optical microscopy software.
Several thousand standard alloys are currently in use and new ones with better performance are developed all the time to meet new demands. For example, there are multiple alloys of steel and aluminum which are used to build automobiles, trucks, planes, and trains.
When exploiting alloys, it is important to understand their properties which are related to the composition and microstructure. The microstructure - phases, grains, or inclusions - has a significant effect on tensile strength, elongation, and both thermal and electrical conductivity. Understanding well the relationship between the composition, microstructure, and macroscopic properties is important for the design and manufacture of alloys. Grains are crystallites (microscopic crystals) which form in alloys during the cooling stage of production.
It has been well known for a long time that as the grain size increases, the alloy’s (refer to figure 1) :
- tensile strength (Rm) and yield strength (Re) decrease;
- elongation at fracture (A%) increases; and
- ductile-brittle transition temperature increases.
In order to characterize the microstructure of an alloy, a sample has to be prepared from the alloy material, then ground and polished, imaged with a microscope, and finally the images are analyzed. Figure 2 shows a diagram illustrating the typical workflow for sample preparation and microstructural analysis.
Different types of experimental techniques are used to investigate the microstructure of alloys. For more than 100 years, optical microscopy, using incident brightfield, darkfield, differential interference contrast (DIC), and polarized light illumination along with color etching, has been the most common method. At the present time, computer automated microscopes and image analysis systems provide a rapid and accurate way to evaluate such alloys.
The setup and analytical capabilities of the imaging software used has a very important effect on the accuracy, reliability, reproducibility, and efficiency of:
- image capture and analysis;
- grain size and microstructure evaluation; and
- report generation from the results.
A Leica microscope using the