Shawn Wallace, Applications Specialist, EDAX
One of the powers of EBSD is showing how microstructures are created by the processing of a material and how these microstructures can change the material properties of a sample. Explaining this connection to novice users or potential customers can be difficult. Luckily for me, my sidewalk has given me a perfect example.
It is made up of oriented bricks. Some are placed square side up. Some are placed rectangular side up. But look at the color? Why are some bricks wet while others are dry? The square sides tend to be dry, rectangular still wet.
Now let’s start building up a case as to why this happens.
The first step is understanding how these bricks are made and what they are made of. You take clay, you slap it into a mold. You press the rectangular side to compress it to fill the mold. Fire it and tada, you have a brick. A lot is going on in these steps that you can’t see with the naked eye.
The main thing is that the squeezing step is really having a profound effect on the brick. You are taking randomly oriented platy minerals (Figure 1) and giving them a preferred orientation by squeezing them (Figure 2). It is like a house of cards that has fallen down. You now have grains lying down. Water can’t break through the new “sheets”, but turn the brick on its side and you have pathways to drain the water.
Figure 1. Clay minerals in bricks are often platy in shape. Without any outside forces, the grains are randomly orientated.
FIgure 2. By compressing the material to form a brick shape, the grains are laid flat relative to each other. This is much like EBSD samples with a texture.
This is what you are seeing here. The square bricks have clays that are oriented to wick the water deeper in to the brick, while on the rectangular faces, the water has nowhere to go (Figure 3). Square bricks wick away and are dry, while rectangular faces are still wet.
Figure 3. After the brick is pressed, there are no connecting paths for the water to flow into the material from the top. But from the side, there are channels leading into the bricks. This is what allows the water to wick inside the brick from the square side, but not the rectangular side.
On a high level, this is what EBSD is all about. You are seeing how these processes forming a material are now controlling how the material behaves. For EBSD, these can be electrical, thermal, or mechanical properties, but EBSD is the driving force to truly understanding how and why your material behaves the way it does.