Grain boundaries

Grain boundaries are interfaces between the grains.  These boundaries can be very important in determining the way in which materials perform.  This is because the atomic structure in the narrow region near the grain boundary is different from that in the grain itself.  What's more, when the orientation of grains changes, the structure of the boundary will also change.  Because EBSD measures the crystal orientation, it can characterise the boundary by the difference in orientation on either side of the boundary.  Of course, there are a lot of grain boundaries in a sample of material and measuring just one is not enough.  Running unattended, EBSD can easily measure many thousands of grain boundary orientations so that good statistical information on  the types of grain boundaries present can be built up.

Information of this type is important because  grain boundary orientations can influence many materials properties.  For example, corrosion and fracture can be initiated at grain boundaries.   Some grain boundary orientations are more resistant to these phenomena than others.   Material processing routes which encourage the formation of the more resistant boundaries can lead to materials with improved properties.   This is sometimes referred to as grain boundary engineering.  An example of this is the improved lifetime of lead electrodes in battery acid when processed to have a higher fraction of a particular boundary type.

The model shows the crystal orientation at points A and B on either side of a grain boundary.  Click the button to switch between the orientation at A and B.

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You may be able to see that the crystal is rotating around the longest diagonal in the cube.   This is a special type of boundary called a twin boundary.

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