Rules of Grain Growth
Grain growth has long been studied primarily by the examination of sectioned, polished and etched samples under the optical microscope. Although such methods enabled the collection of a great deal of empirical evidence, particular with regard to factors such as temperature or composition, the lack of crystallographic information limited the development of an understanding of the fundamental physics. Nevertheless, the following became well-established features of grain growth:
- Grain growth occurs by the movement of grain boundaries and not by coalescence (i.e. like water droplets)
- Boundary movement is discontinuous and the direction of motion may change suddenly.
- One grain may grow into another grain whilst being consumed from the other side
- The rate of consumption often increases when the grain is nearly consumed
- A curved boundary typically migrates towards its centre of curvature
- When grain boundaries in a single phase meet at angles other than 120 degrees, the grain included by the more acute angle will be consumed so that the angles approach 120 degrees.
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