A3 Devitrification (Undesired Crystallization)

Topic

When this glass is reheated or remains in service at high temperatures for an extended period, it can undergo an unwanted crystallization process known as devitrification. This process—triggered by reheating or prolonged high-temperature exposure—causes crystallization nuclei to appear within what should remain an amorphous structure; these nuclei then grow to form tiny crystals (crystallites) inside the glass.

The fraction of the glass that crystallizes in this manner increases over time and with temperature, as described by the Avrami equation. This equation plots a curve that rises slowly at first—when few crystallization nuclei exist—accelerates as those nuclei grow and new ones appear, and then slows down again toward the end, when little amorphous glass remains to be transformed. Higher temperatures cause this crystallized fraction to grow more rapidly for a given duration, while a longer duration results in a larger crystallized fraction for a given temperature.

The formation of these crystallites within the glass, as the crystallized fraction grows, causes a loss of transparency due to Rayleigh scattering. Each crystallite has a refractive index different from that of the surrounding amorphous glass; when the size of these crystallites approaches the wavelength of visible light, this difference in refractive index scatters the light rather than allowing it to pass through in a straight line. A larger crystallized fraction intensifies this light scattering, thereby resulting in a greater loss of transparency.

For this reason, the model—which began by addressing devitrification—utilizes the same equations employed by another model in this set to describe (via Avrami) the nucleation rate and crystallized fraction during a deliberate manufacturing process; However, here he uses them in the opposite sense—as a defect occurring during reheating or prolonged service of the glass, rather than as a controlled manufacturing process. The fact that the same equations describe a desired process in one model and an unwanted defect in the other means that the difference between the two models lies not in the mathematics they share, but in whether or not the crystallization occurs in a controlled manner.

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