Devitrification

Devitrification is the process by which a glassy, non-crystalline material partially or fully crystallizes, forming crystalline phases within the amorphous matrix. It can occur during heating, aging, or long-term storage when the glass remains in a temperature range that allows nucleation and crystal growth. Devitrification is distinct from the initial crystallization that can occur during melt cooling; it refers to crystallization that happens after a glass has formed.

The mechanism of devitrification involves nucleation of crystals inside the supercooled liquid, followed by growth of those crystals. Kinetic factors such as diffusion control the rate, and the temperature determines whether diffusion is fast enough for crystallization to proceed. The tendency to devitrify depends on the glass composition, including the presence of crystal-forming sectors or nucleating agents, as well as the thermal history, impurities, and the duration of exposure to elevated temperatures.

Detection and effects vary. Devitrification often manifests as opacity or whitening due to crystalline inclusions, and it can be identified by differential scanning calorimetry as an exothermic crystallization peak on heating, or by X-ray diffraction and microscopy revealing crystalline phases. In many applications, devitrification is undesirable because it degrades optical clarity, mechanical integrity, and chemical durability. However, controlled devitrification is exploited in the production of glass-ceramics, where deliberate crystallization yields materials with tailored properties such as increased strength or thermal resistance.

Prevention and control focus on avoiding processing conditions that fall within the crystallization window. Strategies include rapid cooling to bypass crystallization, adjusting composition to reduce crystallization tendency, adding inhibitors or carefully selecting nucleation agents, and optimizing heat-treatment schedules to prevent unintended crystallization while enabling desired glass-ceramic development.