Vitrocerâmicas de silicato de lítio com gradientes microestruturais - simulações e experimentos
Resumen
The microstructure of glass-ceramics (GCs) depends on their chemical
composition and thermal history, from the cooling of the precursor glass to the
heat treatments. In glass production, the cooling rate on the surface is much
higher than in the center, allowing the appearance of crystal nuclei in the central
parts that cause microstructural heterogeneities in the GC. Furthermore, during
the heat treatments, temperature gradients appear in relatively thick or large
samples and, consequently, microstructures with different numbers and sizes of
crystals will be obtained along the part, causing deterioration of its mechanical
properties. In this work, vitreous samples based on the Li2O-SiO2 system were
prepared to evaluate the effects of different sample sizes on the appearance of
thermal gradients along the two stages of obtaining GCs (cooling and thermal
treatments). The heat exchange in these steps and the thermal gradients
generated were numerically simulated and the impact of these gradients on the
microstructure and mechanical properties of the GCs was analyzed. For that, the
glass-forming liquid was poured into copper and 304 stainless steel molds and,
from a thermal treatment of crystal growth and optical microscopy analysis, the
existence of microstructural gradients caused by nuclei formed in the cooling was
evaluated. After nucleation and growth heat tratament, samples from each mold
were analyzed by Scanning Electron Microscopy, Biaxial Bending and X-Ray
Diffraction. The results indicated that an increase in the size of the mold and the
glass produced leads to the appearance of crystalline nuclei inside the part during
cooling. However, after the heat treatments of nucleation and crystal growth, the
existence of pre-nucleation does not cause significant heterogeneities in the GC.
On the other hand, heat treatments in samples with 14 x 45 mm (diameter x
height), promote significant thermal gradients and the existence of
microstructural gradients, reducing their mechanical properties.
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