EXPERIMENTAL TEST OF THE GENERAL-THEORY OF TRANSFORMATION KINETICS - HOMOGENEOUS NUCLEATION IN A NA2O.2CAO.3SIO2 GLASS

Experiments were carried out to test the validity of the Johnson-Mehl-Avrami theory of transformation kinetics. The isothermal crystallization of a nearly stoichiometric Na//2 O multiplied by (times) 2CaO multiplied by (times) 3SiO//2 glass was studied at 627 degree C and 629 degree C by optical microscopy, density measurements and X-ray diffraction. Both nucleation and growth rates were measured by single and double stage heat treatments up to very high volume fractions transformed and the experimental data for crystallinity were compared with the calculated values at the two temperatures. The early crystallization stages were well described by theory for the limiting case of homogeneous nucleation and interface controlled growth. For higher degrees of crystallinity, both growth and overall crystallization rate decreased due to compositional changes of the glassy matrix and the experimental kinetics could be described by theory if diffusion controlled growth was assumed. It was concluded that if proper precautions are taken the general theory predicts the glass-crystal transformation well.