The TTT Curves of the Heterogeneous and Homogeneous Crystallization of Lithium Disilicate - A Stochastic Approach to Crystal Nucleation
The present study explores the temperature and time dependence of heterogeneous (HET) crystal nucleation in a lithium disilicate glass using the stochastic approach. In particular, a single lithium disilicate sample was repeatedly (284 runs) undercooled to 1173 K in a PtRh-crucible and the crystallization onset time during an isothermal hold was detected in each run. The statistical distribution of the times elapsed before crystallization is described by a first-order reaction with a HET crystal nucleation rate of (9.19 ± 0.04) × 10−4 s−1 while individually shaped crystallization exotherms of each run were recorded, which indicate growth of a single or only few crystals during crystallization of the entire volume. The data were used together with results of previous stochastic experiments and those of double-stage heat treatments to calculate the crystallization time of a fraction of 10−4 percent for all temperatures between glass transition and melting. The derived TTT diagram shows a double-nose of crystallization in the volume at large undercoolings (0.53–0.61 Tm) and crystallization at the surface at small undercoolings (0.62–0.92 Tm) initiated by homogeneous and HET crystal nucleation, respectively. The critical cooling rate at the HET nose is approximately 73 K s−1.